Guide for the
Use of the International
System of Units (SI)

m
kg

s

SI molcd

K

NIST Special Publication 811

A2008 Edition

Ambler Thompson and Barry N. Taylor

NIST Special Publication 811
2008 Edition

Guide for the Use of the International
System of Units (SI)
Ambler Thompson
Technology Services

andBarry N. Taylor
Physics Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899(Supersedes NIST Special Publication 811, 1995 Edition, April 1995)
March 2008

U.S. Department of Commerce
Carlos M. Gutierrez, Secretary
National Institute of Standards and Technology
James M. Turner, Acting Director

National Institute of Standards and Technology Special Publication 811, 2008 Edition(Supersedes NIST Special Publication 811, April 1995 Edition)
Natl. Inst. Stand. Technol. Spec. Publ. 811, 2008 Ed., 85 pages (March 2008; 2nd printing November 2008)
CODEN: NSPUE3

Note on 2nd printing: This 2nd printing dated November 2008 of NIST SP811
stcorrects a number of minor typographical errors present in the 1 printing dated
March 2008.

Guide for the Use of the International System of Units (SI)

Preface
The International System of Units, universally abbreviated SI (from the French Le SystèmeInternational d’Unités), is the modern metric system of measurement. Long the dominant measurement
system used in science, the SI is becoming the dominant measurement system used in internationalcommerce.
The Omnibus Trade and Competitiveness Act of August 1988 [Public Law (PL) 100-418] changed
the name of the National Bureau of Standards (NBS) to the National Institute of Standards and Technology(NIST) and gave to NIST the added task of helping U.S. industry increase its competitiveness in the global
marketplace. It also recognized the rapidly expanding use of the SI by amending the Metric Conversion Act
of 1975 (PL 94-168). In particular, section 5164 (Metric Usage) of PL 100-418 designates
the metric system of measurement as the preferred system of weights and measures for UnitedStates trade and commerce . . .
and requires thateach Federal agency, by a date certain and to the extent economically feasible by the end of
fiscal year 1992, use the metric system of measurement in its procurements, grants, and otherbusiness-related activities, except to the extent that such use is impractical or is likely to causesignificant inefficiencies or loss of markets for United States firms . . .
In January 1991, the Department of Commerce issued an addition to the Code of Federal
Regulations entitled “Metric Conversion Policy for Federal Agencies,” 15 CFR 1170, which removes thevoluntary aspect of the conversion to the SI for Federal agencies and gives in detail the policy for thatconversion. Executive Order 12770, issued in July 1991, reinforces that policy by providing Presidential
authority and direction for the use of the metric system of measurement by Federal agencies anddepartments.*
The Metric Act of 1866 allowed use of the metric system of measurement in the United States. In
2007, the 1866 law was amended by PL 110–69, also known as the America COMPETES Act. This
amendment updated the definition of the metric system:“The metric system of measurement shall be defined as the International System of Units asestablished in 1960, and subsequently maintained, by the General Conference of Weights andMeasures, and as interpreted or modified for the United States by the Secretary of Commerce.”
The America COMPETES Act also repealed separate legislation on electrical and photometric units,
as they are included in SI, and it established UTC (Coordinated Universal Time) as the basis for standard
time in the United States.
Because of the importance of the SI to both science and technology, NIST has over the years
published documents to assist NIST authors and other users of the SI, especially to inform them of changes
in the SI and in SI usage. For example, this third edition of the Guide replaces the second edition (1995)
prepared by Barry N. Taylor, which replaced the first edition (1991) prepared by Arthur O. McCoubrey.
That edition, in turn, replaced NBS Letter Circular LC 1120 (1979), which was widely distributed in the
United States and which was incorporated into the NBS Communications Manual for Scientific, Technical,
and Public Information, a manual of instructions issued in 1980 for the preparation of technicalpublications at NBS.
* Executive Order 12770 was published in the Federal Register, Vol. 56, No. 145, p. 35801, July 29, 1991; 15 CFR 1170 was
originally published in the Federal Register, Vol. 56, No. 1, p. 160, January 2, 1991, as 15 CFR Part 19, but was redesignated
15 CFR 1170.

iii

Guide for the Use of the International System of Units (SI)
It is quite natural for NIST to publish documents on the use of the SI. First, NIST coordinates theFederal Government policy on the conversion to the SI by Federal agencies and on the use of the SI by U.S.
industry and the public. Second, NIST provides official U.S. representation in the various international
bodies established by the Meter Convention (Convention du Mètre, often called the Treaty of the Meter in
the United States), which was signed in Paris in 1875 by seventeen countries, including the United States(51 countries are now members of the Convention).
One body created by the Meter Convention is the General Conference on Weights and Measures(CGPM, Conférence Générale des Poids et Mesures), a formal diplomatic organization.** TheInternational System was in fact established by the 11th CGPM in 1960, and it is the responsibility of the
CGPM to ensure that the SI is widely disseminated and that it reflects the latest advances in science andtechnology.
This 2008 edition of the Guide corrects a small number of misprints in the 1995 edition, incorporates
the modifications made to the SI by the CGPM and CIPM in the last 13 years, and updates thebibliography. Some minor changes in format have also been made in an attempt to improve the ease of useof the Guide.
In keeping with U.S. and International practice (see Sec. C.2), this Guide uses the dot on the line as
the decimal marker. In addition this Guide utilizes the American spellings “meter,” “liter,” and “deka”
rather than “metre,” “litre,” and “deca,” and the name “metric ton” rather than “tonne.”
March 2008

Ambler Thompson

Barry N. Taylor__________________** See Ref. [1] or [2] for a brief description of the various bodies established by the Meter Convention: The International Bureau of
Weights and Measures (BIPM, Bureau International des Poids et Mesures), the International Committee for Weights and Measures(CIPM, Comité International des Poids et Mesures ), and the CGPM. The BIPM, which is located in Sèvres, a suburb of Paris, France,
and which has the task of ensuring worldwide unification of physical measurements, operates under the exclusive supervision of the
CIPM, which itself comes under the authority of the CGPM. In addition to a complete description of the SI, Refs. [1] and [2] also givethe various CGPM and CIPM resolutions on which it is based.

iv

Guide for the Use of the International System of Units (SI)

Check List for Reviewing Manuscripts
The following check list is intended to help NIST authors review the conformity of theirmanuscripts with proper SI usage and the basic principles concerning quantities and units. (The chapter or
section numbers in parentheses indicate where additional information may be found.)(1)

Only SI units and those units recognized for use with the SI are used to express the values ofquantities. Equivalent values in other units are given in parentheses following values in
acceptable units only when deemed necessary for the intended audience. (See Chapter 2.)(2)

Abbreviations such as sec (for either s or second), cc (for either cm3 or cubic centimeter), or
mps (for either m/s or meter per second), are avoided and only standard unit symbols, SI
prefix symbols, unit names, and SI prefix names are used. (See Sec. 6.1.8.)(3)

The combinations of letters “ppm,” “ppb,” and “ppt,” and the terms part per million, part perbillion, and part per trillion, and the like, are not used to express the values of quantities. The
following forms, for example, are used instead: 2.0 µL/L or 2.0 × 10–6 V, 4.3 nm/m or
4.3 × 10–9 l, 7 ps/s or 7 × 10−12 t, where V, l, and t are, respectively, the quantity symbols forvolume, length, and time. (See Sec. 7.10.3.)(4)

Unit symbols (or names) are not modified by the addition of subscripts or other information.
The following forms, for example, are used instead. (See Secs. 7.4 and 7.10.2.)
Vmax = 1000 V
a mass fraction of 10 %but not:but not:

V = 1000 Vmax
10 % (m/m) or 10 % (by weight)(5)

Statements such as “the length l1 exceeds the length l2 by 0.2 %” are avoided because it isrecognized that the symbol % represents simply the number 0.01. Instead, forms such as“l1 = l2 (1 + 0.2 %)” or “Δ = 0.2 %” are used, where Δ is defined by the relationΔ = (l1 − l2)/l2. (See Sec. 7.10.2.)(6)

Information is not mixed with unit symbols (or names). For example, the form “the watercontent is 20 mL/kg” is used and not “20 mL H2O/kg” or “20 mL of water/kg.” (SeeSec. 7.5.)(7)

It is clear to which unit symbol a numerical value belongs and which mathematical operation
applies to the value of a quantity because forms such as the following are used. (SeeSec. 7.7.)
35 cm × 48 cm
1MHz to 10 MHz or (1 to 10) MHz
20 ºC to 30 ºC or (20 to 30) ºC
123 g ± 2 g or (123 ± 2) g
70 % ± 5 % or (70 ± 5) %240 × (1 ± 10 %) V

(8)

but not:but not:but not:but not:but not:but not:

35 × 48 cm
1 MHz – 10 MHz or 1 to 10 MHz
20 ºC – 30 ºC or 20 to 30 ºC
123 ± 2 g
70 ± 5 %240 V ± 10 % (one cannot add
240 V and 10 %)Unit symbols and unit names are not mixed and mathematical operations are not applied tounit names. For example, only forms such as kg/m3, kg · m−3, or kilogram per cubic meterare used and not forms such as kilogram/m3, kg/cubic meter, kilogram/cubic meter, kg per
m3, or kilogram per meter3. (See Secs. 6.1.7, 9.5, and 9.8.)v

Guide for the Use of the International System of Units (SI)(9)

Values of quantities are expressed in acceptable units using Arabic numerals and thesymbols for the units. (See Sec. 7.6.)
m = 5 kg
the current was 15 A

(10)

but not:but not:

m = five kilograms or m = five kg
the current was 15 amperes.

There is a space between the numerical value and unit symbol, even when the value is used
as an adjective, except in the case of superscript units for plane angle. (See Sec. 7.2.)
a 25 kg spherean angle of 2º3'4"

but not:but not:a 25-kg spherean angle of 2 º3 '4 "If the spelled-out name of a unit is used, the normal rules of English are applied: “a roll of
35-millimeter film.” (See Sec. 7.6, note 3.)(11)

The digits of numerical values having more than four digits on either side of the decimalmarker are separated into groups of three using a thin, fixed space counting from both theleft and right of the decimal marker. For example, 15 739.012 53 is highly preferred to15739.01253. Commas are not used to separate digits into groups of three. (See Sec. 10.5.3.)(12)Equations between quantities are used in preference to equations between numerical values,
and symbols representing numerical values are different from symbols representing thecorresponding quantities. When a numerical-value equation is used, it is properly written
and the corresponding quantity equation is given where possible. (See Sec. 7.11.)(13)

Standardized quantity symbols such as those given in Refs. [4] and [5] are used, forexample, R for resistance and Ar for relative atomic mass, and not words, acronyms, or ad
hoc groups of letters. Similarly, standardized mathematical signs and symbols such as are
given in Ref. [4: ISO 31-11] are used, for example, “tan x” and not “tg x.” More specifically,
the base of “log” in equations is specified when required by writing loga x (meaning log tothe base a of x), lb x (meaning log2 x ), ln x (meaning loge x), or lg x (meaning log10 x ). (SeeSecs. 10.1.1 and 10.1.2.)(14)

Unit symbols are in roman type, and quantity symbols are in italic type with superscripts andsubscripts in roman or italic type as appropriate. (See Sec. 10.2 and Secs. 10.2.1 to 10.2.4.)(15)

When the word “weight” is used, the intended meaning is clear. (In science and technology,
weight is a force, for which the SI unit is the newton; in commerce and everyday use, weightis usually a synonym for mass, for which the SI unit is the kilogram.) (See Sec. 8.3.)(16)

A quotient quantity, for example, mass density, is written “mass divided by volume” rather
than “mass per unit volume.” (See Sec. 7.12.)(17)

An object and any quantity describing the object are distinguished. (Note the difference
between “surface” and “area,” “body” and “mass,” “resistor” and “resistance,” “coil” and“inductance.”) (See Sec. 7.13.)(18)

The obsolete term normality and the symbol N, and the obsolete term molarity and thesymbol M, are not used, but the quantity amount-of-substance concentration of B (morecommonly called concentration of B), and its symbol cB and SI unit mol/m3 (or a related
acceptable unit), are used instead. Similarly, the obsolete term molal and the symbol m are
not used, but the quantity molality of solute B, and its symbol bB or mB and SI unit mol/kg(or a related SI unit), are used instead. (See Secs. 8.6.5 and 8.6.8.)vi

Guide for the Use of the International System of Units (SI)

Contents
Preface.................................................................................................................................................. iii
Check List for Reviewing Manuscripts ................................................................................................. v
1 Introduction ....................................................................................................................................... 1
1.1 Purpose of Guide..................................................................................................................... 1
1.2 Outline of Guide .................................................................................................................... 1
2 NIST policy on the Use of the SI ....................................................................................................... 2
2.1 Essential data .......................................................................................................................... 2
2.1.1 Tables and graphs .......................................................................................................... 2
2.2 Descriptive information ......................................................................................................... 2
3 Other Sources of Information on the SI.............................................................................................. 3
3.1 Publications ........................................................................................................................... 3
3.2 Fundamental Constants Data Center ..................................................................................... 3
3.3 Metric Program ..................................................................................................................... 3
4 The Two Classes of SI Units and the SI Prefixes .............................................................................. 3
4.1 SI base units ..........................................................................................................................
4.2 SI derived units .....................................................................................................................
4.2.1 SI derived units with special names and symbols .........................................................
4.2.1.1 Degree Celsius .................................................................................................
4.2.2 Use of SI derived units with special names and symbols...............................................
4.3 Decimal multiples and submultiples of SI units: SI prefixes .................................................

4
4
4
5
6
7

5 Units Outside the SI............................................................................................................................ 7
5.1 Units accepted for use with the SI .......................................................................................... 7
5.1.1 Hour, degree, liter, and the like ...................................................................................... 8
5.1.2 Neper, bel, shannon, and the like ................................................................................... 8
5.1.3 Electronvolt, astronomical unit, and unified atomic mass unit ...................................... 9
5.1.4 Natural and atomic units ................................................................................................ 9
5.2 Non-SI units accepted for use with the SI............................................................................. 10
5.3 Units not accepted for use with the SI ................................................................................... 10
5.3.1 CGS units ..................................................................................................................... 10
5.3.2 Other unacceptable units .............................................................................................. 11
5.4 The terms “SI units” and “acceptable units”.......................................................................... 11
6 Rules and Style Conventions for Printing and Using Units .............................................................. 12
6.1 Rules and style conventions for unit symbols ....................................................................... 12
6.1.1 Typeface ....................................................................................................................... 12
6.1.2 Capitalization ................................................................................................................ 12
6.1.3 Plurals............................................................................................................................ 12
6.1.4 Punctuation.................................................................................................................... 12
6.1.5 Unit symbols obtained by multiplication ...................................................................... 126.1.6 Unit symbols obtained by division................................................................................ 13
6.1.7 Unacceptability of unit symbols and unit names together ............................................ 13
6.1.8 Unacceptability of abbreviations for units ................................................................... 13

vii

Guide for the Use of the International System of Units (SI)
6.2 Rules and style conventions for SI prefixes ........................................................................... 13
6.2.1 Typeface and spacing ................................................................................................... 13
6.2.2 Capitalization ................................................................................................................ 14
6.2.3 Inseparability of prefix and unit .................................................................................... 14
6.2.4 Unacceptability of compound prefixes ......................................................................... 14
6.2.5 Use of multiple prefixes ................................................................................................ 14
6.2.6 Unacceptability of stand-alone prefixes ....................................................................... 14
6.2.7 Prefixes and the kilogram ............................................................................................. 14
6.2.8 Prefixes with the degree Celsius and units accepted for use with the SI....................... 15
7 Rules and Style Conventions for Expressing Values of Quantities .................................................. 15
7.1 Value and numerical value of a quantity ............................................................................ 15
7.2 Space between numerical value and unit symbol ............................................................... 16
7.3 Number of units per value of a quantity ............................................................................. 16
7.4 Unacceptability of attaching information to units .............................................................. 16
7.5 Unacceptability of mixing information with units ............................................................. 17
7.6 Symbols for numbers and units versus spelled-out names of numbers and units ............... 17
7.7 Clarity in writing values of quantities ................................................................................ 18
7.8 Unacceptability of stand-alone unit symbols ..................................................................... 18
7.9 Choosing SI prefixes ......................................................................................................... 18
7.10 Values of quantities expressed simply as numbers: the unit one, symbol 1 ....................... 19
7.10.1 Decimal multiples and submultiples of the unit one ........................................... 19
7.10.2 %, percentage by, fraction ................................................................................... 19
7.10.3 ppm, ppb, and ppt ................................................................................................ 20
7.10.4 Roman numerals .................................................................................................. 21
7.11 Quantity equations and numerical-value equations ............................................................ 21
7.12 Proper names of quotient quantities ................................................................................... 22
7.13 Distinction between an object and its attribute ................................................................... 22
7.14 Dimension of a quantity .................................................................................................... 22
8 Comments on Some Quantities and Their Units .............................................................................. 23
8.1
8.2
8.3
8.4
8.5
8.6

8.7
8.8
8.9

Time and rotational frequency............................................................................................ 23
Volume ............................................................................................................................... 23
Weight ................................................................................................................................ 23
Relative atomic mass and relative molecular mass............................................................. 24
Temperature interval and temperature difference............................................................... 24
Amount of substance, concentration, molality, and the like ............................................... 25
8.6.1
Amount of substance ........................................................................................... 25
8.6.2
Mole fraction of B; amount-of-substance fraction of B ...................................... 25
8.6.3
Molar volume ...................................................................................................... 26
8.6.4
Molar mass ........................................................................................................... 27
8.6.5
Concentration of B; amount-of-substance concentration of B ............................ 27
8.6.6
Volume fraction of B ........................................................................................... 27
8.6.7
Mass density; density .......................................................................................... 28
8.6.8
Molality of solute B.............................................................................................. 28
8.6.9
Specific volume ................................................................................................... 28
8.6.10 Mass fraction of B ............................................................................................... 28
Logarithmic quantities and units: level, neper, bel ............................................................. 28
Viscosity ............................................................................................................................ 30
Massic, volumic, areic, lineic ............................................................................................ 30

viii

Guide for the Use of the International System of Units (SI)
9 Rules and Style Conventions for Spelling Unit Names ..................................................................... 31
9.1
9.2
9.3
9.4
9.5
9.6
9.7
9.8

Capitalization...................................................................................................................... 31
Plurals .............................................................................................................................. 31
Spelling unit names with prefixes ...................................................................................... 31
Spelling unit names obtained by multiplication ................................................................. 31
Spelling unit names obtained by division ........................................................................... 32
Spelling unit names raised to powers ................................................................................ 32
Other spelling conventions ................................................................................................ 32
Unacceptability of applying mathematical operations to unit names ................................ 32

10 More on Printing and Using Symbols and Numbers in Scientific and Technical Documents ........ 32
10.1 Kinds of symbols ............................................................................................................... 33
10.1.1 Standardized quantity symbols ............................................................................ 33
10.1.2 Standardized mathematical signs and symbols .................................................... 33
10.2 Typefaces for symbols........................................................................................................ 33
10.2.1 Quantities and variables—italic .......................................................................... 34
10.2.2 Units— roman ..................................................................................................... 35
10.2.3 Descriptive terms—roman ................................................................................... 35
10.2.4 Sample equations showing correct type .............................................................. 35
10.3 Greek alphabet in roman and italic type ............................................................................. 35
10.4 Symbols for the elements .................................................................................................. 36
10.4.1 Typeface and punctuation for element symbols ................................................... 36
10.4.2 Subscripts and superscripts on element symbols.................................................. 36
10.5 Printing numbers ................................................................................................................ 37
10.5.1 Typeface for numbers .......................................................................................... 37
10.5.2 Decimal sign or marker ....................................................................................... 37
10.5.3 Grouping digits..................................................................................................... 37
10.5.4 Multiplying numbers ........................................................................................... 37
Appendix A. Definitions of the SI Base Units...................................................................................... 39
A.1
A.2
A.3
A.4
A.5
A.6
A.7
A.8

Introduction ........................................................................................................................ 39
Meter ................................................................................................................................. 39
Kilogram............................................................................................................................. 39
Second ............................................................................................................................... 39
Ampere .............................................................................................................................. 39
Kelvin ............................................................................................................................... 39
Mole.................................................................................................................................... 39
Candela ............................................................................................................................... 39

Appendix B. Conversion Factors ......................................................................................................... 40
B.1
B.2
B.3
B.4
B.5
B.6
B.7

Introduction ....................................................................................................................... 40
Notation ............................................................................................................................. 40
Use of conversion factors ................................................................................................... 40
Organization of entries and style ....................................................................................... 41
Factor for converting motor vehicle efficiency ................................................................. 42
U.S. survey foot and mile ................................................................................................... 42
Rules for rounding numbers and converted numerical values of quantities ...................... 43
B.7.1 Rounding numbers .................................................................................................. 43
B.7.2 Rounding converted numerical values of quantities ............................................... 43
B.8 Factors for units listed alphabetically ................................................................................. 45
B.9 Factors for units listed by kind of quantity or field of science ........................................... 57

ix

Guide for the Use of the International System of Units (SI)Appendix C. Comments on the References of Appendix D—Bibliography ....................................... 70
C.1
C.2
C.3
C.4
C.5
C.6
C.7
C.8

Defining document for the SI: BIPM SI Brochure ............................................................ 70
United States version of defining document for the SI: NIST SP 330 ............................... 70
ISO and IEC ...................................................................................................................... 70
ANSI/IEEE SI-10 ............................................................................................................... 70
Federal Register notices ..................................................................................................... 71
Federal Standard 376B ...................................................................................................... 72
2006 CODATA values of the fundamental constants ........................................................ 72
Uncertainty in measurement .............................................................................................. 72

Appendix D. Bibliography .................................................................................................................. 73xGuide for the Use of the International System of Units (SI)1

Introduction

1.1

Purpose of Guide

The International System of Units was established in 1960 by the 11th General Conference on
Weights and Measures (CGPM— see Preface). Universally abbreviated SI (from the French Le SystèmeInternational d’Unités), it is the modern metric system of measurement used throughout the world. This
Guide has been prepared by the National Institute of Standards and Technology (NIST) to assist membersof the NIST staff, as well as others who may have need of such assistance, in the use of the SI in theirwork, including the reporting of results of measurements.
1.2

Outline of Guide

The Preface gives the principal Federal Government actions taken since 1988 regarding the SI andintroduces the international body— the CGPM—that is responsible for the SI.
A check list immediately follows the Preface to help NIST authors review the conformity of theirmanuscripts with proper SI usage and the basic principles concerning quantities and units.
A detailed Contents, the aim of which is to simplify the use of the Guide, follows the check list.
This introductory chapter gives the purpose of the Guide and its outline, while Chapter 2 summarizes
and clarifies the NIST policy on the use of the SI in NIST publications.
Chapter 3 notes the existence of a number of publications on the SI and gives the two organizational
units at NIST to which questions concerning the SI may be directed and from which additional information
about the SI may be obtained.
Chapter 4 discusses the fundamental aspects of the SI, including the two current classes of SI units:base, and derived; those derived units that have special names and symbols, including the degree Celsius;
and the SI prefixes that are used to form decimal multiples and submultiples of units.
Chapter 5 discusses units that are outside the SI and indicates those that may be used with it andthose that may not. It also gives (see Sec. 5.4) precise definitions of the terms “SI units” and “acceptable
units” as used in this Guide.
Chapter 6 gives the rules and style conventions for printing and using units, especially unit symbols
and SI prefix symbols.
Chapters 7 and 8, which some readers may view as the most important parts of this Guide, provide,
respectively, the rules and style conventions for expressing the values of quantities, and clarifyingcomments on some often troublesome quantities and their units.
Chapter 9 gives the rules and style conventions for spelling unit names.
Chapter 10 further elaborates on printing and using symbols and numbers in scientific and technicaldocuments and is intended to assist NIST authors prepare manuscripts that are consistent with accepted
typesetting practice.
Appendix A gives the definitions of the SI base units, while Appendix B gives conversion factors forconverting values of quantities expressed in units that are mainly unacceptable for use with the SI to values
expressed mainly in SI units. Appendix B also includes a simplified discussion of rounding numbers androunding converted numerical values of quantities. Appendix C discusses in some detail most of thereferences included in Appendix D—Bibliography, which concludes the Guide.

1

Guide for the Use of the International System of Units (SI)2

NIST Policy on the Use of the SI

In accordance with various Federal Acts, the Code of Federal Regulations, and Executive Order
12770 (see Preface), it is NIST policy that the SI shall be used in all NIST publications. 1 When the field of
application or the special needs of users of NIST publications require the use of other units, the values ofquantities shall first be expressed in acceptable units, where it is to be understood that acceptable units
include the SI units and those units recognized for use with the SI; the corresponding values expressed in
the other units shall then follow in parentheses. (For precise definitions of the terms “SI units” and“acceptable units” as used in this Guide, see Sec. 5.4.) Exceptions to this policy require the prior approvalof the NIST Director. The following three sections—2.1 Essential data, 2.1.1 Tables and graphs, and
2.2 Descriptive information—elaborate upon this policy.
2.1

Essential data

Essential data express or interpret quantitative results. All such data shall be given in acceptable
units. In those cases where— the sole use of acceptable units would compromise good communication, or— units other than acceptable units have been specified as a contractual requirement,
values of quantities shall be given in acceptable units followed, in parentheses, by the values of the same
quantities given in the other units.
Exceptions may sometimes be necessary for commercial devices, technical standards, or quantities
having special legal significance; examples include commercial weights and measures devices and therelated laws and regulations. However, even in such cases, values of quantities expressed in acceptable
units should be used when possible with the same values expressed in other units following in parentheses.
2.1.1 Tables and graphs
In tables, values of quantities expressed in acceptable units and the corresponding values expressedin other units may be shown in parallel columns, with the acceptable-unit column preceding the other-unit
column. In graphs, axes labeled in other units shall be given secondary status. This may preferably be doneby placing scale marks on and labeling the left-hand ordinate and bottom abscissa in acceptable units, andplacing scale marks on and labeling the right-hand ordinate and top abscissa in other units. Alternatively,
lighter-weight scale marks and smaller type may be employed to indicate other units using the sameordinate and abscissa as is used for the acceptable units.
2.2

Descriptive information

Descriptive information characterizes arrangements, environments, the generalized dimensions ofobjects, apparatus, or materials, and other attributes that do not enter directly into calculations or results.
When necessary for effective communication, such information may be expressed using customary terms
that are widely used and recognized. Examples include common drill sizes and traditional tools used in the
United States, U.S. standard fastener sizes, commercial pipe sizes, and other common terms used in the
trades, the professions, the marketplace, sports, and various social activities. When such descriptive
information is given, values in acceptable units are not required. For example, it is permissible to refer to a
“36-inch pipeline” or a “half-inch drill” without first giving the value in an acceptable unit.

1

The NIST policy on the use of the SI is set forth in the NIST Administration Manual, Chapter 4, Communications, Subchapter 4.09,
NIST Technical Communications Program, Appendix D—Use of Metric Units.

2Guide for the Use of the International System of Units (SI)3

Other Sources of Information on the SI

3.1

Publications

Appendix C briefly describes a number of publications that deal with the SI and related topics;
citations for these publications are given in Appendix D—Bibliography. Additional information about the
SI is also available from the two NIST organizational units indicated in Secs. 3.2 and 3.3.
3.2

Fundamental Constants Data Center

Questions concerning the more fundamental aspects of the SI and subtle aspects of proper SI usage
may be directed to:
Fundamental Constants Data Center
Physics Laboratory
National Institute of Standards and Technology
100 Bureau Drive, Stop 8420
Gaithersburg, MD 20899-8420
Telephone: (301) 975-3200
Fax: (301)-990-1350
SI Units Web Page: http://physics.nist.gov/cuu/Units/index.html
3.3

Metric Program

Questions concerning Federal Government use of the SI and Federal Government policy on the useof the SI by U.S. industry and the public may be directed to:Metric Program
Technology Services
National Institute of Standards and Technology
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http://nist.gov/metric

4

The Two Classes of SI Units and the SI Prefixes

Since the 1995 edition of this Guide, the 20th CGPM, which met October 9 - 12, 1995, decided toeliminate the class of supplementary units as a separate unit class in the SI. The SI now consists of only twoclasses of units: base units and derived units. The radian and steradian, which were the two supplementary
units, are now subsumed into the class of SI derived units. Thus the SI units are currently divided into baseunits and derived units, which together form what is called “the coherent system of SI units.” 2 The SI also
includes the prefixes to form decimal multiples and submultiples of SI units.

2
According to Ref. [4: ISO 31-0], a system of units is coherent with respect to a system of quantities and equations if the system ofunits is chosen in such a way that the equations between numerical values have exactly the same form (including the numericalfactors) as the corresponding equations between the quantities (see Secs. 7.11 and 7.14). In such a coherent system, of which the SI isan example, no numerical factor other than the number 1 ever occurs in the expressions for the derived units in terms of the base units.

3

Guide for the Use of the International System of Units (SI)4.1

SI base units

Table 1 gives the seven base quantities, assumed to be mutually independent, on which the SI isfounded, and the names and symbols of their respective units, called “SI base units.” Definitions of the SI
base units are given in Appendix A. The kelvin and its symbol K are also used to express the value of atemperature interval or a temperature difference (see Sec. 8.5).
Table 1. SI base units
SI base unit
Base quantity
lengthmass
timeelectric current
thermodynamic temperature
amount of substanceluminous intensity
4.2

Name
meter
kilogram
secondamperekelvinmole
candelaSymbol
m
kg
s
A
K
mol
cd

SI derived units

Derived units are expressed algebraically in terms of base units or other derived units. The symbols
for derived units are obtained by means of the mathematical operations of multiplication and division. For
example, the derived unit for the derived quantity molar mass (mass divided by amount of substance) is thekilogram per mole, symbol kg/mol. Additional examples of derived units expressed in terms of SI baseunits are given in Table 2. (The rules and style conventions for printing and using SI unit symbols are given
in Secs. 6.1.1 to 6.1.8.)
Table 2. Examples of SI coherent derived units expressed in terms of SI base units
SI coherent derived unit
Derived quantity
Name
Symbolarea
square meterm2
volumecubic meterm3
speed, velocity
meter per second
m/s
acceleration
meter per second squared
m/s2
wavenumber
reciprocal meterm−1
density, mass density
kilogram per cubic meter
kg/m3
specific volume
cubic meter per kilogram
m3/kg
current density
ampere per square meterA/m2
magnetic field strength
ampere per meterA/mluminance
candela per square meter
cd/m2
amount-of-substance concentration
amount concentration , concentration
mole per cubic meter
mol/m34.2.1 SI coherent derived units with special names and symbols
Certain SI coherent derived units have special names and symbols; these are given in Table 3.
Consistent with the discussion in Sec. 4, the radian and steradian, which are the two former supplementary
units, are included in Table 3. The last four units in Table 3 were introduced into the SI for reasons ofsafeguarding human health.

4

Guide for the Use of the International System of Units (SI)
Table 3. The 22 SI coherent derived units with special names and symbols.
SI coherent derived unit (a)Special nameSpecial Expression in
Expression insymbol terms of otherterms of SI base
SI unitsunits
plane angle
radian (b)
rad
1 (b)
m/m
solid angle
steradian (b)sr (c)
1 (b)
m2/m2
Hz
s−1
frequency
hertz (d)force
newtonNm · kg · s−22
m−1 · kg · s−2
pressure, stresspascalPa
N/menergy, work,
joule
J
N·m
m2 · kg · s−2
amount of heat
power, radiant flux
watt
W
J/s
m2 · kg · s−3
electric charge,
coulombC
s·A
amount of electricity
electric potential difference(e),
volt
V
W/A
m2 · kg · s−3 · A–1
electromotive force
capacitance
farad
F
C/V
m−2 · kg−1 · s4 · A2electric resistance
ohm
Ω
V/A
m2 · kg · s−3 · A−2
electric conductancesiemensS
A/Vm−2 · kg−1 · s3 · A2
magnetic flux
weber
Wb
V·s
m2 · kg · s−2 · A−1
2
kg · s−2 · A−1
magnetic flux density
tesla
T
Wb/m
inductance
henry
H
Wb/A
m2 · kg · s−2 · A−2(f)ºCKCelsius temperature
degree Celsius
Cdluminous flux
lumenlm
cd · sr(c)m−2 · cd
illuminance
lux
lx
lm/m2
(d)
becquerelBqs−1
activity referred to(g)a radionuclide
absorbed dose,
gray
Gy
J/kg
m2 · s−2
specific energy (imparted),kerma
dose equivalent,
Sv
J/kg
m2 · s−2
sievert (h)ambient dose equivalent,
directional dose equivalent,
personal dose equivalentcatalytic activity
katal
kat
s−1 · mol
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)

The SI prefixes may be used with any of the special names and symbols, but when this is done the resulting unit will no longer
be coherent. (See Sec. 6.2.8.)
The radian and steradian are special names for the number one that may be used to convey information about the quantity
concerned. In practice the symbols rad and sr are used where appropriate, but the symbol for the derived unit one is generally
omitted in specifying the values of dimensionless quantities. (See Sec 7.10.)
In photometry the name steradian and the symbol sr are usually retained in expressions for units.
The hertz is used only for periodic phenomena, and the becquerel is used only for stochastic processes in activity referred to aradionuclide.
Electric potential difference is also called “voltage” in the United States.
The degree Celsius is the special name for the kelvin used to express Celsius temperatures. The degree Celsius and the kelvin are
equal in size, so that the numerical value of a temperature difference or temperature interval is the same when expressed in either
degrees Celsius or in kelvins. (See Secs. 4.2.1.1 and 8.5.)
Activity referred to a radionuclide is sometimes incorrectly called radioactivity.
See Refs. [1, 2], on the use of the sievert.

4.2.1.1 Degree Celsius
In addition to the quantity thermodynamic temperature (symbol T), expressed in the unit kelvin, use
is also made of the quantity Celsius temperature (symbol t) defined by the equationt = T − T0,5

Guide for the Use of the International System of Units (SI)where T0 = 273.15 K by definition. To express Celsius temperature, the unit degree Celsius, symbol ºC,
which is equal in magnitude to the unit kelvin, is used; in this case, “degree Celsius” is a special name usedin place of “kelvin.” An interval or difference of Celsius temperature, however, can be expressed in the unit
kelvin as well as in the unit degree Celsius (see Sec. 8.5). (Note that the thermodynamic temperature T0 isexactly 0.01 K below the thermodynamic temperature of the triple point of water (see Sec. A.6).)
4.2.2 Use of SI derived units with special names and symbols
Examples of SI derived units that can be expressed with the aid of SI derived units having specialnames and symbols are given in Table 4.
Table 4. Examples of SI coherent derived units expressed with the aid of SI derived units having specialnames and symbols.
SI coherent derived unit
Derived quantity
Name
SymbolExpression in terms of
SI base units
dynamic viscosity
pascal second
Pa · sm−1 · kg · s−1
moment of force
newton meterN·m
m2 · kg · s−2
surface tension
newton per meterN/mkg · s–2
rad/s
m · m−1 · s−1 = s−1
angular velocity
radian per second
2m · m−1 · s−2 = s−2angular acceleration
radian per second squared rad/s
2
kg · s−3
heat flux density,
watt per square meter
W/mirradianceheat capacity, entropy
joule per kelvin
J/K
m2 · kg · s−2 · K−1
specific heat capacity,
joule per kilogram kelvin
J/(kg · K)
m2 · s−2 · K−1
specific entropy
specific energy
joule per kilogram
J/kg
m2 · s−2
thermal conductivity
watt per meter kelvinW(m · K)m · kg · s−3 · K−1
3m−1 · kg · s−2
energy density
joule per cubic meter
J/melectric field strength
volt per meterV/mm · kg · s−3 · A−1
3m−3 · s · Aelectric charge density
coulomb per cubic meterC/m2
m−2 · s · A
surface charge density
coulomb per square meter C/m2m−2 · s · Aelectric flux density,
coulomb per square meter C/melectric displacement
permittivity
farad per meterF/m
m−3 · kg−1 · s4 · A2
permeability
henry per meterH/mm · kg · s−2 · A−2
molar energy
joule per mole
J/mol
m2 · kg · s−2 · mol−1
molar entropy,
joule per mole kelvinJ/(mol · K)
m2 · kg · s−2 · K−1 · mol−1
molar heat capacity
exposure (x and γ rays)
coulomb per kilogram
C/kg
kg−1 · s · A
absorbed dose rategray per second
Gy/s
m2 · s−3
radiant intensity
watt per steradianW/srm4 · m−2 · kg · s−3 = m2 · kg · s−3
2m2 · m−2 · kg · s−3 = kg · s−3
radiance
watt per square meter
W/(m · sr)
steradiancatalytic activity
katal per cubic meterkat/m3m−3 · s−1 · molconcentration
The advantages of using the special names and symbols of SI derived units are apparent in Table 4.
Consider, for example, the quantity molar entropy: the unit J/(mol · K) is obviously more easily understoodthan its SI base-unit equivalent, m2 · kg · s−2 · K−1 · mol−1. Nevertheless, it should always be recognized thatthe special names and symbols exist for convenience; either the form in which special names or symbols
are used for certain combinations of units or the form in which they are not used is correct. For example,
because of the descriptive value implicit in the compound-unit form, communication is sometimes
facilitated if magnetic flux (see Table 3) is expressed in terms of the volt second (V · s) instead of theweber (Wb) or the combination of SI base units, m2 · kg · s−2 · A−1.

6

Guide for the Use of the International System of Units (SI)
Tables 3 and 4 also show that the values of several different quantities are expressed in the same SI
unit. For example, the joule per kelvin (J/K) is the SI unit for heat capacity as well as for entropy. Thus thename of the unit is not sufficient to define the quantity measured.
A derived unit can often be expressed in several different ways through the use of base units andderived units with special names. In practice, with certain quantities, preference is given to using certain
units with special names, or combinations of units, to facilitate the distinction between quantities whose
values have identical expressions in terms of SI base units. For example, the SI unit of frequency is
specified as the hertz (Hz) rather than the reciprocal second (s−1), and the SI unit of moment of force is
specified as the newton meter (N · m) rather than the joule (J).
Similarly, in the field of ionizing radiation, the SI unit of activity is designated as the becquerel (Bq)rather than the reciprocal second (s−1), and the SI units of absorbed dose and dose equivalent are designatedas the gray (Gy) and the sievert (Sv), respectively, rather than the joule per kilogram (J/kg).
4.3

Decimal multiples and submultiples of SI units: SI prefixes

Table 5 gives the SI prefixes that are used to form decimal multiples and submultiples of units. They
allow very large or very small numerical values (see Sec. 7.1) to be avoided. A prefix name attaches
directly to the name of a unit, and a prefix symbol attaches directly to the symbol for a unit. For example,
one kilometer, 1 km, is equal to one thousand meters, 1000 m or 103 m. When prefixes are used to form
multiples and submultiples of SI base and derived units, the resulting units are no longer coherent. (Seefootnote 2 for a brief discussion of coherence.) The rules and style conventions for printing and using SI
prefixes are given in Secs. 6.2.1 to 6.2.8. The special rule for forming decimal multiples and submultiplesof the unit of mass is given in Sec. 6.2.7.
Table 5. SI prefixes
FactorPrefix Name
1024 = (103)8
yotta
1021 = (103)7
zetta
1018 = (103)6
exa
1015 = (103)5
peta
1012 = (103)4
tera
109 = (103)3
giga
106 = (103)2
mega
103 = (103)1
kilo
102
hecto
101
deka

Symbol
Y
Z
E
P
T
G
M
k
h
da

Factor10−1
10−2
10−3 = (103)−1
10−6 = (103)−210−9 = (103)−310−12 = (103)−4
10−15 = (103)−510−18 = (103)−610−21 = (103)−710−24 = (103)−8Prefix Name
deci
centi
milli
micro
nano
pico
femto
atto
zepto
yocto

Symbol
d
c
m
µ
n
p
f
a
z
y

Note:

Alternative definitions of the SI prefixes and their symbols are not permitted. For example, it is unacceptable to use kilo (k) torepresent 210 = 1024, mega (M) to represent 220 = 1 048 576, or giga (G) to represent 230 = 1 073 741 824. See the note toRef. [5] on page 74 for the prefixes for binary powers adopted by the IEC.

5

Units Outside the SI
Units that are outside the SI, that is, non-SI units, may be divided into three categories:— those units that are accepted for use with the SI by the CIPM and hence this Guide;— those units that are not accepted for use with the SI by the CIPM, but are temporarily accepted
for use with the SI by this Guide; and— those units that are not accepted for use with the SI by either the CIPM or this Guide and in theview of this Guide must strictly be avoided.

5.1

Units accepted for use with the SI
The following four sections discuss in detail the units this Guide accepts for use with the SI.

7

Guide for the Use of the International System of Units (SI)5.1.1 Hour, degree, liter, and the like
Certain units that are not part of the SI are essential and used so widely that they are accepted by the
CIPM, and thus by this Guide, for use with the SI [2, 3]. These units are given in Table 6. The combinationof units of this table with SI units to form derived units should be restricted to special cases in order not tolose the advantages of the coherence of SI units. (The use of SI prefixes with the units of Table 6 is
discussed in Sec. 6.2.8.)
Additionally, this Guide recognizes that situations on occasion will require the use of time-related
units other than those given in Table 6; such as using intervals of time be expressed in weeks, months, or
years. In such cases, if a standardized symbol for the unit is not available, the name of the unit should be
written out in full. (See Sec. 8.1 for a suggestion regarding the symbol for year and Chapter 9 for the rules
and style conventions for spelling unit names.)
Table 6. Non-SI units accepted for use with the SI by the CIPM and this Guide
Name
SymbolValue in SI unitsminminute1 min = 60 s
time
h
hour
1 h = 60 min = 3600 sdday
1 d = 24 h
= 86 400 sºdegree1º= (π/180) rad
plane angle (a)'minute1'= (1/60)º = (π/10 800) rad"second1"= (1/60)' = (π/648 000) radhahectare(h)1ha = 1 hm2 = 104 m2
liter
1 L = 1 dm3 = 10−3 m3
L(b), l(c)
1t
= 103 kg
metric ton
T[see footnote (g) regarding the numericalneper
Np(d, f )(e, f )
value of logarythmic ratio quantities suchBbel(e, f )
as the neper, the bel, and the decibel]dB
decibel}}(a) See also Sec. 7.2.
(b) The alternative symbol for the liter, L, was adopted by the CGPM in order to avoid the risk of confusion between the letter l andthe number 1 (see Ref. [1] or [2]). Thus, although both l and L are internationally accepted symbols for the liter, to avoid this risk
the symbol to be used in the United States is L (see Refs. [2] and [6]). The script letter l is not an approved symbol for the liter.
(c) This is the name to be used for this unit in the United States (see Refs. [2] and [6]); it is also used in some other English-speaking
countries. However, this unit is called “tonne” in Ref. [1] and is the name used in many countries.
(d) The statement LA = n Np (where n is a number) is interpreted to mean that ln(A2 / A1) = n. Thus when LA = 1 Np, A2 / A1 = e. The
symbol A is used here to denote the amplitude of a sinusoidal signal, and LA is then called the Napierian logarithmic amplitude
ratio, or the Napierian amplitude level difference.
(e) The statement LX = m dB = (m / 10) B (where m is a number) is interpreted to mean that lg(X / X0) = m/10. Thus when LX = 1 B,
X / X0 = 10, and when LX = 1 dB, X / X0 = 101/10. If X denotes a mean square signal or power-like quantity, LX is called a power
level referred to X0. (See Sec. 8.7.)(f) In using these units it is important that the nature of the quantity be specified, and that any reference value used be specified.
These units are not SI units, but they have been accepted by the CIPM for use with the SI. For additional information on theneper and bel, see Ref. [5: IEC 60027-3], and Sec. 8.7 of this Guide.
(g) The numerical values of the neper, bel, and decibel (and hence the relation of the bel and the decibel to the neper) are rarely
required. They depend on the way in which the logarithmic quantities are defined.
(h) This unit and its symbol are used to express agrarian area.

5.1.2 Electronvolt, astronomical unit, and unified atomic mass unit
The CIPM, and thus this Guide, accepts for use with the SI the units given in Table 7 [1, 2]. These
units are used in specialized fields; their values in SI units must be obtained from experiment and,
therefore, are not known exactly. (The use of SI prefixes with the units of Table 7 is discussed in Sec.
6.2.8.)

8

Guide for the Use of the International System of Units (SI)
Table 7. Non-SI Units accepted for use with the SI by the CIPM and this Guide, whose values in SI units
are obtained experimentally
Name
SymbolDefinition and Value in SI units
electronvolteV(a)astronomical unitua(b)
unified atomic mass unitu(c)daltonDa(d)(a) The electronvolt is the kinetic energy acquired by an electron in passing through a potential difference of 1 V in vacuum,
1.602 176 487(40) × 10−19 J. This value of 1 eV is the 2006 CODATA recommended value with the standard uncertainty in thelast two digits given in parenthesis [19, 20].
(b) The astronomical unit is approximately equal to the mean Earth-Sun distance. It is the radius of an unperturbed circular
Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a mean motion of 0.017 202 098 95 radians
per day (known as the Gaussian constant). The value and standard uncertainty of the astronomical unit, ua, is 1.495 978 706
91(6) × 1011 m. This is cited from the IERS Conventions 2003 (D.D. McCarthy and G. Petit eds., IERS Technical Note 32,
Frankfurt am Main: Verlag des Bundesamts für Kartographie und Geodäsie, 2004, 12). The value of the astronomical unit inmeters comes from the JPL ephemerides DE403 (Standish E.M., Report of the IAU WGAS Sub-Group on Numerical Standards,
Highlights of Astronomy, Appenzeller ed., Dordrecht: Kluwer Academic Publishers, 1995, 180-184).
(c) The unified atomic mass unit is equal to 1/12 times the mass of a free carbon 12 atom, at rest and in its ground state,
1.660 538 782(83) × 10−27 kg. This value of 1 u is the 2006 CODATA recommended value with the standard uncertainty in thelast two digits given in parenthesis [19, 20].
(d) The dalton (Da) and the unified atomic mass unit (u) are alternative names (and symbols) for the same unit, equal to 1/12 times
the mass of a free carbon 12 atom, at rest and in its ground state. The dalton is often combined with SI prefixes, for example toexpress the masses of large molecules in kilodaltons, kDa, or megadaltons, MDa.
Note: The abbreviation, AMU is not an acceptable unit symbol for the unified atomic mass unit. The only allowed name is “unified
atomic mass unit” and the only allowed symbol is u.

5.1.3 Units from International Standards
There are a few highly specialized units that are given by the International Organization forStandardization (ISO) or the International Electrotechnical Commission (IEC) and which in the view of thisGuide are also acceptable for use with the SI. They include the octave, phon, and sone, and units used in
information technology, including the baud (Bd), bit (bit), erlang (E), hartley (Hart), and shannon (Sh)3. It
is the position of this Guide that the only such additional units NIST authors may use with the SI are thosegiven in either the International Standards on quantities and units of ISO (Ref. [4]) or of IEC (Ref. [5]).
5.1.4 Natural and atomic units
In some cases, particularly in basic science, the values of quantities are expressed in terms offundamental constants of nature. The two most important of these unit systems are the natural unit (n.u.)system used in high energy or particle physics, and the atomic unit (a.u.) system used in atomic physics andquantum chemistry. The use of these units with the SI is not formally accepted by the CIPM, but the CIPM
recognizes their existence and importance. Therefore, this Guide formally accepts their use when it isnecessary for effective communication. In such cases, the specific unit system used must be identified.
Examples of physical quantities used as units are given in Table 8.
Table 8. Examples of physical quantities sometimes used as units
Kind of quantity
Physical quantity used as a unit
speed
speed of light in vacuum (n.u.)
actionPlanck constant divided by 2π (n.u.)
masselectron rest mass (n.u. and a.u.)electric chargeelementary charge (a.u.)
lengthBohr radius (a.u.)energyHartree energy (a.u.)timeratio of action to energy (a.u.)3

Symbol
c
ħ
me
e
a0
Eh
ħ/ Eh

The symbol in parentheses following the name of the unit is its internationally accepted unit symbol, but the octave, phon, and sone
have no such unit symbols. For additional information on the neper and bel, see Ref. [5: IEC 60027-3], and Sec. 8.7 of this Guide.

9

Guide for the Use of the International System of Units (SI)
5.2

Other Non-SI units accepted for use with the SI

Because of established practice in certain fields or countries, in 1978 the CIPM considered that it
was permissible for the following units given in Table 9, nautical mile, knot, angstrom, are, barn, bar, andmillimeter of mercury to continue to be used with the SI. However, these units must not be introduced infields where they are not presently used. Further, this Guide strongly discourages the continued use of theseunits by NIST authors except when absolutely necessary. If these units are used by NIST authors thevalues of relevant quantities shall be given in terms of SI units first followed by these non-SI units inparentheses.
The curie, roentgen, rad, and rem have been added to the NIST SP 330 [2] and Table 9 of this Guide,
since they are in wide use in the United States, especially in regulatory documents dealing with health andsafety. Nevertheless, this Guide strongly discourages the continued use of the curie, roentgen, rad, and rem
and recommends that SI units should be used by NIST authors only if necessary. If these units are used by
NIST authors the values of relevant quantities shall be given in terms of SI units first followed by theseoutdated non-SI units in parentheses.
Table 9. Other non-SI units accepted for use with the SI either by the CIPM and this Guide (indicated by*),or by this Guide (indicated by**)Name
SymbolValue in SI units
nautical mile*
1 nautical mile = 1852 m
knot*
1 nautical mile per hour = (1852/3600) m/sångström*Å
1 Å = 0.1 nm = 10−10 m*barn
1 b = 100 fm2 = 10−28 m2b*bar
1 bar = 0.1 MPa = 100 kPa = 1000 hPa = 105 Pabar*millimeter of mercurymmHg
1 mmHg ≈ 133.322 Pa
curie**
roentgen**
rad**
rem**
(a)

5.3

1 Ci = 3.7 x 1010 Bq
1 R = 2.58 x 10−4 C/kg
1 rad = 1 cGy = 10−2 Gy
1 rem = 1 cSv = 10−2 Sv

CiR
rad(a)
rem

When there is risk of confusion with the symbol for the radian, rd may be used as the symbol for rad.

Units not accepted for use with the SI
The following two sections briefly discuss units not accepted for use with the SI.

5.3.1 CGS units
Table 10 gives examples of centimeter-gram-second (CGS) units having special names. These units
are not accepted for use with the SI by this Guide. Further, no other units of the various CGS systems ofunits, which includes the CGS Electrostatic (ESU), CGS Electromagnetic (EMU), and CGS Gaussian
systems, are accepted for use with the SI by this Guide except such units as the centimeter, gram, andsecond that are also defined in the SI.

10

Guide for the Use of the International System of Units (SI)
Table 10. Examples of CGS units with special names (not accepted for use with the SI by this Guide)
Name
SymbolValue in SI unitserg
erg
1 erg = 10−7 Jdyne
1 dyn = 10−5 N
dyn
(a)
poise
P
1 P = 1 dyn · s/cm2 = 0.1 Pa · s(b)stokesSt
1 St = 1 cm2/s = 10−4 m2/s
(c)
gauss
Gs, G
1 Gs corresponds to 10−4 T(c)
1 Oe corresponds to (1000/4π) A/m
oerstedOemaxwell(c)Mx
1 Mx corresponds to 10−8 Wb
stilbsb
1 sb = 1 cd/cm2 = 104 cd/m2
1 ph = 104 lx
phot
ph
(d)
gal
Gal
1 Gal = 1 cm s−2 = 10−2 m s−2(a) The poise (P) is the CGS unit for viscosity (also called dynamic viscosity). The SI unit is the pascal second (Pa · s).
(b) The stokes (St) is the CGS unit for kinematic viscosity. The SI unit is the meter squared per second (m2/s).
(c) This unit is part of the so-called electromagnetic three-dimensional CGS system and cannot strictly speaking be compared to thecorresponding SI unit, which has four dimensions when only mechanical and electric quantities are considered.
(d) The gal is employed in geodesy and geophysics to express acceleration due to gravity.

5.3.2 Other unacceptable units
There are many units besides CGS units that are outside the SI and not accepted for use with it,
including, of course, all of the U.S. customary (that is, inch-pound) units. In the view of this Guide such
units must strictly be avoided and SI units, their multiples or submultiples, or those units accepted or
temporarily accepted for use with the SI (including their appropriate multiples and submultiples), must be
used instead. This restriction also applies to the use of unaccepted special names for SI units or specialnames for multiples or submultiples of SI units, such as mho for siemens (S) and micron for micrometer(μm). Table 11 gives a few examples of some of these other unacceptable units.
Table 11. Examples of other unacceptable units
Name
Symbolfermi
fermi
photometric carat
metric carat
torr
Torr
standard atmosphereatmkilogram-force
kgf
micronμcalorie (various)
calth (thermochemical)
xu
x unit
st
stere
γ
gamma
γ
gamma (mass)
λlambda (volume)
5.4

Value in SI units
1 fermi = 1 fm = 10−15 m
1 metric carat = 200 mg = 2 × 10−4 kg
1 Torr = (101 325/760) Pa
1 atm = 101 325 Pa
1 kgf = 9.806 65 N
1 m = 1 µm = 10−6 m
1 calth = 4.184 J
1 xu ≈ 0.1002 pm = 1.002 × 10−13 m
1 st = 1 m3
1 γ = 1 nT = 10−9 T
1 γ = 1 µg = 10−9 kg
1 λ = 1 µL = 10−6 L = 10−9 m3

The terms “SI units” and “acceptable units”

Consistent with accepted practice [1, 2], this Guide uses either the term “SI units” or “units of theSI” to mean the SI base units and SI coherent derived units, and multiples and submultiples of these units
formed by using the SI prefixes. The term “acceptable units,” which is introduced in this Guide forconvenience, is used to mean the SI units plus (a) those non-SI units accepted for use with the SI (seeTables 6 - 9); and (b) appropriate multiples and submultiples of such accepted non-SI units.

11Guide for the Use of the International System of Units (SI)6

Rules and Style Conventions for Printing and Using Units

6.1

Rules and style conventions for unit symbols
The following eight sections give rules and style conventions related to the symbols for units.

6.1.1 Typeface
Unit symbols are printed in roman (upright) type regardless of the type used in the surrounding text.
(See also Sec. 10.2 and Secs. 10.2.1 to 10.2.4.)
6.1.2 Capitalization
Unit symbols are printed in lower-case letters except that:(a) the symbol or the first letter of the symbol is an upper-case letter when the name of the unit is derived
from the name of a person; and(b) the recommended symbol for the liter in the United States is L. (See Table 6, footnote (b).)
Examples:m (meter) s (second)
Pa (pascal) lm (lumen)

V (volt)
Wb (weber)

6.1.3 Plurals
Unit symbols are unaltered in the plural.
Example:l = 75 cm

but not:l = 75 cms

Note: l is the quantity symbol for length. (The rules and style conventions for expressing the values ofquantities are discussed in detail in Chapter 7.)
6.1.4 Punctuation
Unit symbols are not followed by a period unless at the end of a sentence.
Example:“Its length is 75 cm.” or “It is 75 cm long.” but not: “It is 75 cm. long.”

6.1.5 Unit symbols obtained by multiplication
Symbols for units formed from other units by multiplication are indicated by means of either a halfhigh (that is, centered) dot or a space. However, this Guide, as does Ref. [6], prefers the half-high dot
because it is less likely to lead to confusion.
Example:N · m or N m

Notes:
1.

A half-high dot or space is usually imperative. For example, m · s−1 is the symbol for the meter per
second while ms−1 is the symbol for the reciprocal millisecond (103 s−1— see Sec. 6.2.3).

2.

Reference [4: ISO 31-0] suggests that if a space is used to indicate units formed by multiplication,
the space may be omitted if it does not cause confusion. This possibility is reflected in thecommon practice of using the symbol kWh rather than kW · h or kW h for the kilowatt hour.
Nevertheless, this Guide takes the position that a half-high dot or a space should always be used to12

Guide for the Use of the International System of Units (SI)avoid possible confusion; for this same reason, only one of these two allowed forms should be
used in any given manuscript.
6.1.6 Unit symbols obtained by division
Symbols for units formed from other units by division are indicated by means of a solidus (obliquestroke, / ) , a horizontal line, or negative exponents.
Example:m/s, m , or m · s−1sHowever, to avoid ambiguity, the solidus must not be repeated on the same line unless parentheses
are used.
Examples:m/s2 or m · s−2 but not: m/s/s
m · kg/(s3 · A) or m · kg · s−3 · A−1

but not:m · kg/s3/A

Negative exponents should be used in complicated cases.
6.1.7 Unacceptability of unit symbols and unit names together
Unit symbols and unit names are not used together. (See also Secs. 9.5 and 9.8.)
Example:C/kg, C · kg−1, or coulomb per kilogram

but not:

coulomb/kg; coulomb per kg;
C/kilogram; coulomb · kg−1; C perkg; coulomb/kilogram

6.1.8 Unacceptability of abbreviations for units
Because acceptable units generally have internationally recognized symbols and names, it is not
permissible to use abbreviations for their unit symbols or names, such as sec (for either s or second), sq.
mm (for either mm2 or square millimeter), cc (for either cm3 or cubic centimeter), mins (for either min or
minutes), hrs (for either h or hours), lit (for either L or liter), amps (for either A or amperes), AMU (foreither u or unified atomic mass unit), or mps (for either m/s or meter per second). Although the values ofquantities are normally expressed using symbols for numbers and symbols for units (see Sec. 7.6), if forsome reason the name of a unit is more appropriate than the unit symbol (see Sec. 7.6, note 3), the name of
the unit should be spelled out in full.
6.2

Rules and style conventions for SI prefixes
The following eight sections give rules and style conventions related to the SI prefixes.

6.2.1 Typeface and spacing
Prefix names and symbols are printed in roman (upright) type regardless of the type used in thesurrounding text, and are attached to unit symbols without a space between the prefix name or symbol andthe unit name or symbol. This last rule also applies to prefixes attached to unit names.
Examples:mL (milliliter)pm (picometer)

13GΩ (gigaohm)THz (terahertz)

Guide for the Use of the International System of Units (SI)6.2.2 Capitalization
The prefix symbols Y (yotta), Z (zetta), E (exa), P (peta), T (tera), G (giga), and M (mega) are printed inupper-case letters while all other prefix symbols are printed in lower-case letters (see Table 5). Prefixnames are normally printed in lowercase letters.
6.2.3 Inseparability of prefix and unit
The grouping formed by a prefix symbol attached to a unit symbol constitutes a new inseparable
symbol (forming a multiple or submultiple of the unit concerned) which can be raised to a positive or
negative power and which can be combined with other unit symbols to form compound unit symbols.
Examples:

2.3 cm3 = 2.3 (cm)3 = 2.3 (10−2 m)3 = 2.3 × 10−6 m3
1 cm−1 = 1 (cm)−1 = 1 (10−2 m)−1 = 102 m−1
5000 µs−1 = 5000 (µs)−1 = 5000 (10−6 s)−1 = 5000 × 106 s−1 = 5 × 109 s−1
1 V/cm = (1 V)/(10−2 m) = 102 V/mPrefix names are also inseparable from the unit names to which they are attached. Thus, for example,
millimeter, micropascal, and meganewton are single words.
6.2.4 Unacceptability of compound prefixes
Compound prefix names or symbols, that is, prefix names or symbols formed by the juxtaposition of
two or more prefix names or symbols, are not permitted.
Example:nm (nanometer)

but not: mµm (millimicrometer)

6.2.5 Use of multiple prefixes
In a derived unit formed by division, the use of a prefix symbol (or a prefix name) in both thenumerator and the denominator can cause confusion. Thus, for example, 10 kV/mm is acceptable, but
10 MV/m is often considered preferable because it contains only one prefix symbol and it is in the
numerator.
In a derived unit formed by multiplication, the use of more than one prefix symbol (or more than one
prefix name) can also cause confusion. Thus, for example, 10 MV · ms is acceptable, but 10 kV · s is often
considered preferable.
Note:

Such considerations usually do not apply if the derived unit involves the kilogram. For example,
0.13 mmol/g is not considered preferable to 0.13 mol/kg.

6.2.6 Unacceptability of stand-alone prefixes
Prefix symbols cannot stand alone and thus cannot be attached to the number 1, the symbol for theunit one. In a similar vein, prefix names cannot be attached to the name of the unit one, that is, to the word
“one.” (See Sec. 7.10 for a discussion of the unit one.)
Example:the number density of Pb atoms is 5 ×106/m3

but not:

the number density of Pb atoms
is 5 M/m3

6.2.7 Prefixes and the kilogram
For historical reasons, the name “kilogram” for the SI base unit of mass contains the name “kilo,”
the SI prefix for 103. Thus, because compound prefixes are unacceptable (see Sec. 6.2.4), symbols for
decimal multiples and submultiples of the unit of mass are formed by attaching SI prefix symbols to g, the14

Guide for the Use of the International System of Units (SI)
unit symbol for gram, and the names of such multiples and submultiples are formed by attaching SI prefixnames to the name “gram.”
Example: 10−6 kg = 1 mg (1 milligram)but not: 10−6 kg = 1 µkg (1 microkilogram)

6.2.8 Prefixes with the degree Celsius and units accepted for use with the SI
Prefix symbols may be used with the unit symbol ºC and prefix names may be used with the unit
name “degree Celsius.” For example, 12 mºC (12 millidegrees Celsius) is acceptable. However, to avoid
confusion, prefix symbols (and prefix names) are not used with the time-related unit symbols (names) min(minute), h (hour), d (day); nor with the angle-related symbols (names) º (degree), ' (minute), and" (second) (see Table 6).
Prefix symbols (and prefix names) may be used with the unit symbols (names) L (liter), t (metricton), eV (electronvolt), u (unified atomic mass unit), Da (dalton) (see Tables 6 and 7). However, althoughsubmultiples of the liter such as mL (milliliter) and dL (deciliter) are in common use, multiples of the litersuch as kL (kiloliter) and ML (megaliter) are not. Similarly, although multiples of the metric ton such as kt(kilometric ton) are commonly used, submultiples such as mt (millimetric ton), which is equal to thekilogram (kg), are not. Examples of the use of prefix symbols with eV and u are 80 MeV (80megaelectronvolts) and 15 nu (15 nanounified atomic mass units).

7

Rules and Style Conventions for Expressing Values of Quantities

7.1

Value and numerical value of a quantity

The value of a quantity is its magnitude expressed as the product of a number and a unit, and thenumber multiplying the unit is the numerical value of the quantity expressed in that unit.
More formally, the value of quantity A can be written as A = {A}[A], where {A} is the numericalvalue of A when the value of A is expressed in the unit [A]. The numerical value can therefore be written as{A} = A / [A], which is a convenient form for use in figures and tables. Thus, to eliminate the possibility of
misunderstanding, an axis of a graph or the heading of a column of a table can be labeled “t/ºC” instead of“t (ºC)” or “Temperature (ºC).” Similarly, an axis or column heading can be labeled “E/(V/m)” instead of“E (V/m)” or “Electric field strength (V/m).”Examples:1.

In the SI, the value of the velocity of light in vacuum is c = 299 792 458 m/s exactly. The number
299 792 458 is the numerical value of c when c is expressed in the unit m/s, and equals c/(m/s).

2.

The ordinate of a graph is labeled T/(103 K), where T is thermodynamic temperature and K is theunit symbol for kelvin, and has scale marks at 0, 1, 2, 3, 4, and 5. If the ordinate value of a point
on a curve in the graph is estimated to be 3.2, the corresponding temperature is T / (103 K) = 3.2 orT = 3200 K. Notice the lack of ambiguity in this form of labeling compared with “Temperature(103 K).”

3.

An expression such as ln(p/MPa), where p is the quantity symbol for pressure and MPa is the unit
symbol for megapascal, is perfectly acceptable, because p/MPa is the numerical value of p when p
is expressed in the unit MPa and is simply a number.

Notes:
1.

For the conventions concerning the grouping of digits, see Sec. 10.5.3.

15

Guide for the Use of the International System of Units (SI)2.
7.2

An alternative way of writing c/(m/s) is {c}m/s, meaning the numerical value of c when c isexpressed in the unit m/s.
Space between numerical value and unit symbol

In the expression for the value of a quantity, the unit symbol is placed after the numerical value anda space is left between the numerical value and the unit symbol.
The only exceptions to this rule are for the unit symbols for degree, minute, and second for planeangle: º, ', and ", respectively (see Table 6), in which case no space is left between the numerical value andthe unit symbol.
Example:Note:

α = 30º22'8"

α is a quantity symbol for plane angle.

This rule means that:(a) The symbol ºC for the degree Celsius is preceded by a space when one expresses the values of Celsius
temperatures.
Example:t = 30.2 ºC

but not:t = 30.2ºCort = 30.2º C(b) Even when the value of a quantity is used as an adjective, a space is left between the numerical value
and the unit symbol. (This rule recognizes that unit symbols are not like ordinary words orabbreviations but are mathematical entities, and that the value of a quantity should be expressed in a
way that is as independent of language as possible—sees Secs. 7.6 and 7.10.3.)
Examples:a 1 m end gauge
a 10 kΩ resistor

but not:but not:

a 1-m end gauge
a 10-kΩ resistor

However, if there is any ambiguity, the words should be rearranged accordingly. For example, thestatement “the samples were placed in 22 mL vials” should be replaced with the statement “the samples
were placed in vials of volume 22 mL.”
Note:

When unit names are spelled out, the normal rules of English apply. Thus, for example, “a roll of
35-millimeter film” is acceptable (see Sec. 7.6, note 3).

7.3 Number of units per value of a quantity
The value of a quantity is expressed using no more than one unit.
Example:
Note:

7.4

l = 10.234 m

but not:l = 10 m 23 cm 4 mm

Expressing the values of time intervals and of plane angles are exceptions to this rule. However, it
is preferable to divide the degree decimally. Thus one should write 22.20º rather than 22º12',
except in fields such as cartography and astronomy.
Unacceptability of attaching information to units

When one gives the value of a quantity, it is incorrect to attach letters or other symbols to the unit inorder to provide information about the quantity or its conditions of measurement. Instead, the letters orother symbols should be attached to the quantity.
Example:Vmax = 1000 V

but not:

V = 1000 Vmax

16

Guide for the Use of the International System of Units (SI)Note:

V is a quantity symbol for potential difference.

7.5

Unacceptability of mixing information with units

When one gives the value of a quantity, any information concerning the quantity or its conditions ofmeasurement must be presented in such a way as not to be associated with the unit. This means thatquantities must be defined so that they can be expressed solely in acceptable units (including the unit
one — see Sec. 7.10).
Examples:the Pb content is 5 ng/L

but not:

5 ng Pb/L or 5 ng of lead/L

the sensitivity for NO3 molecules is 5 × 1010/cm3

but not:

the sensitivity is 5 × 1010
NO3 molecules/cm3

the neutron emission rate is 5 × 1010/s

but not:

the emission rate is 5 × 1010 n/s

the number density of O2 atoms is 3 × 1018/cm3

but not:

the density is 3 × 1018 O2 atoms/cm3

the resistance per square is 100 Ω

but not:

the resistance is 100 Ω/square

7.6

Symbols for numbers and units versus spelled-out names of numbers and units

This Guide takes the position that the key elements of a scientific or technical paper, particularly the
results of measurements and the values of quantities that influence the measurements, should be presented
in a way that is as independent of language as possible. This will allow the paper to be understood by asbroad an audience as possible, including readers with limited knowledge of English. Thus, to promote thecomprehension of quantitative information in general and its broad understandability in particular, valuesof quantities should be expressed in acceptable units using— the Arabic symbols for numbers, that is, the Arabic numerals, not the spelled-out names of the
Arabic numerals; and— the symbols for the units, not the spelled-out names of the units.
Examples:the length of the laser is 5 m

but not:

the length of the laser is five meters

the sample was annealed at atemperature of 955 K for 12 h

but not:

the sample was annealed at a temperatureof 955 kelvins for 12 hours

Notes:
1.

If the intended audience for a publication is unlikely to be familiar with a particular unit symbol, it
should be defined when first used.

2.

Because the use of the spelled-out name of an Arabic numeral with a unit symbol can cause
confusion, such combinations must strictly be avoided. For example, one should never write “thelength of the laser is five m.”3.

Occasionally, a value is used in a descriptive or literary manner and it is fitting to use the spelledout name of the unit rather than its symbol. Thus, this Guide considers acceptable statements such

17

Guide for the Use of the International System of Units (SI)as “the reading lamp was designed to take two 60-watt light bulbs,” or “the rocket journeyed
uneventfully across 380 000 kilometers of space,” or “they bought a roll of 35-millimeter film fortheir camera.”4.

7.7

The United States Government Printing Office Style Manual (Ref. [3], pp. 181-189) gives the rulethat symbols for numbers are always to be used when one expresses (a) the value of a quantity interms of a unit of measurement, (b) time (including dates), and (c) an amount of money. Thispublication should be consulted for the rules governing the choice between the use of symbols fornumbers and the spelled-out names of numbers when numbers are dealt with in general.
Clarity in writing values of quantities

The value of a quantity is expressed as the product of a number and a unit (see Sec. 7.1). Thus, toavoid possible confusion, this Guide takes the position that values of quantities must be written so that it iscompletely clear to which unit symbols the numerical values of the quantities belong. Also to avoid
possible confusion, this Guide strongly recommends that the word “to” be used to indicate a range ofvalues for a quantity instead of a range dash (that is, a long hyphen) because the dash could be
misinterpreted as a minus sign. (The first of these recommendations once again recognizes that unit
symbols are not like ordinary words or abbreviations but are mathematical entities—see Sec. 7.2.)
Examples:
51 mm × 51 mm × 25 mm

but not:

51 × 51 × 25 mm

225 nm to 2400 nm or (225 to 2400) nm

but not:

225 to 2400 nm

0 ºC to 100 ºC or (0 to 100) ºC

but not:

0 ºC − 100 ºC

0 V to 5 V or (0 to 5) V

but not:

0−5V(8.2, 9.0, 9.5, 9.8, 10.0) GHz

but not:

8.2, 9.0, 9.5, 9.8, 10.0 GHz

63.2 m ± 0.1 m or (63.2 ± 0.1) m

but not:

63.2 ± 0.1 m or 63.2 m ± 0.1

129 s − 3 s = 126 s or (129 − 3) s = 126 s

but not:

129 − 3 s = 126 s

Note: For the conventions concerning the use of the multiplication sign, see Sec. 10.5.4.
7.8

Unacceptability of stand-alone unit symbols

Symbols for units are never used without numerical values or quantity symbols (they are notabbreviations).
Examples:7.9

but not: there are many mm in a km
there are 106 mm in 1 km
it is sold by the cubic meterbut not: it is sold by the m3
t/ºC, E/(V/m), p/MPa, and the like are perfectly acceptable (see Sec. 7.1).

Choosing SI prefixes

The selection of the appropriate decimal multiple or submultiple of a unit for expressing the value ofa quantity, and thus the choice of SI prefix, is governed by several factors.
These include:— the need to indicate which digits of a numerical value are significant,— the need to have numerical values that are easily understood, and18

Guide for the Use of the International System of Units (SI)— the practice in a particular field of science or technology.
A digit is significant if it is required to express the numerical value of a quantity. In the expression
l = 1200 m, it is not possible to tell whether the last two zeroes are significant or only indicate themagnitude of the numerical value of l. However, in the expression l = 1.200 km, which uses the SI prefix
symbol for 103 (kilo, symbol k), the two zeroes are assumed to be significant because if they were not, thevalue of l would have been written l = 1.2 km.
It is often recommended that, for ease of understanding, prefix symbols should be chosen in such a
way that numerical values are between 0.1 and 1000, and that only prefix symbols that represent the
number 10 raised to a power that is a multiple of 3 should be used.
Examples:

3.3 × 107 Hz may be written as 33 × 106 Hz = 33 MHz
0.009 52 g may be written as 9.52 × 10−3 g = 9.52 mg
2703 W may be written as 2.703 × 103 W = 2.703 kW
5.8 × 10−8 m may be written as 58 × 10−9 m = 58 nm

However, the values of quantities do not always allow this recommendation to be followed, nor is it
mandatory to try to do so.
In a table of values of the same kind of quantities or in a discussion of such values, it is usually
recommended that only one prefix symbol should be used even if some of the numerical values are not
between 0.1 and 1000. For example, it is often considered preferable to write “the size of the sample is
10 mm × 3 mm × 0.02 mm” rather than “the size of the sample is 1 cm × 3 mm × 20 µm.”
In certain kinds of engineering drawings it is customary to express all dimensions in millimeters.
This is an example of selecting a prefix based on the practice in a particular field of science or technology.
7.10 Values of quantities expressed simply as numbers: the unit one, symbol 1
Certain quantities, such as refractive index, relative permeability, and mass fraction, are defined as
the ratio of two mutually comparable quantities and thus are of dimension one (see Sec. 7.14). The coherent
SI unit for such a quantity is the ratio of two identical SI units and may be expressed by the number 1.
However, the number 1 generally does not appear in the expression for the value of a quantity of dimension
one. For example, the value of the refractive index of a given medium is expressed as n = 1.51 × 1 = 1.51.
On the other hand, certain quantities of dimension one have units with special names and symbols
which can be used or not depending on the circumstances. Plane angle and solid angle, for which the SI
units are the radian (rad) and steradian (sr), respectively, are examples of such quantities (see Sec. 4.2.1).
7.10.1

Decimal multiples and submultiples of the unit one

Because SI prefix symbols cannot be attached to the unit one (see Sec. 6.2.6), powers of 10 are used
to express decimal multiples and submultiples of the unit one.
Example:µr = 1.2 × 10-6

but not:µr = 1.2 µNote:µr is the quantity symbol for relative permeability.

7.10.2%, percentage by, fraction

In keeping with Ref. [4: ISO 31-0], this Guide takes the position that it is acceptable to use theinternationally recognized symbol % (percent) for the number 0.01 with the SI and thus to express thevalues of quantities of dimension one (see Sec. 7.14) with its aid. When it is used, a space is left between

19

Guide for the Use of the International System of Units (SI)the symbol % and the number by which it is multiplied [4: ISO 31-0]. Further, in keeping with Sec. 7.6, thesymbol % should be used, not the name “percent.”
Example:Note:

xB = 0.0025 = 0.25 %but not:xB = 0.0025 = 0.25% or xB = 0.25 percent

xB is the quantity symbol for amount-of-substance fraction of B (see Sec. 8.6.2).

Because the symbol % represents simply a number, it is not meaningful to attach information to it(see Sec. 7.4). One must therefore avoid using phrases such as “percentage by weight,” “percentage bymass,” “percentage by volume,” or “percentage by amount of substance.” Similarly, one must avoid
writing, for example, “% (m/m),” “% (by weight),” “% (V/V),” “% (by volume),” or “% (mol/mol).” Thepreferred forms are “the mass fraction is 0.10,” or “the mass fraction is 10 %,” or “wB = 0.10,” or “wB =10 %” (wB is the quantity symbol for mass fraction of B—see Sec. 8.6.10); “the volume fraction is 0.35,” or“the volume fraction is 35 %,” or “ φB = 0.35,” or “φB = 35 %” (φB is the quantity symbol for volume
fraction of B—see Sec. 8.6.6); and “the amount-of-substance fraction is 0.15,” or “the amount-of-substancefraction is 15 %,” or “xB = 0.15,” or “xB = 15 %.” Mass fraction, volume fraction, and amount-of-substancefraction of B may also be expressed as in the following examples: wB = 3 g/kg; φB = 6.7 mL/L; xB =185 mmol/mol. Such forms are highly recommended (see also Sec. 7.10.3).
In the same vein, because the symbol % represents simply the number 0.01, it is incorrect to write,
for example, “where the resistances R1 and R2 differ by 0.05 %,” or “where the resistance R1 exceeds theresistance R2 by 0.05 %.” Instead, one should write, for example, “where R1 = R2 (1 + 0.05 %),” or define aquantity Δ via the relation Δ = (R1 − R2) / R2 and write “where Δ = 0.05 %.” Alternatively, in certain cases,
the word “fractional” or “relative” can be used. For example, it would be acceptable to write “the fractional
increase in the resistance of the 10 kΩ reference standard in 2006 was 0.002 %.”7.10.3

ppm, ppb, and ppt

In keeping with Ref. [4: ISO 31-0], this Guide takes the position that the language-dependent terms
part per million, part per billion, and part per trillion, and their respective abbreviations “ppm,” “ppb,” and“ppt” (and similar terms and abbreviations), are not acceptable for use with the SI to express the values ofquantities. Forms such as those given in the following examples should be used instead.
Examples:a stability of 0.5 (µA/A)/min

but not:a stability of 0.5 ppm/mina shift of 1.1 nm/m

but not:a shift of 1.1 ppb

a frequency change of 0.35 × 10−9 f

but not:a frequency change of 0.35 ppb

a sensitivity of 2 ng/kg

but not:a sensitivity of 2 ppt

the relative expanded uncertainty of the resistance R is Ur = 3 µΩ/Ω
orthe expanded uncertainty of the resistance R is U = 3 × 10−6 R
orthe relative expanded uncertainty of the resistance R is Ur = 3 × 10−6
but not:

20

Guide for the Use of the International System of Units (SI)the relative expanded uncertainty of the resistance R is Ur = 3 ppm
Because the names of numbers 109 and larger are not uniform worldwide, it is best that they be
avoided entirely (in many countries, 1 billion = 1 × 1012, not 1 × 109 as in the United States); the preferred
way of expressing large numbers is to use powers of 10. This ambiguity in the names of numbers is one of
the reasons why the use of ppm, ppb, ppt, and the like is deprecated. Another, and a more important one, isthat it is inappropriate to use abbreviations that are language dependent together with internationallyrecognized signs and symbols, such as MPa, ln, 1013, and %, to express the values of quantities and inequations or other mathematical expressions (see also Sec. 7.6).
Note:This Guide recognizes that in certain cases the use of ppm, ppb, and the like may be required by alaw or a regulation. Under these circumstances, Secs. 2.1 and 2.1.1 apply.

7.10.4

Roman numerals

It is unacceptable to use Roman numerals to express the values of quantities. In particular, one
should not use C, M, and MM as substitutes for 102, 103, and 106, respectively.
7.11 Quantity equations and numerical-value equations
A quantity equation expresses a relation among quantities. An example is l = υt, where l is the
distance a particle in uniform motion with velocity υ travels in the time t.
Because a quantity equation such as l = υt is independent of the units used to express the values of
the quantities that compose the equation, and because l, υ, and t represent quantities and not numericalvalues of quantities, it is incorrect to associate the equation with a statement such as “where l is in meters, υ
is in meters per second, and t is in seconds.”
On the other hand, a numerical value equation expresses a relation among numerical values ofquantities and therefore does depend on the units used to express the values of the quantities. For example,{l}m = 3.6−1 {υ}km/h {t}s expresses the relation among the numerical values of l, υ, and t only when thevalues of l, υ, and t are expressed in the units meter, kilometer per hour, and second, respectively. (Here{A}X is the numerical value of quantity A when its value is expressed in the unit X—see Sec. 7.1, note 2.)
An alternative way of writing the above numerical value equation, and one that is preferred becauseof its simplicity and generality, is l/m = 3.6−1 [υ/(km/h)](t / s). NIST authors should consider using thispreferred form instead of the more traditional form “l = 3.6−1 υt, where l is in meters, υ is in kilometers perhour, and t is in seconds.” In fact, this form is still ambiguous because no clear distinction is made between
a quantity and its numerical value. The correct statement is, for example, “l* = 3.6−1 υ* t *, where l* is thenumerical value of the distance l traveled by a particle in uniform motion when l is expressed in meters, υ*is the numerical value of the velocity υ of the particle when υ is expressed in kilometers per hour, and t* isthe numerical value of the time of travel t of the particle when t is expressed in seconds.” Clearly, as isdone here, it is important to use different symbols for quantities and their numerical values to avoid
confusion.
It is the strong recommendation of this Guide that because of their universality, quantity equations
should be used in preference to numerical-value equations. Further, if a numerical-value equation is used, itshould be written in the preferred form given in the above paragraph, and if at all feasible the quantity
equation from which it was obtained should be given.
Notes:
1.

Two other examples of numerical-value equations written in the preferred form are as follows,
where Eg is the gap energy of a compound semiconductor and k is the conductivity of anelectrolytic solution:

21

Guide for the Use of the International System of Units (SI)
Eg/eV = 1.425 − 1.337x + 0.270x 2, 0 ≤ x ≤ 0.15,
where x is an appropriately defined amount-of-substance fraction (see Sec. 8.6.2).
k /(S / cm) = 0.065 135 + 1.7140 × 10−3(t / ºC) + 6.4141 × 10−6(t / ºC)2 − 4.5028 × 10−8(t / ºC)3,0 ºC ≤ t ≤ 50 ºC, where t is Celsius temperature.
2.

Writing numerical-value equations for quantities expressed in inch-pound units in the preferred
form will simplify their conversion to numerical-value equations for the quantities expressed in SI
units.

7.12 Proper names of quotient quantities
Derived quantities formed from other quantities by division are written using the words “divided by” or per
rather than the words “per unit” in order to avoid the appearance of associating a particular unit with thederived quantity.
Example:pressure is force divided by areaor pressure is force per area

but not:pressure is force per unit area

7.13 Distinction between an object and its attributeTo avoid confusion, when discussing quantities or reporting their values, one should distinguishbetween a phenomenon, body, or substance, and an attribute ascribed to it. For example, one should
recognize the difference between a body and its mass, a surface and its area, a capacitor and its capacitance,
and a coil and its inductance. This means that although it is acceptable to say “an object of mass 1 kg was
attached to a string to form a pendulum,” it is not acceptable to say “a mass of 1 kg was attached to a string
to form a pendulum.”7.14 Dimension of a quantity
Any SI derived quantity Q can be expressed in terms of the SI base quantities length (l ) , mass (m),time (t), electric current (l ) , thermodynamic temperature (T ) , amount of substance (n), and luminous
intensity (I v ) by an equation of the formK

Q = l m t I T n Iv ∑
α

β γ δ

ε

ζ

η

ak ,

k =1

where the exponents α, β, γ, . . . are numbers and the factors ak are also numbers. The dimension of Q isdefined to be
dim Q = LαMβTγIδθεNζJη ,where L, M, T, I, θ, N, and J are the dimensions of the SI base quantities length, mass, time, electric
current, thermodynamic temperature, amount of substance, and luminous intensity, respectively. The
exponents α, β, γ, . . . are called “dimensional exponents.” The SI derived unit of Q is mα·kgβ·sγ ·Aδ·Kεζη·mol ·cd , which is obtained by replacing the dimensions of the SI base quantities in the dimension of Q
with the symbols for the corresponding base units.
Example:Consider a nonrelativistic particle of mass m in uniform motion which travels a distance l in atime t . Its velocity is υ = l / t and its kinetic energy is Ek = mυ2 / 2 = l.2 mt.−2 / 2. The
dimension of Ek is dim Ek = L2MT−2 and the dimensional exponents are 2, 1, and −2. The SI
derived unit of Ek is then m2·kg·s−2, which is given the special name “joule” and special
symbol J.

22

Guide for the Use of the International System of Units (SI)A derived quantity of dimension one, which is sometimes called a “dimensionless quantity,” is one
for which all of the dimensional exponents are zero: dim Q = 1. It therefore follows that the derived unit forsuch a quantity is also the number one, symbol 1, which is sometimes called a “dimensionless derived
unit.”
Example:The mass fraction wB of a substance B in a mixture is given by wB = mB / m, where mB is themass of B and m is the mass of the mixture (see Sec. 8.6.10). The dimension of wB is dim
wB = M1M−1 = 1; all of the dimensional exponents of wB are zero, and its derived unit iskg1·kg−1 = 1 also.

8

Comments on Some Quantities and Their Units

8.1

Time and rotational frequency

The SI unit of time (actually time interval) is the second (s) and should be used in all technicalcalculations. When time relates to calendar cycles, the minute (min), hour (h), and day (d) might benecessary. For example, the kilometer per hour (km/h) is the usual unit for expressing vehicular speeds.
Although there is no universally accepted symbol for the year, Ref. [4: ISO 80000-3] suggests the
symbol a.
The rotational frequency n of a rotating body is defined to be the number of revolutions it makes in atime interval divided by that time interval [4: ISO 80000-3]. The SI unit of this quantity is thus thereciprocal second (s−1). However, as pointed out in Ref. [4: ISO 80000-3], the designations “revolutions per
second” (r/s) and “revolutions per minute” (r/min) are widely used as units for rotational frequency inspecifications on rotating machinery.
8.2

VolumeThe SI unit of volume is the cubic meter (m3) and may be used to express the volume of any
substance, whether solid, liquid, or gas. The liter (L) is a special name for the cubic decimeter (dm3), but
the CGPM recommends that the liter not be used to give the results of high accuracy measurements ofvolumes [1, 2]. Also, it is not common practice to use the liter to express the volumes of solids nor to use
multiples of the liter such as the kiloliter (kL) [see Sec. 6.2.8, and also Table 6, footnote (b)].
8.3

WeightIn science and technology, the weight of a body in a particular reference frame is defined as theforce that gives the body an acceleration equal to the local acceleration of free fall in that reference frame[4: ISO 80000-4]. Thus the SI unit of the quantity weight defined in this way is the newton (N). When thereference frame is a celestial object, Earth for example, the weight of a body is commonly called the local
force of gravity on the body.
Example:Note:

The local force of gravity on a copper sphere of mass 10 kg located on the surface of the
Earth, which is its weight at that location, is approximately 98 N.

The local force of gravity on a body, that is, its weight, consists of the resultant of all thegravitational forces acting on the body and the local centrifugal force due to the rotation of thecelestial object. The effect of atmospheric buoyancy is usually excluded, and thus the weight of abody is generally the local force of gravity on the body in vacuum.

In commercial and everyday use, and especially in common parlance, weight is usually used as asynonym for mass. Thus the SI unit of the quantity weight used in this sense is the kilogram (kg) and theverb “to weigh” means “to determine the mass of” or “to have a mass of.”
Examples:the child’s weight is 23 kg

the briefcase weighs 6 kg
23

Net wt. 227 g

Guide for the Use of the International System of Units (SI)
Inasmuch as NIST is a scientific and technical organization, the word “weight” used in the everyday
sense (that is, to mean mass) should appear only occasionally in NIST publications; the word “mass”
should be used instead. In any case, in order to avoid confusion, whenever the word “weight” is used, itshould be made clear which meaning is intended.
8.4 Relative atomic mass and relative molecular mass
The terms atomic weight and molecular weight are obsolete and thus should be avoided. They havebeen replaced by the equivalent but preferred terms relative atomic mass, symbol Ar, and relative molecularmass, symbol Mr, respectively [4: ISO 31-8], which better reflect their definitions. Similar to atomic weightand molecular weight, relative atomic mass and relative molecular mass are quantities of dimension one
and are expressed simply as numbers. The definitions of these quantities are as follows [4: ISO 31-8]:
Relative atomic mass (formerly atomic weight): ratio of the average mass per atom of an element to 1/12of the mass of the atom of the nuclide 12C.
Relative molecular mass (formerly molecular weight): ratio of the average mass per molecule or specified
entity of a substance to 1/12 of the mass of an atom of the nuclide 12C.
Examples:Ar(Si) = 28.0855

Mr(H2) = 2.0159

Ar(12C) = 12 exactly

Notes:
1.

It follows from these definitions that if X denotes a specified atom or nuclide and B a specified
molecule or entity (or more generally, a specified substance), then Ar(X) = m(X) / [m(12C) / 12]
and Mr(B) = m(B) / [m(12C) / 12], where m(X) is the mass of X, m(B) is the mass of B, and m(12C)
is the mass of an atom of the nuclide 12C. It should also be recognized that m(12C) / 12 = u, theunified atomic mass unit, which is approximately equal to 1.66 × 10−27 kg [see Table 7,
footnote (d)].

2.

It follows from the examples and note 1 that the respective average masses of Si, H2, and 12C arem(Si) = Ar(Si) u, m(H2) = Mr(H2) u, and m(12C) = Ar(12C) u.

3.

In publications dealing with mass spectrometry, one often encounters statements such as “themass-to-charge ratio is 15.” What is usually meant in this case is that the ratio of the nucleon
number (that is, mass number—see Sec. 10.4.2) of the ion to its number of charges is 15. Thus
mass-to-charge ratio is a quantity of dimension one, even though it is commonly denoted by thesymbol m / z. For example, the mass-to-charge ratio of the ion 12C71H7+ + is 91/2 = 45.5.

8.5 Temperature interval and temperature difference
As discussed in Sec. 4.2.1.1, Celsius temperature (t) is defined in terms of thermodynamic
temperature (T) by the equation t = T − T0, where T0 = 273.15 K by definition. This implies that thenumerical value of a given temperature interval or temperature difference whose value is expressed in theunit degree Celsius (ºC) is equal to the numerical value of the same interval or difference when its value isexpressed in the unit kelvin (K); or in the notation of Sec. 7.1, note 2, {Δt }ºC = {ΔT}K. Thus temperatureintervals or temperature differences may be expressed in either the degree Celsius or the kelvin using thesame numerical value.
Example:The difference in temperature between the freezing point of gallium and the triple point ofwater is Δt = 29.7546 ºC = ΔT = 29.7546 K.

24

Guide for the Use of the International System of Units (SI)8.6

Amount of substance, concentration, molality, and the like

The following section discusses amount of substance, and the subsequent nine sections, which are
based on Ref. [6: ISO 31-8] and which are succinctly summarized in Table 12, discuss quantities that are
quotients involving amount of substance, volume, or mass. In the table and its associated sections, symbols
for substances are shown as subscripts, for example, xB, nB, bB. However, it is generally preferable to place
symbols for substances and their states in parentheses immediately after the quantity symbol, for examplen(H2SO4). (For a detailed discussion of the use of the SI in physical chemistry, see the book cited in Ref.
[6], note 3.)
8.6.1 Amount of substance
Quantity symbol: n (also v).

SI unit: mole (mol).

Definition: See Sec. A.7.
Notes:
1.

Amount of substance is one of the seven base quantities upon which the SI is founded (seeSec. 4.1 and Table 1).

2.

In general, n(xB) = n(B) / x, where x is a number. Thus, for example, if the amount of substance of
H2SO4 is 5 mol, the amount of substance of (1/3)H2SO4 is 15 mol: n[(1/3)H2SO4] = 3n(H2SO4).

Example:The relative atomic mass of a fluorine atom is Ar(F) = 18.9984. The relative molecular massof a fluorine molecule may therefore be taken as Mr(F2) = 2Ar(F) = 37.9968. The molar massof F2 is then M(F2) = 37.9968 × 10−3 kg/mol = 37.9968 g/mol (see Sec. 8.6.4). The amount of
substance of, for example, 100 g of F2 is then n(F2) = 100 g / (37.9968 g/mol) = 2.63 mol.

8.6.2 Mole fraction of B; amount-of-substance fraction of B
Quantity symbol: xB (also yB).
dimension one).
Definition:

SI unit: one (1) (amount-of-substance fraction is a quantity ofratio of the amount of substance of B to the amount of substance of the mixture: xB = nB/n.

25

Guide for the Use of the International System of Units (SI)
Table 12. Summary description of nine quantities that are quotients involving amount of substance,
volume, or mass(a)Quantity in numerator

Amount of Substance
Symbol: n

Quantity in denominator

SI unit: mol(a)VolumeSymbol: V
SI unit: m3

Mass

Amount of Substance

VolumeMass

Symbol: n

Symbol: V

Symbol: m

SI unit: mol

SI unit: m3

SI unit: kg

amount-of-substancefraction
n B
x B =
n

molar volume

molar mass

SI unit: mol/mol = 1

SI unit: m3/mol

SI unit: kg/mol

amount-of-substanceconcentration

volume fraction

mass density

cB =

Vm =

ϕB =

nB
V

V
n

M

x BV m,* Bρ∑ x AV m,* A

=

=

m
n

m
V

SI unit: mol/mol3

SI unit: m3/m3 = 1

SI unit: kg/m3

molality

specific volume

mass fractionnB
mA

Symbol: mbB =SI unit: kg

SI unit: mol/kg

υ=

V
m

wB =

SI unit: m3/kg

mBm

SI unit: kg/kg = 1

Adapted from Canadian Metric Practice Guide (see Ref. [6], note 2; the book cited in Ref. [6], note 3, may also be consulted).

Notes:
1.

This quantity is commonly called “mole fraction of B” but this Guide prefers the name “amountof-substance fraction of B,” because it does not contain the name of the unit mole (compare
kilogram fraction to mass fraction).

2.

For a mixture composed of substances A, B, C, . . . , nA + nB + nC + ... ≡

∑n

A

A

3.

A related quantity is amount-of-substance ratio of B (commonly called “mole ratio of solute B”),
symbol rB. It is the ratio of the amount of substance of B to the amount of substance of the solvent
substance: rB = nB/nS. For a single solute C in a solvent substance (a one-solute solution),rC = xC/(1 − xC). This follows from the relations n = nC + nS, xC = nC / n, and rC = nC / nS, where thesolvent substance S can itself be a mixture.

8.6.3 Molar volume
Quantity symbol: Vm.

SI unit: cubic meter per mole (m3/mol).

Definition: volume of a substance divided by its amount of substance: Vm = V/n.
Notes:
1.

The word “molar” means “divided by amount of substance.”

26

Guide for the Use of the International System of Units (SI)2.

For a mixture, this term is often called “mean molar volume.”

3.

The amagat should not be used to express molar volumes or reciprocal molar volumes. (Oneamagat is the molar volume Vm of a real gas at p = 101 325 Pa and T = 273.15 K and isapproximately equal to 22.4 × 10−3 m3/mol. The name “amagat” is also given to 1/Vm of a real gas
at p = 101 325 Pa and T = 273.15 K and in this case is approximately equal to 44.6 mol/m3.)

8.6.4 Molar mass
Quantity symbol: M.

SI unit: kilogram per mole (kg/mol).

Definition: mass of a substance divided by its amount of substance: M = m/n.
Notes:
1.

For a mixture, this term is often called “mean molar mass.”

2.

The molar mass of a substance B of definite chemical composition is given by M(B) =Mr(B) × 10−3 kg/mol = Mr(B) kg/kmol = Mr g/mol, where Mr(B) is the relative molecular mass of
B (see Sec. 8.4). The molar mass of an atom or nuclide X is M(X) = Ar(X) × 10−3 kg/mol =Ar(X) kg/kmol = Ar(X) g/mol, where Ar(X) is the relative atomic mass of X (see Sec. 8.4).

8.6.5 Concentration of B; amount-of-substance concentration of B
Quantity symbol: cB.

SI unit: mole per cubic meter (mol/m3).

Definition: amount of substance of B divided by the volume of the mixture: cB = nB/V.
Notes:
1.

This Guide prefers the name “amount-of-substance concentration of B” for this quantity because it
is unambiguous. However, in practice, it is often shortened to amount concentration of B, or evensimply to concentration of B. Unfortunately, this last form can cause confusion because there areseveral different “concentrations,” for example, mass concentration of B, ρB = mB/V; andmolecular concentration of B, CB = NB/V, where NB is the number of molecules of B.

2.

The term normality and the symbol N should no longer be used because they are obsolete. One
should avoid writing, for example, “a 0.5 N solution of H2SO4” and write instead “a solution
having an amount-of-substance concentration of c [(1/2)H2SO4]) = 0.5 mol/dm3” (or 0.5 kmol/m3
or 0.5 mol/L since 1 mol/dm3 = 1 kmol/m3 = 1 mol/L).

3.

The term molarity and the symbol M should no longer be used because they, too, are obsolete. One
should use instead amount-of-substance concentration of B and such units as mol/dm3, kmol/m3,
or mol/L. (A solution of, for example, 0.1 mol/dm3 was often called a 0.1 molar solution, denoted
0.1 M solution. The molarity of the solution was said to be 0.1 M.)

8.6.6 Volume fraction of B
Quantity symbol: φB.

SI unit: one (1) (volume fraction is a quantity of dimension one).

Definition: for a mixture of substances A, B, C, . . . ,27

Guide for the Use of the International System of Units (SI)ϕB=

x BV m,* B∑ x A V m,* A***
where xA, xB, xC, . . . are the amount-of-substance fractions of A, B, C, . . ., Vm,
A , Vm, B , Vm,C , . . . are themolar volumes of the pure substances A, B, C, . . . at the same temperature and pressure, and where thesummation is over all the substances A, B, C, . . . so that∑ x = 1.
A

8.6.7 Mass density; density
Quantity symbol: ρ.

SI unit: kilogram per cubic meter (kg/m3).

Definition: mass of a substance divided by its volume: ρ = m / V.
Notes:
1.

This Guide prefers the name “mass density” for this quantity because there are several different“densities,” for example, number density of particles, n = N / V; and charge density, ρ = Q / V.

2.

Mass density is the reciprocal of specific volume (see Sec. 8.6.9): ρ = 1 / υ.

8.6.8 Molality of solute B
Quantity symbol: bB (also mB).

SI unit: mole per kilogram (mol/kg).

Definition: amount of substance of solute B in a solution divided by the mass of the solvent: bB = nB / mA.
Note:

The term molal and the symbol m should no longer be used because they are obsolete. One should
use instead the term molality of solute B and the unit mol/kg or an appropriate decimal multiple or
submultiple of this unit. (A solution having, for example, a molality of 1 mol/kg was often called a
1 molal solution, written 1 m solution.)

8.6.9 Specific volume
Quantity symbol: υ.

SI unit: cubic meter per kilogram (m3/kg).

Definition: volume of a substance divided by its mass: υ = V / m.
Note:

Specific volume is the reciprocal of mass density (see Sec. 8.6.7): υ = 1 / ρ.

8.6.10 Mass fraction of B
Quantity symbol: wB.

SI unit: one (1) (mass fraction is a quantity of dimension one).

Definition: mass of substance B divided by the mass of the mixture: wB = mB / m.
8.7

Logarithmic quantities and units: level, neper, belThis section briefly introduces logarithmic quantities and units. It is based on Ref. [5: IEC 60027-3],
which should be consulted for further details. Two of the most common logarithmic quantities are level-ofa-field-quantity, symbol LF, and level-of-a-power-quantity, symbol LP; and two of the most common
logarithmic units are the units in which the values of these quantities are expressed: the neper, symbol Np,
or the bel, symbol B, and decimal multiples and submultiples of the neper and bel formed by attaching SI

28

Guide for the Use of the International System of Units (SI)prefixes to them, such as the millineper, symbol mNp (1 mNp = 0.001 Np), and the decibel, symbol dB(1 dB = 0.1 B).
Level-of-a-field-quantity is defined by the relation LF = ln(F/F0), where F/F0 is the ratio of two
amplitudes of the same kind, F0 being a reference amplitude. Level-of-a-power-quantity is defined by the
relation LP = (1/2) ln(P/P0), where P/P0 is the ratio of two powers, P0 being a reference power. (Note that if
P/P0 = (F/F0)2, then LP = LF.) Similar names, symbols, and definitions apply to levels based on otherquantities which are linear or quadratic functions of the amplitudes, respectively. In practice, the name of
the field quantity forms the name of LF and the symbol F is replaced by the symbol of the field quantity.
For example, if the field quantity in question is electric field strength, symbol E, the name of the quantity is“level-of-electric-field-strength” and it is defined by the relation LE = ln(E/E0).
The difference between two levels-of-a-field-quantity (called “field-level difference”) having thesame reference amplitude F0 is ΔLF = LF1 − LF2 = ln(F1/F0) − ln(F2/F0) = ln(F1/F2), and is independent of
F0. This is also the case for the difference between two levels-of-a-power-quantity (called “power-level
difference”) having the same reference power P0: ΔLP = LP1 − LP2 = ln(P1/P0) − ln(P2/P0) = ln(P1/P2).
It is clear from their definitions that both LF and LP are quantities of dimension one and thus have as
their units the unit one, symbol 1. However, in this case, which recalls the case of plane angle and theradian (and solid angle and the steradian), it is convenient to give the unit one the special name “neper” or“bel” and to define these so-called dimensionless units as follows:
One neper (1 Np) is the level-of-a-field-quantity when F/F0 = e, that is, when ln(F/F0) = 1.
Equivalently, 1 Np is the level-of-a-power-quantity when P/P0 = e2, that is, when (1/2) ln(P/P0) = 1. These
definitions imply that the numerical value of LF when LF is expressed in the unit neper is {LF}Np = ln(F/F0),
and that the numerical value of LP when LP is expressed in the unit neper is {LP}Np = (1/2) ln(P/P0); that is
LF = ln(F/F0) Np
LP = (1/2) ln(P/P0) Np.
One bel (1 B) is the level-of-a-field-quantity when F/F0 = 10, that is, when 2 lg(F/F0) = 1 (note thatlg x = log10x – see Sec. 10.1.2). Equivalently, 1 B is the level- of-a-power-quantity when P/P0 = 10, that is,when lg(P/P0) = 1. These definitions imply that the numerical value of LF when LF is expressed in the unit
bel is {LF}B = 2 lg(F/F0) and that the numerical value of LP when LP is expressed in the unit bel is{LP}B = lg(P/P0); that is
LF = 2 lg(F/F0) B = 20 lg(F/F0) dB
LP = lg(P/P0) B = 10 lg(P/P0) dB.
Since the value of LF (or LP) is independent of the unit used to express that value, one may equate LF
in the above expressions to obtain ln(F/F0) Np = 2 lg(F/F0) B, which implies
1 B=

ln 10
Np exactly2≈ 1.151 293 Np
1 dB ≈ 0.115 129 3 Np.
When reporting values of LF and LP, one must always give the reference level. According to Ref. [5:
IEC 60027-3], this may be done in one of two ways: Lx (re xref) or L x / xref where x is the quantity symbol for29

Guide for the Use of the International System of Units (SI)the quantity whose level is being reported, for example, electric field strength E or sound pressure p, andxref is the value of the reference quantity, for example, 1 μV/m for E0, and 20 μPa for p0. Thus
LE (re 1 μV/m) = − 0.58 Np

orLE/(1 μV/m) = − 0.58 Np

means that the level of a certain electric field strength is 0.58 Np below the reference electric field strength
E0 = 1 μV/m. Similarly
Lp (re 20 μPa) = 25 dB

orLp/(20 μPa) = 25 dBmeans that the level of a certain sound pressure is 25 dB above the reference pressure p0 = 20 μPa.
Notes:

8.8

1.

When such data are presented in a table or in a figure, the following condensed notation may be
used instead: − 0.58 Np (1 μV/m); 25 dB (20 μPa).

2.

When the same reference level applies repeatedly in a given context, it may be omitted if its value
is clearly stated initially and if its planned omission is pointed out.

3.

The rules of Ref. [5: IEC 60027-3] preclude, for example, the use of the symbol dBm to indicate areference level of power of 1 mW. This restriction is based on the rule of Sec. 7.4, which does not
permit attachments to unit symbols.
Viscosity

The proper SI units for expressing values of viscosity η (also called dynamic viscosity) and values ofkinematic viscosity ν are, respectively, the pascal second (Pa·s) and the meter squared per second (m2/s)(and their decimal multiples and submultiples as appropriate). The CGS units commonly used to express
values of these quantities, the poise (P) and the stoke (St), respectively [and their decimal submultiples thecentipoise (cP) and the centistoke (cSt)], are not to be used; see Sec. 5.3.1 and Table 10, which gives therelations 1 P = 0.1 Pa·s and 1 St = 10−4 m2/s.
8.9

Massic, volumic, areic, lineic

Reference [4: ISO 31-0] has introduced the new adjectives “massic,” “volumic,” “areic,” and“lineic” into the English language based on their French counterparts: “massique,” “volumique,”“surfacique,” and “linéique.” They are convenient and NIST authors may wish to use them. They areequivalent, respectively, to “specific,” “density,” “surface . . . density,” and “linear . . . density,” asexplained below.
(a) The adjective massic, or the adjective specific, is used to modify the name of a quantity to indicate thequotient of that quantity and its associated mass.
Examples:massic volume or specific volume: υ = V / mmassic entropy or specific entropy: s = S / m(b) The adjective volumic is used to modify the name of a quantity, or the term density is added to it, toindicate the quotient of that quantity and its associated volume.
Examples:volumic mass or (mass) density: ρ = m / V
volumic number or number density: n = N / V

30

Guide for the Use of the International System of Units (SI)
Note:

Parentheses around a word means that the word is often omitted.

(c) The adjective areic is used to modify the name of a quantity, or the terms surface . . . density are added
to it, to indicate the quotient of that quantity (a scalar) and its associated surface area.
Examples:areic mass or surface (mass) density: ρA = m / A
areic charge or surface charge density: σ = Q / A(d) The adjective lineic is used to modify the name of a quantity, or the terms linear . . . density are added
to it, to indicate the quotient of that quantity and its associated length.
Examples:lineic mass or linear (mass) density: ρl = m / llineic electric current or linear electric current density: A = I / b

9Rules and Style Conventions for Spelling Unit Names
The following eight sections give rules and style conventions related to spelling the names of units.

9.1

Capitalization

When spelled out in full, unit names are treated like ordinary English nouns. Thus the names of allunits start with a lower-case letter, except at the beginning of a sentence or in capitalized material such as atitle.
In keeping with this rule, the correct spelling of the name of the unit ºC is “degree Celsius” (the unit
“degree” begins with a lowercase “d” and the modifier “Celsius” begins with an uppercase “C” because itis the name of a person).
9.2

Plurals

Plural unit names are used when they are required by the rules of English grammar. They arenormally formed regularly, for example, “henries” is the plural of henry. According to Ref. [6], thefollowing plurals are irregular: Singular —lux, hertz, siemens; Plural —lux, hertz, siemens. (See alsoSec. 9.7.)
9.3

Spelling unit names with prefixes

When the name of a unit containing a prefix is spelled out, no space or hyphen is used between theprefix and unit name (see Sec. 6.2.3).
Examples:milligrambut not:

milli-gramkilopascal

but not:

kilo-pascal

Reference [6] points out that there are three cases in which the final vowel of an SI prefix iscommonly omitted: megohm (not megaohm), kilohm (not kiloohm), and hectare (not hectoare). In all othercases in which the unit name begins with a vowel, both the final vowel of the prefix and the vowel of theunit name are retained and both are pronounced.
9.4

Spelling unit names obtained by multiplication

When the name of a derived unit formed from other units by multiplication is spelled out, a space,which is preferred by Ref. [6] and this Guide, or a hyphen is used to separate the names of the individual
units.

31

Guide for the Use of the International System of Units (SI)
Example:9.5

pascal second or pascal-second

Spelling unit names obtained by division

When the name of a derived unit formed from other units by division is spelled out, the word “per” is usedand not a solidus. (See also Secs. 6.1.7 and 9.8.)
Example:9.6

ampere per meter (A/m)

but not:

ampere/meter

Spelling unit names raised to powers

When the names of units raised to powers are spelled out, modifiers such as “squared” or “cubed” are used
and are placed after the unit name.
Example:meter per second squared (m/s2)

The modifiers “square” or “cubic” may, however, be placed before the unit name in the case of area orvolume.
Examples:

9.7

square centimeter (cm2)cubic millimeter (mm3)ampere per square meter (A/m2)

kilogram per cubic meter (kg/m3)

Other spelling conventions

A derived unit is usually singular in English, for example, the value 3 m2·K/W is usually spelled out
as “three square meter kelvin per watt,” and the value 3 C·m2/V is usually spelled out as “three coulomb
meter squared per volt.” However, a “single” unit may be plural; for example, the value 5 kPa is spelled out
as “five kilopascals,” although “five kilopascal” is acceptable. If in such a single-unit case the number isless than one, the unit is always singular when spelled out; for example, 0.5 kPa is spelled out as “fivetenths kilopascal.”
Note:

9.8

These other spelling conventions are given for completeness; as indicated in Sec. 7.6, it is the
position of this Guide that symbols for numbers and units should be used to express the values ofquantities, not the spelled-out names of numbers and units. Reference [3] also requires that asymbol for a number be used whenever the value of a quantity is expressed in terms of a unit ofmeasurement.
Unacceptability of applying mathematical operations to unit names

Because it could possibly lead to confusion, mathematical operations are not applied to unit names
but only to unit symbols. (See also Secs. 6.1.7 and 9.5.)
Example:joule per kilogram or J/kg or J·kg−1

but not:joule/kilogram orjoule·kilogram−1

10 More on Printing and Using Symbols and Numbers in Scientific and TechnicalDocuments 4
By following the guidance given in this chapter, NIST authors can prepare manuscripts that are consistent
with accepted typesetting practice.

4

This chapter is adapted in part from Refs. [4: ISO 31-0], and [4: ISO 31-11].

32

Guide for the Use of the International System of Units (SI)10.1 Kinds of symbols
Letter symbols are of three principal kinds: (a) symbols for quantities, (b) symbols for units, and (c)symbols for descriptive terms. Quantity symbols, which are always printed in italic (that is, sloping) type,
are, with few exceptions, single letters of the Latin or Greek alphabets that may have subscripts or
superscripts or other identifying signs. Symbols for units, in particular those for acceptable units, have been
discussed in detail in earlier portions of this Guide. Symbols for descriptive terms include the symbols forthe chemical elements, certain mathematical symbols, and modifying superscripts and subscripts on
quantity symbols.
10.1.1

Standardized quantity symbols

The use of words, acronyms, or other ad hoc groups of letters as quantity symbols should be avoided
by NIST authors. For example, use the quantity symbol Zm for mechanical impedance, not MI. In fact, there
are nationally and internationally accepted symbols for literally hundreds of quantities used in the physicalsciences and technology. Many of these are given in Refs. [4] and [5], and it is likely that symbols for thequantities used in most NIST publications can be found in these international standards or can readily be
adapted from the symbols and principles given in these standards. Because of their international
acceptance, NIST authors are urged to use the symbols of Refs. [4] and [5] to the fullest extent possible.5
Examples:10.1.2Ω (solid angle)
LP (level of a power quantity)p (pressure)ĸT (isothermal compressibility)
E (electric field strength)Zm (mechanical impedance)Δr (relative mass excess)σtot (total cross section)
Eu (Euler number)
TN (Néel temperature)

Standardized mathematical signs and symbols

As is the case for quantity symbols, most of the mathematical signs and symbols used in the physicalsciences and technology are standardized. They may be found in Ref. [4: ISO 31-11] and should be used by
NIST authors to the fullest possible extent.5
Examples:

^

≠
def

(conjunction sign, p ^ q means p and q)(a ≠ b, a is not equal to b)def=( a = b, a is by definition equal to b)≈~
arcsin x
loga x
lb x
ln x
lg x

(a ≈ b, a is approximately equal to b)(a ~ b, a is proportionally equal to b)(arc sine of x)(logarithm to the base a of x)(lb x = log2 x)(ln x = loge x)(lg x = log10 x)10.2 Typefaces for symbols
Most word processing systems now in use at NIST are capable of producing lightface (that is, regular) or
boldface letters of the Latin or Greek alphabets in both roman (upright) and italic (sloping) types. The
5

In addition to Refs. [4] and [5], quantity symbols can also be found in ANSI/IEEE Std 280-1985, IEEE Standard Letter Symbols for
Quantities Used in Electrical Science and Electrical Engineering. Similarly, in addition to Ref. [4: ISO 31-11], mathematical signs andsymbols are also given in ANSI/IEEE Std 260.3-1993, Mathematical Signs and Symbols for Use in Physical Sciences andTechnology. Another publication is the book, Symbols, Units, Nomenclature and Fundamental Constants in Physics, 1987 Revision,
by E. R. Cohen and P. Giacomo, International Union of Pure and Applied Physics, SUNAMCO Commission [reprinted from Physica,
Vol. 146A, Nos. 1-2, p. 1 (November, 1987)]. See also Ref. [6], Note 3.

33

Guide for the Use of the International System of Units (SI)understandability of NIST typed and typeset scientific and technical publications is facilitated if symbols
are in the correct typeface. The typeface in which a symbol appears helps to define what the symbol
represents. For example, irrespective of the typeface used in the surrounding text, “A” would be typed or
typeset in:— italic type for the scalar quantity area: A;— roman type for the unit ampere: A;— italic boldface for the vector quantity vector potential: A.
More specifically, the three major categories of symbols found in scientific and technical publications
should be typed or typeset in either italic or roman type, as follows:— symbols for quantities and variables: italic;— symbols for units: roman;— symbols for descriptive terms: roman.
These rules imply that a subscript or superscript on a quantity symbol is in roman type if it is descriptive(for example, if it is a number or represents the name of a person or a particle); but it is in italic type if it
represents a quantity, or is a variable such as x in Ex or an index such as i in ∑i xi that represents a number(see Secs. 10.2.1, 10.2.3, and 10.2.4). An index that represents a number is also called a “running number”[4: ISO 31-0].
Notes:
1.

The above rules also imply, for example, that μ, the symbol for the SI prefix micro (10!6), that Ω,
the symbol for the SI derived unit ohm, and that F, the symbol for the SI derived unit farad, are inroman type; but they are in italic type if they represent quantities (μ, Ω, and F are therecommended symbols for the quantities magnetic moment of a particle, solid angle, and force,
respectively).

2.

The typeface for numbers is discussed in Sec. 10.5.1. The following four sections give examplesof the proper typefaces for these three major categories.

10.2.1

Quantities and variables — italic
Symbols for quantities are italic, as are symbols for functions in general, for example, f (x):t=3sr = 11 cm

time, s second
r radius, cm centimeter

T = 22 K
λ = 633 nm

T temperature, K kelvin
λ wavelength, nm nanometer

Constants are usually physical quantities and thus their symbols are italic; however, in general,symbols used as subscripts and superscripts are roman if descriptive (see Sec. 10.2.3):NA
θDe

Avogadro constant, A Avogadro
Debye temperature, D Debye
elementary charge

R molar gas constant
Z atomic number
me m mass, e electron

Running numbers and symbols for variables in mathematical equations are italic, as are symbols forparameters such as a and b that may be considered constant in a given context:
m

y=

∑x z

i i

x2 = ay2 = bz2

i =1

Symbols for vectors are boldface italic, symbols for tensors are sans-serif bold italic, and symbols
for matrices are italic:

34

Guide for the Use of the International System of Units (SI)A·B = C (vectors)a a
A = ⎛⎜ 11 21 ⎞⎟ (matrices)⎝ a 21 a 22 ⎠T (tensors)

Symbols used as subscripts and superscripts are italic if they represent quantities or variables:
p pressure

Cp10.2.2qm

m massσΩω z z z coordinateΩ solid angle

Units— roman
The symbols for units and SI prefixes are roman:mcm

10.2.3

meter
centimeter

g
μg

gram
microgram

L
mL

liter
milliliter

Descriptive terms — roman

Symbols representing purely descriptive terms (for example, the chemical elements) are roman, as
are symbols representing mathematical constants that never change (for example, π) and symbols
representing explicitly defined functions or well defined operators (for example, Γ(x) or div):
Chemical elements:
Ar

argon

B

boron

C

carbonMathematical constants, functions, and operators:
e
exp x
dx/dt

base of natural logarithms
exp exponential ofd
1st derivative ofΣxiloga x
sin x

Σ
loga
sin

sum oflogarithm to the base a ofsine of

Symbols used as subscripts and superscripts are roman if descriptive:

ε 0( ir ) ir
V
10.2.41m

m

irrationalEkkkinetic

molar, l liquid phase

μB

B

Bohr

Sample equations showing correct type
F=

q1 q2
4πε 0 r 2*

φB = xBV m,B /E = mc2∑x V*
A m ,AF = mapV = nRT

Ea = RT2 d(1n k) / dT

c1 = λ−5/[exp(c2 / λT) − 1]~
pB = λB lim
p→0 ( xB p / λB )F
= − grad VQ

10.3 Greek alphabet in roman and italic type
Table 13 shows the proper form, in both roman and italic type, of the upper-case and lower-case letters of
the Greek alphabet.

35

Guide for the Use of the International System of Units (SI)
Table 13. Greek alphabet in roman and italic type
Greek Letter Name
Roman

(a)
(b)

Italic

alpha
beta
gamma
delta
epsilon
zeta
eta

Α
Β
Γ
Δ
Ε
Ζ
Η

α
β
γ
δ
ε
ζ
η

theta
iota
kappa
lambdamu
nu
xi
omicron
pi
rho
sigma
tau
upsilon
phi
chi
psi
omega

Θ, Ө(a)θ, ϑ (b)ικ, ϰ (β)λμνξοπ, ϖ(b)
ρ
σ
τ
υ
ϕ, φ
χ
ψ
ω

Ι
Κ
Λ
Μ
Ν
Ξ
Ο
Π
Ρ
Σ
Τ
Υ
Φ
Χ
Ψ
Ω

Α

α

Β

β

Γ
Δ
Ε
Ζ
Η

γ
δ
ε
ζ
η

Θ, Ө(a)θ, ϑ (b)iκ, ϰ (b)λμνξοπ, ϖ(b)
ρ
σ
τ
υ
φ, φ
χ
ψ
ω

Ι
Κ
Λ
Μ
Ν
Ξ
Ο
Π
Ρ
Σ
Τ
Υ
Φ
Χ
Ψ
Ω

ISO (see Ref. [4: ISO 31-0]) gives only the first of these two letters.
ISO (see Ref. [4: ISO 31-0]) gives these two letters in the reverse order.

10.4 Symbols for the elements
The following two sections give the rules and style conventions for the symbols for the elements.
10.4.1

Typeface and punctuation for element symbols

Symbols for the elements are normally printed in roman type without regard to the type used in thesurrounding text (see Sec. 10.2.3). They are not followed by a period unless at the end of a sentence.
10.4.2

Subscripts and superscripts on element symbols
The nucleon number (mass number) of a nuclide is indicated in the left superscript position:

28

Si.

The number of atoms in a molecule of a particular nuclide is shown in the right subscript
position: 1H2.
The proton number (atomic number) is indicated in the left subscript position: 29Cu.
The state of ionization or excitation is indicated in the right superscript position, some examples ofwhich are as follows:36

Guide for the Use of the International System of Units (SI)State of ionization: Ba++Co(NO2)6− − −.
orCo ( NO 2 ) 36 −Electronic excited state:

Ne*, CO*

Nuclear excited state:

15

or[Co(NO2)6]3 −N* or 15Nm

10.5 Printing numbers
The following three sections give rules and style conventions related to the printing of numbers.
10.5.1

Typeface for numbers

Arabic numerals expressing the numerical values of quantities (see Sec. 7.6) are generally printed inlightface (that is, regular) roman type irrespective of the type used for the surrounding text. Arabic
numerals other than numerical values of quantities may be printed in lightface or bold italics, or in boldroman type, but lightface roman type is usually preferred.
10.5.2

Decimal sign or marker

The recommended decimal sign or marker for use in the United States is the dot on the line [3, 6].
For numbers less than one, a zero is written before the decimal marker. For example, 0.25 s is the correct
form, not .25 s.
10.5.3

Grouping digits

Because the comma is widely used as the decimal marker outside the United States, it should not be
used to separate digits into groups of three. Instead, digits should be separated into groups of three,counting from the decimal marker towards the left and right, by the use of a thin, fixed space. However,this practice is not usually followed for numbers having only four digits on either side of the decimalmarker except when uniformity in a table is desired.
Examples:76 483 522
43 279.168 29
8012 or 8 012
0.491 722 3
0.5947 or 0.594 7
8012.5947 or 8 012.594 7

but not:but not:but not:
is highly preferred to:
but not:but not:

76,483,522
43,279.168 29
8,012
0.4917223
0.59 47
8 012.5947 or 8012.594 7

Note:

The practice of using a space to group digits is not usually followed in certain specialized
applications, such as engineering drawings and financial statements.

10.5.4Multiplying numbers

When the dot is used as the decimal marker as in the United States, the preferred sign for themultiplication of numbers or values of quantities is a cross (that is, multiplication sign) (×), not a half-high(that is, centered) dot (·).
Examples:

25 × 60.5
53 m/s × 10.2 s
15 × 72 kg

but not:but not:but not:

37

25·60.5
53 m/s·10.2 s
15·72 kg

Guide for the Use of the International System of Units (SI)
Notes:
1.

When the comma is used as the decimal marker, the preferred sign for the multiplication of
numbers is the half-high dot. However, even when the comma is so used, this Guide prefers the
cross for the multiplication of values of quantities.

2.

The multiplication of quantity symbols (or numbers in parentheses or values of quantities inparentheses) may be indicated in one of the following ways: ab, a b, a·b, a × b.

38

Guide for the Use of the International System of Units (SI)Appendix A. Definitions of the SI Base Units
A.1

Introduction

The following definitions of the SI base units are taken from Refs. [1, 2]; it should be noted that SI
derived units are uniquely defined only in terms of SI base units; for example, 1 V = 1 m2·kg·s−3·A−1.
Meter (17th CGPM, 1983)

A.2

The meter is the length of the path traveled by light in vacuum during a time interval of 1/299 792
458 of a second.
Kilogram (3d CGPM, 1901)

A.3

The kilogram is the unit of mass; it is equal to the mass of the international prototype of thekilogram.
Second (13th CGPM, 1967)

A.4

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition
between the two hyperfine levels of the ground state of the cesium-133 atom.
Ampere (9th CGPM, 1948)

A.5

The ampere is that constant current which, if maintained in two straight parallel conductors of
infinite length, of negligible circular cross section, and placed 1 meter apart in vacuum, would produce
between these conductors a force equal to 2 × 10−7 newton per meter of length.
Kelvin (13th CGPM, 1967)

A.6

The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic
temperature of the triple point of water.
Mole (14th CGPM, 1971)

A.7
1.

The mole is the amount of substance of a system which contains as many elementary entities as
there are atoms in 0.012 kilogram of carbon 12.

2.

When the mole is used, the elementary entities must be specified and may be atoms, molecules,
ions, electrons, other particles, or specified groups of such particles.

In the definition of the mole, it is understood that unbound atoms of carbon 12, at rest and in their
ground state, are referred to.
Note: This definition specifies at the same time the nature of the quantity whose unit is the mole.
A.8

Candela (16th CGPM, 1979)

The candela is the luminous intensity, in a given direction, of a source that emits monochromaticradiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of (1/683) watt per
steradian.

39

Guide for the Use of the International System of Units (SI)Appendix B. Conversion Factors 6
B.1

Introduction

Sections B.8 and B.9 give factors for converting values of quantities expressed in various units—
predominantly units outside the SI that are unacceptable for use with it—to values expressed either in (a) SI
units, (b) units that are accepted for use with the SI (especially units that better reflect the nature of theunconverted units), (c) units formed from such accepted units and SI units, or (d) decimal multiples or
submultiples of the units of (a) to (c) that yield numerical values of convenient magnitudes.
An example of (d) is the following: the values of quantities expressed in ångströms, such as the
wavelengths of visible laser radiations, are usually converted to values expressed in nanometers, notmeters. More generally, if desired, one can eliminate powers of 10 that appear in converted values as aresult of using the conversion factors (or simply factors for brevity) of Secs. B.8 and B.9 by selecting anappropriate SI prefix (see Sec. B.3).
B.2

Notation

The factors given in Secs. B.8 and B.9 are written as a number equal to or greater than 1 and lessthan 10, with 6 or fewer decimal places. The number is followed by the letter E, which stands for exponent,a plus (+) or minus (–) sign, and two digits that indicate the power of 10 by which the number is multiplied.
Examples:

3.523 907 E−02 means 3.523 907 × 10−2 = 0.035 239 07
3.386 389 E+03 means 3.386 389 × 103 = 3386.389

A factor in boldface is exact. All other factors have been rounded to the significant digits given inaccordance with accepted practice (see Secs. 7.9, B.7.2, and Refs. [4: ISO 31-0] and [6]). Where less thansix digits after the decimal place are given, the unit does not warrant a greater number of digits in its
conversion. However, for the convenience of the user, this practice is not followed for all such units,
including the cord, cup, quad, and teaspoon.
B.3

Use of conversion factors
Each entry in Secs. B.8 and B.9 is to be interpreted as in these two examples:

To convert fromtoMultiply byatmosphere, standard (atm) ...........................pascal (Pa).......................................1.01325 ....................E+05
cubic foot per second (ft3/s) ..........................cubic meter per second (m3/s) .........2.831 685 ................. E−02
means

1 atm = 101 325 Pa (exactly);
1 ft3/s = 0.028 316 85 m3/s.

Thus to express, for example, the pressure p = 11.8 standard atmospheres (atm) in pascals (Pa), write
p = 11.8 atm × 101 325 Pa/atm and obtain the converted numerical value 11.8 × 101 325 = 1 195 635 andthe converted value p = 1.20 MPa.

6This appendix is essentially the same as Appendix B of the 1995 Edition. That appendix was significantly revised version ofAppendix C of the 1991 Edition, which was reprinted from ANSI/IEEE Std 268-1982, American National Standard Metric Practice,©1982 by the Institute of Electrical and Electronics Engineers, Inc., with the permission of the IEEE. The origin of this material is E.
A. Mechtly, The International System of Units — Physical Constants and Conversion Factors, NASA SP-7012, Second Revision,
National Aeronautics and Space Administration (U.S. Government Printing Office, Washington, DC, 1973).

40

Guide for the Use of the International System of Units (SI)
Notes:
1.

Guidance on rounding converted numerical values of quantities is given in Sec. B.7.2.

2.

If the value of a quantity is expressed in a unit of the center column of Sec. B.8 or B.9 and it isnecessary to express it in the corresponding unit of the first column, divide by the factor.
The factors for derived units not included in Secs. B.8 and B.9 can readily be found from thefactors given.

Examples:B.9

To find the factor for converting values in lb·ft/s to values in kg·m/s, obtain from Sec. B.8 or

1 lb = 4.535 924 E−01 kg
1 ft = 3.048 E−01 m
and substitute these values into the unit lb·ft/s to obtain
1 lb·ft/s = 0.453 592 4 kg × 0.3048 m/s= 0.138 255 0 kg·m/s
and the factor is 1.382 550 E−01.
To find the factor for converting values in (avoirdupois) oz·in2 to values in kg·m2, obtain fromSec. B.8 or B.9
1 oz = 2.834 952 E−02 kg
1 in2 = 6.4516 E−04 m2
and substitute these values into the unit oz·in2 to obtain
1 oz·in2 = 0.028 349 52 kg × 0.000 645 16 m2= 0.000 018 289 98 kg·m2
and the factor is 1.828 998 E−05.
B.4

Organization of entries and style

In Sec. B.8 the units for which factors are given are listed alphabetically, while in Sec B.9 the sameunits are listed alphabetically within the following alphabetized list of kinds of quantities and fields ofscience:
ACCELERATION
ANGLE
AREA AND SECOND MOMENTOF AREACAPACITY (see VOLUME)
DENSITY (that is, MASS DENSITY)(see MASS DIVIDED BY VOLUME)
ELECTRICITY and MAGNETISMENERGY (includes WORK)ENERGY DIVIDED BY AREA TIMEFLOW (see MASS DIVIDED BY TIME
or VOLUME DIVIDED BY TIME)FORCEFORCE DIVIDED BY LENGTHHEAT
Available Energy
Coefficient of Heat Transfer
Density of Heat
Density of Heat Flow Rate
Fuel Consumption
Heat Capacity and Entropy
Heat Flow Rate
Specific Heat Capacity and Specific Entropy
Thermal Conductivity
Thermal Diffusivity
Thermal Insulance41

Guide for the Use of the International System of Units (SI)FORCE DIVIDED BY AREA
(see PRESSURE)
LENGTH
LIGHT
MASS and MOMENT OF INERTIAMASS DENSITY (see MASS DIVIDED
BY VOLUME)
MASS DIVIDED BY AREA
MASS DIVIDED BY CAPACITY(see MASS DIVIDED BY VOLUME)
MASS DIVIDED BY LENGTHMASS DIVIDED BY TIME
(includes FLOW)MASS DIVIDED BY VOLUME(includes MASS DENSITY and
MASS CONCENTRATION)
MOMENT OF FORCE or TORQUE
MOMENT OF FORCE or TORQUE,DIVIDED BY LENGTHPERMEABILITY

Thermal Resistance
Thermal Resistivity
POWER
PRESSURE or STRESS (FORCEDIVIDED BY AREA)
RADIOLOGY
SPEED (see VELOCITY)
STRESS (see PRESSURE)TEMPERATURETEMPERATURE INTERVALTIMETORQUE (see MOMENT OF FORCE)VELOCITY (includes SPEED)VISCOSITY, DYNAMIC
VISCOSITY, KINEMATIC
VOLUME (includes CAPACITY)
VOLUME DIVIDED BY TIME
(includes FLOW)WORK (see ENERGY)

In Secs. B.8 and B.9, the units in the left-hand columns are written as they are often usedcustomarily; the rules and style conventions recommended in this Guide are not necessarily observed.
Further, many are obsolete and some are not consistent with good technical practice. The corresponding
units in the center columns are, however, written in accordance with the rules and style conventions
recommended in this Guide.
B.5

Factor for converting motor vehicle efficiency

The efficiency of motor vehicles in the United States is commonly expressed in miles per U.S.
gallon, while in most other countries it is expressed in liters per one hundred kilometers. To convert fueleconomy stated in miles per U.S. gallon to fuel consumption expressed in L/(100 km), divide 235.215 by
the numerical value of the stated fuel economy. Thus 24 miles per gallon corresponds to 9.8 L/(100 km).
B.6

U.S. survey foot and mile

The U.S. Metric Law of 1866 gave the relationship 1 m = 39.37 in (in is the unit symbol for theinch). From 1893 until 1959, the yard was defined as being exactly equal to (3600/3937) m, and thus thefoot was defined as being exactly equal to (1200/3937) m.
In 1959 the definition of the yard was changed to bring the U.S. yard and the yard used in othercountries into agreement; see Ref. [7: FR 1959]. Since then the yard has been defined as exactly equal to0.9144 m, and thus the foot has been defined as exactly equal to 0.3048 m. At the same time it was decided
that any data expressed in feet derived from geodetic surveys within the United States would continue tobear the relationship as defined in 1893, namely, 1 ft = (1200/3937) m (ft is the unit symbol for the foot).
The name of this foot is “U.S. survey foot,” while the name of the new foot defined in 1959 is“international foot.” The two are related to each other through the expression 1 internationalfoot = 0.999 998 U.S. survey foot exactly.
In Secs. B.8 and B.9, the factors given are based on the international foot unless otherwise indicated.
Users of this Guide will also find the following summary of exact relationships helpful, where forconvenience in this section, the symbols ft and mi, that is, ft and mi in italic type, indicate that it is the U.S.
survey foot or U.S. survey mile that is meant rather than the international foot (ft) or international mile (mi),and where rd is the unit symbol for the rod and fur is the unit symbol for the furlong.

42

Guide for the Use of the International System of Units (SI)
1 ft = (1200/3937) m
1 ft = 0.3048 m
1 ft = 0.999 998 ft
1 rd, pole, or perch = 16 ½ ft
40 rd = 1 fur = 660 ft
8 fur = 1 U.S. survey mile (also called “statute mile”) = 1 mi = 5280 ft
1 fathom = 6 ft
1 international mile = 1 mi = 5280 ft
272 1/4 ft2 = 1 rd2
160 rd2 = 1 acre = 43 560 ft2
640 acre = 1 mi2
B.7

Rules for rounding numbers and converted numerical values of quantities

Rules for rounding numbers are discussed in Refs. [4: ISO 31-0] and [6]; the latter reference also
gives rules for rounding the converted numerical values of quantities whose values expressed in units thatare not accepted for use with the SI (primarily customary or inch-pound units) are converted to values
expressed in acceptable units. This Guide gives the principal rules for rounding numbers in Sec. B.7.1, andthe basic principle for rounding converted numerical values of quantities in Sec. B.7.2. The cited references
should be consulted for additional details.
B.7.1 Rounding numbers
To replace a number having a given number of digits with a number (called the rounded number)having a smaller number of digits, one may follow these rules:
1.

If the digits to be discarded begin with a digit less than 5, the digit preceding the 5 is not changed.

Example:2.

If the digits to be discarded begin with a 5 and at least one of the following digits is greater than 0,
the digit preceding the 5 is increased by 1.

Examples:3.

6.974 951 5 rounded to 3 digits is 6.97

6.974 951 5 rounded to 2 digits is 7.0
6.974 951 5 rounded to 5 digits is 6.9750

If the digits to be discarded begin with a 5 and all of the following digits are 0, the digit preceding
the 5 is unchanged if it is even and increased by 1 if it is odd. (Note that this means that the final
digit is always even.)

Examples:

6.974 951 5 rounded to 7 digits is 6.974 952
6.974 950 5 rounded to 7 digits is 6.974 950

B.7.2 Rounding converted numerical values of quantities
The use of the factors given in Secs. B.8 and B.9 to convert values of quantities was demonstrated inSec. B.3. In most cases the product of the unconverted numerical value and the factor will be a numericalvalue with a number of digits that exceeds the number of significant digits (see Sec. 7.9) of the unconverted
numerical value. Proper conversion procedure requires rounding this converted numerical value to thenumber of significant digits that is consistent with the maximum possible rounding error of the unconverted
numerical value.
Example:To express the value l = 36 ft in meters, use the factor 3.048 E−01 from Sec. B.8 or Sec. B.9
and writel = 36 ft × 0.3048 m/ft = 10.9728 m = 11.0 m.

43

Guide for the Use of the International System of Units (SI)The final result, l = 11.0 m, is based on the following reasoning: The numerical value “36” has two
significant digits, and thus a relative maximum possible rounding error (abbreviated RE in this Guide forsimplicity) of ± 0.5/36 = ± 1.4 %, because it could have resulted from rounding the number 35.5, 36.5, or
any number between 35.5 and 36.5. To be consistent with this RE, the converted numerical value“10.9728” is rounded to 11.0 or three significant digits because the number 11.0 has an RE
of ± 0.05/11.0 = ± 0.45 %. Although this ± 0.45 % RE is one-third of the ± 1.4 % RE of the unconverted
numerical value “36,” if the converted numerical value “10.9728” had been rounded to 11 or two
significant digits, information contained in the unconverted numerical value “36” would have been lost.
This is because the RE of the numerical value “11” is ± 0.5/11 = ± 4.5 %, which is three times the± 1.4 % RE of the unconverted numerical value “36.” This example therefore shows that when selecting thenumber of digits to retain in the numerical value of a converted quantity, one must often choose between
discarding information or providing unwarranted information. Consideration of the end use of theconverted value can often help one decide which choice to make.
Note: Consider that one had been told initially that the value l = 36 ft had been rounded to the nearest inch.
Then in this case, since l is known to within ± 1 in, the RE of the numerical value “36” is ± 1
in/(36 ft × 12 in/ft) = ± 0.23 %. Although this is less than the ± 0.45 % RE of the number 11.0, it iscomparable to it. Therefore, the result l = 11.0 m is still given as the converted value. (Note that thenumerical value “10.97” would give excessive unwarranted information because it has an RE that isone-fifth of ± 0.23 %.)44

Guide for the Use of the International System of Units (SI)B.8 Factors for units listed alphabeticallyCaution: The units listed in column 1 are in general not to be used in NIST publications, with the exceptionof those few in italic type.

Factors in boldface are exact
To convert fromtoMultiply byabampere .............................................................. ampere (A) ...................................................... 1.0 E+01
abcoulomb ........................................................... coulomb (C) .................................................... 1.0 E+01
abfarad.................................................................. farad (F) .......................................................... 1.0 E+09
abhenry ................................................................ henry (H) ......................................................... 1.0 E−09
abmho ................................................................... siemens (S) ...................................................... 1.0 E+09
abohm ................................................................... ohm (Ω) ........................................................... 1.0 E−09
abvolt ................................................................... volt (V) ............................................................ 1.0 E−08
acceleration of free fall, standard (gn)................... meter per second squared (m / s2) ........... 9.806 65 E+00
acre (based on U.S. survey foot)7 ........................ square meter (m2).................................. 4.046 873 E+03
acre foot (based on U.S. survey foot)7 ................. cubic meter (m3) ................................... 1.233 489 E+03
ampere hour (A·h) ............................................... coulomb (C) .................................................... 3.6 E+03ångström (Å)......................................................... meter (m) ........................................................ 1.0 E−10ångström (Å)......................................................... nanometer (nm)................................................ 1.0 E−01
are (a) ................................................................... square meter (m2) ............................................ 1.0 E+02
astronomical unit (ua) ......................................... meter (m) ............................................. 1.495 979 E+11
atmosphere, standard (atm) .................................. pascal (Pa) .............................................. 1.013 25 E+05
atmosphere, standard (atm) ................................. kilopascal (kPa) ...................................... 1.013 25 E+02
atmosphere, technical (at)8 ................................... pascal (Pa) .............................................. 9.806 65 E+04
atmosphere, technical (at)8 ................................... kilopascal (kPa) ...................................... 9.806 65 E+01
bar (bar) ............................................................... pascal (Pa) ....................................................... 1.0 E+05
bar (bar) ............................................................... kilopascal (kPa) ............................................... 1.0 E+02
barn (b) ................................................................ square meter (m2)............................................. 1.0 E−28
barrel [for petroleum, 42 gallons (U.S.)](bbl) ...... cubic meter (m3) ................................... 1.589 873 E−01
barrel [for petroleum, 42 gallons (U.S.)](bbl) ..... liter (L) ................................................. 1.589 873 E+02
biot (Bi) ................................................................ ampere (A) ...................................................... 1.0 E+01
British thermal unitIT (BtuIT)9 ............................... joule (J) ................................................ 1.055 056 E+03
British thermal unitth (BtuIT)9................................ joule (J) ................................................ 1.054 350 E+03
British thermal unit (mean) (Btu) ........................ joule (J) .................................................. . 1.055 87 E+03
British thermal unit (39 ºF) (Btu) ........................ joule (J) .................................................. . 1.059 67 E+03
British thermal unit (59 ºF) (Btu) ........................ joule (J) .................................................. . 1.054 80 E+03
British thermal unit (60 ºF) (Btu) ........................ joule (J) .................................................. . 1.054 68 E+03
British thermal unitIT foot per hour square foot degree Fahrenheit[BtuIT·ft/(h·ft2·ºF)].......................................... watt per meter kelvin [W / (m · K)] ...... 1.730 735 E+00
British thermal unitth foot per hour square foot degree Fahrenheit[Btuth·ft/(h·ft2·ºF)] .........................................watt per meter kelvin [W / (m · K)] ...... 1.729 577 E+00
British thermal unitIT inch per hour square foot degree Fahrenheit[BtuIT·in/(h·ft2·ºF)] ........................................ watt per meter kelvin [W / (m · K)] ...... 1.442 279 E−01
British thermal unitth inch per hour square foot degree Fahrenheit[Btuth·in/(h·ft2·ºF)] ........................................ watt per meter kelvin [W / (m · K)] ..... 1.441 314 E−01
British thermal unitIT inch per second square foot degree Fahrenheit[BtuIT·in/(s·ft2·ºF)] ......................................... watt per meter kelvin [W / (m · K)] ..... 5.192 204 E+02
7

For remarks on U.S. survey foot, see Sec. B.6.
One technical atmosphere equals one kilogram-force per square centimeter (1 at = 1 kgf/cm2).
9
The Fifth International Conference on the Properties of Steam (London, July 1956) defined the International Table calorie as
4.1868 J. Therefore, the exact conversion factor for the International Table Btu is 1.055 055 852 62 kJ. Note that the notation forInternational Table used in this listing is subscript “IT.” Similarily, the notation for thermochemical is subscript “th.” Further, thethermochemical Btu, Btuth, is based on the thermochemical calorie, calth, where calth = 4.184 J exactly.
845

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply by

British thermal unitth inch per second square foot degree Fahrenheit[Btuth·in/(s·ft2·ºF)] ......................................... watt per meter kelvin [W / (m · K)] ..... 5.188 732 E+02
British thermal unitIT per cubic foot(BtuIT/ft3) ...................................................... joule per cubic meter (J / m3) ................ 3.725 895 E+04
British thermal unitth per cubic foot(Btuth/ft3) ...................................................... joule per cubic meter (J / m3) ............... 3.723 403 E+04
British thermal unitIT per degree Fahrenheit(BtuIT/ºF) ...................................................... joule per kelvin (J / K) ......................... 1.899 101 E+03
British thermal unitth per degree Fahrenheit(Btuth/ºF)........................................................ joule per kelvin (J / K) ........................ 1.897 830 E+03
British thermal unitIT per degree Rankine(BtuIT / ºR) ..................................................... joule per kelvin (J / K) ......................... 1.899 101 E+03
British thermal unitth per degree Rankine(Btuth / ºR) ..................................................... joule per kelvin (J / K) ......................... 1.897 830 E+03
British thermal unitIT per hour (BtuIT/h) .............. watt (W) ................................................ 2.930 711 E−01
British thermal unitth per hour (Btuth/h) ............... watt (W) ................................................ 2.928 751 E−01
British thermal unitIT per hour square foot degree Fahrenheit[BtuIT / (h·ft2·ºF)] .......................................... watt per square meter kelvin[W / (m2 · K)] ........................................ 5.678 263 E+00
British thermal unitth per hour square foot degree Fahrenheit[Btuth / (h·ft2·ºF)] ........................................... watt per square meter kelvin[W / (m2 · K)] ........................................ 5.674 466 E+00
British thermal unitth per minute (Btuth / min) ...... watt (W) ............................................... 1.757 250 E+01
British thermal unitIT per pound (BtuIT / lb) ......... joule per kilogram (J / kg) ........................... 2.326 E+03
British thermal unitth per pound (Btuth / lb) .......... joule per kilogram (J / kg) .................... 2.324 444 E+03
British thermal unitIT per pound degree Fahrenheit[BtuIT / (lb·ºF)] .............................................. joule per kilogram kelvin (J / (kg · K)] ...... 4.1868 E+03
British thermal unitth per pound degree Fahrenheit[Btuth / (lb·ºF)] .............................................. joule per kilogram kelvin [J / (kg · K)] ........ 4.184 E+03
British thermal unitIT per pound degree Rankine[BtuIT / (lb·ºR)] .............................................. joule per kilogram kelvin [J / (kg · K)] ...... 4.1868 E+03
British thermal unitth per pound degree Rankine[Btuth / (lb·ºR)] .............................................. joule per kilogram kelvin [J / (kg · K)] ........ 4.184 E+03
British thermal unitIT per second (BtuIT / s) ......... watt (W) ............................................... 1.055 056 E+03
British thermal unitth per second (Btuth / s) .......... watt (W) ............................................... 1.054 350 E+03
British thermal unitIT per second square foot degree Fahrenheit[BtuIT / (s · ft2·ºF)] ......................................... watt per square meter kelvin[W/(m2 · K)] ......................................... 2.044 175 E+04
British thermal unitth per second square foot degree Fahrenheit[Btuth / (s · ft2·ºF)] .......................................... watt per square meter kelvin[W/(m2 · K)] ......................................... 2.042 808 E+04
British thermal unitIT per square foot(BtuIT / ft2) .................................................... joule per square meter (J / m2) ............. 1.135 653 E+04
British thermal unitth per square foot(Btuth / ft2) ..................................................... joule per square meter (J / m2) ............. 1.134 893 E+04
British thermal unitIT per square foot hour[(BtuIT / (ft2 · h)] ............................................ watt per square meter (W / m2) ............ 3.154 591 E+00
British thermal unitth per square foot hour
[Btuth / (ft2 · h)] .............................................. watt per square meter (W / m2) ............ 3.152 481 E+00
British thermal unitth per square foot minute[Btuth / (ft2 · min)] ......................................... watt per square meter (W / m2) ............ 1.891 489 E+02
British thermal unitIT per square foot second[(BtuIT / (ft2 · s)] ............................................ watt per square meter (W / m2) ............ 1.135 653 E+04

46

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply by

British thermal unitth per square foot second[Btuth / (ft2 · s)] .............................................. watt per square meter (W / m2) ........... . 1.134 893
British thermal unitth per square inch second[Btuth /(in2 · s)] .............................................. watt per square meter (W / m2) ............ 1.634 246
bushel (U.S.) (bu) ................................................ cubic meter (m3) ................................... 3.523 907
bushel (U.S.) (bu) ................................................ liter (L) ................................................. 3.523 907

E+04
E+06
E−02
E+01

calorieIT (calIT)10 .................................................. joule (J) ..................................................... 4.1868 E+00
calorieth (calth)10 ................................................... joule (J) ....................................................... 4.184 E+00
calorie (cal) (mean) .............................................. joule (J) .................................................. 4.190 02 E+00
calorie (15 ºC) (cal15) ........................................... joule (J) ................................................... 4.185 80 E+00
calorie (20 ºC) (cal20) ........................................... joule (J) ................................................... 4.181 90 E+00
calorieIT, kilogram (nutrition)11 ........................... joule (J) ..................................................... 4.1868 E+03
calorieth, kilogram (nutrition)11 ............................ joule (J) ....................................................... 4.184 E+03
calorie (mean), kilogram (nutrition)11 .................. joule (J) .................................................. 4.190 02 E+03
calorieth per centimeter second degree Celsius[calth / (cm · s · ºC)] ....................................... watt per meter kelvin [W / (m · K)] ............. 4.184 E+02
calorieIT per gram (calIT / g) ................................. joule per kilogram (J / kg) ......................... 4.1868 E+03
calorieth per gram (calth / g) .................................. joule per kilogram (J / kg) ........................... 4.184 E+03
calorieIT per gram degree Celsius[calIT / (g·ºC)] ............................................... joule per kilogram kelvin [J / (kg · K)] ...... 4.1868 E+03
calorieth per gram degree Celsius[calth / (g·ºC)] ................................................ joule per kilogram kelvin [J / (kg · K)] ........ 4.184 E+03
calorieIT per gram kelvin [calIT / (g·K)] ............... joule per kilogram kelvin [J / (kg · K)] ...... 4.1868 E+03
calorieth per gram kelvin [calth / (g·K)] ................ joule per kilogram kelvin [J / (kg · K)] ........ 4.184 E+03
calorieth per minute (calth / min) .......................... watt (W) ............................................... 6.973 333 E−02
calorieth per second (calth / s) ............................... watt (W) ...................................................... 4.184 E+00
calorieth per square centimeter (calth / cm2) ......... joule per square meter (J / m2) .................... 4.184 E+04
calorieth per square centimeter minute[calth / (cm2 · min)] ........................................ watt per square meter (W / m2) ............ 6.973 333 E+02
calorieth per square centimeter second
calth / (cm2 · s)] .............................................. watt per square meter (W / m2) ................... 4.184 E+04
candela per square inch (cd / in2) ......................... candela per square meter (cd / m2) ....... 1.550 003 E+03
carat, metric ......................................................... kilogram (kg) .................................................. 2.0 E−04
carat, metric ......................................................... gram (g) .......................................................... 2.0 E−01
centimeter of mercury (0 ºC)12 ............................ pascal (Pa) .............................................. 1.333 22 E+03
centimeter of mercury (0 ºC)12 ............................ kilopascal (kPa)....................................... 1.333 22 E+00
centimeter of mercury, conventional (cmHg)12 ... pascal (Pa) . ......................................... . 1.333 224 E+03
centimeter of mercury, conventional (cmHg)12 ... kilopascal (kPa) .................................... 1.333 224 E+00
centimeter of water (4 ºC)12 ................................. pascal (Pa) .............................................. 9.806 38 E+01
centimeter of water, conventional (cmH2O)12 ..... pascal (Pa) .............................................. 9.806 65 E+01
centipoise (cP) ..................................................... pascal second (Pa·s) ........................................ 1.0 E−03
centistokes (cSt) .................................................. meter squared per second (m2/s) ..................... 1.0 E−06
chain (based on U.S. survey foot) (ch)7 ............... meter (m) ............................................. 2.011 684 E+01
circular mil .......................................................... square meter (m2) ................................. 5.067 075 E−10
10

The small calorie or gram calorie approximates the energy needed to increase the temperature of 1 gram of water by 1 °C.
Subscripts “IT” and “th” refer to International Table and thermochemical calories, respectively; see footnote 9.
11
The kilogram calorie or ‘‘large calorie’’ is an obsolete term used for the kilocalorie, which is the calorie used to express the energy
content of foods. However, in practice, the prefix ‘‘kilo’’ is usually omitted.
12
Conversion factors for mercury manometer pressure units are calculated using the standard value for the acceleration of gravity andthe density of mercury at the stated temperature. Additional digits are not justified because the definitions of the units do not take into
account the compressibility of mercury or the change in density caused by the revised practical temperature scale, ITS-90. Similarcomments also apply to water manometer pressure units. Conversion factors for conventional mercury and water manometer pressure
units are based on Ref. [4: ISO 80000-4].

47

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply by

circular mil .......................................................... square millimeter (mm2) ...................... 5.067 075 E−04
clo ........................................................................ square meter kelvin per watt (m2·K / W) ...... 1.55 E−01
cord (128 ft3) ....................................................... cubic meter (m3) ................................... 3.624 556 E+00
cubic foot (ft3) ..................................................... cubic meter (m3) ................................... 2.831 685 E−02
cubic foot per minute (ft3 / min) .......................... cubic meter per second (m3 / s) ............ 4.719 474 E−04
cubic foot per minute (ft3 / min) .......................... liter per second (L/s) ............................ 4.719 474 E−01
cubic foot per second (ft3 / s) ............................... cubic meter per second (m3 / s) ............ 2.831 685 E−02
cubic inch (in3)13 ................................................... cubic meter (m3) ................................... 1.638 706 E−05
cubic inch per minute (in3 / min) ......................... cubic meter per second (m3 / s) ............ 2.731 177 E−07
cubic mile (mi3) ................................................... cubic meter (m3).................................... 4.168 182 E+09
cubic yard (yd3) ................................................... cubic meter (m3) ................................... 7.645 549 E−01
cubic yard per minute (yd3 / min) ........................ cubic meter per second (m3 / s) ............ 1.274 258 E−02
cup (U.S.) ............................................................ cubic meter (m3) ................................... 2.365 882 E−04
cup (U.S.) ............................................................ liter (L) ................................................. 2.365 882 E−01
cup (U.S.) ............................................................ milliliter (mL) ...................................... 2.365 882 E+02
curie (Ci) ............................................................. becquerel (Bq) ................................................. 3.7 E+10
darcy14 ................................................................. meter squared (m2) ............................... 9.869 233 E−13
day (d) .................................................................. second (s) ..................................................... 8.64 E+04
day (sidereal) ....................................................... second (s) .............................................. 8.616 409 E+04
debye (D) ............................................................. coulomb meter (C · m).......................... 3.335 641 E−30
degree (angle) ( º ) ............................................... radian (rad) ........................................... 1.745 329 E−02
degree Celsius (temperature) (ºC) ....................... kelvin (K) ..................................... T / K = t / º C + 273.15
degree Celsius (temperature interval) (ºC) .......... kelvin (K) ........................................................ 1.0 E+00
degree centigrade (temperature)15 ........................ degree Celsius (ºC) .......................... t / º C ≈ t / deg. cent.
degree centigrade (temperature interval)15 .......... degree Celsius (ºC) ......................................... 1.0 E+00
degree Fahrenheit (temperature) (ºF) ................... degree Celsius (ºC) .................... t / º C = (t / º F − 32)/1.8
degree Fahrenheit (temperature) (ºF) ................... kelvin (K) ............................. T / K = (t / º F + 459.67)/1.8
degree Fahrenheit (temperature interval)(ºF) ...... degree Celsius (ºC) ............................... 5.555 556 E−01
degree Fahrenheit (temperature interval) (ºF) ...... kelvin (K) ............................................. 5.555 556 E−01
degree Fahrenheit hour per British thermal unitIT(ºF · h/BtuIT) .................................................. kelvin per watt (K/W) ......................... 1.895 634 E+00
degree Fahrenheit hour per British thermal unitth(ºF·h / Btuth) .................................................. kelvin per watt (K / W) ........................ 1.896 903 E+00
degree Fahrenheit hour square foot per British
thermal unitIT (ºF · h · ft2 / BtuIT) ................. square meter kelvin per watt
(m2 · K / W) ......................................... 1.761 102 E−01
degree Fahrenheit hour square foot per British
thermal unitth (ºF · h · ft2 / Btuth) .................. square meter kelvin per watt
(m2 · K / W) ....................................... 1.762 280 E−01
degree Fahrenheit hour square foot per British
thermal unitIT inch [ºF · h · ft2 / (BtuIT · in)]. meter kelvin per watt (m · K / W) ........ 6.933 472 E+00
degree Fahrenheit hour square foot per British
thermal unitth inch [ºF · h · ft2 / (Btuth · in)] .. meter kelvin per watt (m · K / W) ....... 6.938 112 E+00
degree Fahrenheit second per British thermal unitIT(ºF · s / BtuIT) ................................................ kelvin per watt (K / W) ........................ 5.265 651 E−04
degree Fahrenheit second per British thermal unitth(ºF · s / Btuth) ................................................ kelvin per watt (K / W) ....................... 5.269 175 E−04
degree Rankine (ºR) ............................................. kelvin (K) ........................................ T / K = (T / ºR) / 1.8
13

The exact conversion factor is 1.638 706 4 E−05.
The darcy is a unit for expressing the permeability of porous solids, not area.
15
The centigrade temperature scale is obsolete; the degree centigrade is only approximately equal to the degree Celsius.
1448

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply bydegree Rankine (temperature interval) (ºR) ......... kelvin (K) ............................................. 5.555 556 E−01
denier ................................................................... kilogram per meter (kg / m) .................. 1.111 111 E−07
denier ................................................................... gram per meter (g / m) ......................... 1.111 111 E−04
dyne (dyn) ............................................................ newton (N) ...................................................... 1.0 E−05
dyne centimeter (dyn·cm) .................................... newton meter (N · m) ...................................... 1.0 E−07
dyne per square centimeter (dyn / cm2) ............... pascal (Pa) ....................................................... 1.0 E−01
electronvolt (eV) .................................................. joule (J) ................................................ 1.602 176 E−19
EMU of capacitance (abfarad) ............................. farad (F) .......................................................... 1.0 E+09
EMU of current (abampere) ................................ ampere (A) ...................................................... 1.0 E+01
EMU of Electric potential (abvolt) .................... volt (V) ............................................................ 1.0 E−08
EMU of inductance (abhenry) ............................. henry (H) ......................................................... 1.0 E−09
EMU of resistance (abohm) ................................. ohm (Ω) ........................................................... 1.0 E−09
erg (erg) ............................................................... joule (J) .............................................................1.0 E−07
erg per second (erg / s) ........................................ watt (W) .......................................................... 1.0 E−07
erg per square centimeter second[erg / (cm2 · s)] ............................................. watt per square meter (W / m2) ....................... 1.0 E−03
ESU of capacitance (statfarad) ............................ farad (F) ............................................... 1.112 650 E−12
ESU of current (statampere) ................................. ampere (A) ........................................... 3.335 641 E−10
ESU of Electric potential (statvolt) ..................... volt (V).................................................. 2.997 925 E+02
ESU of inductance (stathenry) ............................. henry (H) .............................................. 8.987 552 E+11
ESU of resistance (statohm) ................................ ohm (Ω) ............................................... 8.987 552 E+11
faraday (based on carbon 12) ............................... coulomb (C) ......................................... 9.648 534 E+04
fathom (based on U.S. survey foot)7 .................... meter (m) ............................................. 1.828 804 E+00
fermi .................................................................... meter (m) ........................................................ 1.0 E−15
fermi .................................................................... femtometer (fm) .............................................. 1.0 E+00
fluid ounce (U.S.) (fl oz) ..................................... cubic meter (m3) ................................... 2.957 353 E−05
fluid ounce (U.S.) (fl oz) ..................................... milliliter (mL) ...................................... 2.957 353 E+01
foot (ft) ................................................................ meter (m) .................................................... 3.048 E−01
foot (U.S. survey) (ft)7 ......................................... meter (m) ............................................. 3.048 006 E−01
footcandle ............................................................ lux (lx) ................................................. 1.076 391 E+01
footlambert .......................................................... candela per square meter (cd / m2) ....... 3.426 259 E+00
foot of mercury, conventional (ftHg)12 ................ pascal (Pa)............................................. 4.063 666 E+04
foot of mercury, conventional (ftHg)12 ................ kilopascal (kPa) .................................... 4.063 666 E+01
foot of water (39.2 ºF)12 ....................................... pascal (Pa) .............................................. 2.988 98 E+03
foot of water (39.2 ºF)12 ....................................... kilopascal (kPa) ...................................... 2.988 98 E+00
foot of water, conventional (ftH2O)12 .................. pascal (Pa) ............................................ 2.989 067 E+03
foot of water, conventional (ftH2O)12 .................. kilopascal (kPa) .................................... 2.989 067 E+00
foot per hour (ft / h) .............................................. meter per second (m / s) ....................... 8.466 667 E−05
foot per minute (ft / min) ..................................... meter per second (m / s) ................................ 5.08 E−03
foot per second (ft / s) .......................................... meter per second (m / s) .............................. 3.048 E−01
foot per second squared (ft / s2) ........................... meter per second squared (m / s2) ............... 3.048 E−01
foot poundal.......................................................... joule (J) ................................................. 4.214 011 E−02
foot pound-force (ft · lbf) .................................... joule (J) ............................................... 1.355 818 E+00
foot pound-force per hour (ft · lbf / h) ................. watt (W) ............................................... 3.766 161 E−04
foot pound-force per minute (ft · lbf / min) ......... watt (W) ............................................... 2.259 697 E−02
foot pound-force per second (ft · lbf / s)............... watt (W) ............................................... 1.355 818 E+00
foot to the fourth power (ft4) 16 ............................. meter to the fourth power (m4) ............. 8.630 975 E−03
franklin (Fr) ......................................................... coulomb (C) ......................................... 3.335 641 E−10
16This is a unit for the quantity second moment of area, which is sometimes called the “moment of section” or “area moment ofinertia” of a plane section about a specified axis.

49

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply bygal (Gal) ............................................................... meter per second squared (m / s2) .................... 1.0 E−02
gallon [Canadian and U.K. (Imperial)] (gal) ....... cubic meter (m3) ..................................... 4.546 09 E−03
gallon [Canadian and U.K. (Imperial)] (gal) ....... liter (L) ................................................... 4.546 09 E+00
gallon (U.S.) (gal) ................................................ cubic meter (m3) ................................... 3.785 412 E−03
gallon (U.S.) (gal) ................................................ liter (L) ................................................. 3.785 412 E+00
gallon (U.S.) per day (gal / d) .............................. cubic meter per second (m3 / s) ............ 4.381 264 E−08
gallon (U.S.) per day (gal / d) .............................. liter per second (L / s) .......................... 4.381 264 E−05
gallon (U.S.) per horsepower hour[gal / (hp · h)] ................................................ cubic meter per joule (m3 / J) ............... 1.410 089 E−09
gallon (U.S.) per horsepower hour[gal / (hp · h)] ................................................ liter per joule (L / J) ............................. 1.410 089 E−06
gallon (U.S.) per minute (gpm)(gal / min)............ cubic meter per second (m3 / s) ............ 6.309 020 E−05
gallon (U.S.) per minute (gpm)(gal / min) ........... liter per second (L / s) .......................... 6.309 020 E−02
gamma (γ) ............................................................ tesla (T) ........................................................... 1.0 E−09
gauss (Gs, G) ....................................................... tesla (T) ........................................................... 1.0 E−04
gilbert (Gi) ........................................................... ampere (A) ........................................... 7.957 747 E−01
gill [Canadian and U.K. (Imperial)] (gi)............... cubic meter (m3) ................................... 1.420 653 E−04
gill [Canadian and U.K. (Imperial)] (gi) .............. liter (L) ................................................. 1.420 653 E−01
gill (U.S.) (gi) ...................................................... cubic meter (m3) ................................... 1.182 941 E−04
gill (U.S.) (gi) ...................................................... liter (L) ................................................. 1.182 941 E−01
gon (also called grade) (gon) ............................... radian (rad) ........................................... 1.570 796 E−02
gon (also called grade) (gon) ............................... degree (angle) ( º ) ........................................... 9.0 E−01
grain (gr) .............................................................. kilogram (kg) ....................................... 6.479 891 E−05
grain (gr) .............................................................. milligram (mg) ..................................... 6.479 891 E+01
grain per gallon (U.S.) (gr / gal) .......................... kilogram per cubic meter (kg / m3) ...... 1.711 806 E−02
grain per gallon (U.S.) (gr / gal) .......................... milligram per liter (mg / L) .................. 1.711 806 E+01
gram-force per square centimeter (gf / cm2) ........ pascal (Pa) .............................................. 9.806 65 E+01
gram per cubic centimeter (g / cm3) .................... kilogram per cubic meter (kg / m3) ................. 1.0 E+03
hectare (ha) .......................................................... square meter (m2) ............................................ 1.0 E+04
horsepower (550 ft · lbf / s) (hp) .......................... watt (W) ............................................... 7.456 999 E+02
horsepower (boiler) ............................................. watt (W) ................................................. 9.809 50 E+03
horsepower (electric) ........................................... watt (W) ........................................................ 7.46 E+02
horsepower (metric) ............................................. watt (W) ............................................... 7.354 988 E+02
horsepower (U.K.) ............................................... watt (W) .................................................... 7.4570 E+02
horsepower (water) .............................................. watt (W) ................................................. 7.460 43 E+02
hour (h) ................................................................ second (s) ......................................................... 3.6 E+03
hour (sidereal) ...................................................... second (s) ............................................. 3.590 170 E+03
hundredweight (long, 112 lb) ............................... kilogram (kg) ....................................... 5.080 235 E+01
hundredweight (short, 100 lb) ............................. kilogram (kg) ....................................... 4.535 924 E+01
inch (in) ............................................................... meter (m) ...................................................... 2.54 E−02
inch (in) ............................................................... centimeter (cm) ............................................. 2.54 E+00
inch of mercury (32 ºF)12 ..................................... pascal (Pa) .............................................. 3.386 38 E+03
inch of mercury (32 ºF)12 ..................................... kilopascal (kPa) ...................................... 3.386 38 E+00
inch of mercury (60 ºF)12 ..................................... pascal (Pa) .............................................. 3.376 85 E+03
inch of mercury (60 ºF)12 ..................................... kilopascal (kPa)....................................... 3.376 85 E+00
inch of mercury, conventional (inHg)12 ............... pascal (Pa)............................................. 3.386 389 E+03
inch of mercury, conventional (inHg)12 ............... kilopascal (kPa)..................................... 3.386 389 E+00
inch of water (39.2 ºF)12 ...................................... pascal (Pa) .............................................. 2.490 82 E+02
inch of water (60 ºF)12 ......................................... pascal (Pa) ................................................. 2.4884 E+02
inch of water, conventional (inH2O)12 ................. pascal (Pa)............................................. 2.490 889 E+02
inch per second (in / s) ......................................... meter per second (m / s) ................................ 2.54 E−02

50

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply byinch per second squared (in / s2) .......................... meter per second squared (m / s2) ................. 2.54 E−02
inch to the fourth power (in4)16 ............................ meter to the fourth power (m4) ............. 4.162 314 E−07
kayser (K) ............................................................ reciprocal meter (m−1) ..................................... 1.0 E+02
kelvin (K) ............................................................. degree Celsius (ºC) ...................... t / ºC = T / K − 273.15
kilocalorieIT (kcalIT) ............................................. joule (J) ..................................................... 4.1868 E+03
kilocalorieth (kcalth) .............................................. joule (J) ....................................................... 4.184 E+03
kilocalorie (mean) (kcal) ..................................... joule (J) .................................................. 4.190 02 E+03
kilocalorieth per minute (kcalth / min) .................. watt (W ................................................. 6.973 333 E+01
kilocalorieth per second (kcalth / s) ....................... watt (W ........................................................ 4.184 E+03
kilogram-force (kgf) ............................................ newton (N) ............................................. 9.806 65 E+00
kilogram-force meter (kgf · m) ............................ newton meter (N · m) ............................. 9.806 65 E+00
kilogram-force per square centimeter(kgf / cm2) ..................................................... pascal (Pa) .............................................. 9.806 65 E+04
kilogram-force per square centimeter(kgf / cm2) ..................................................... kilopascal (kPa)....................................... 9.806 65 E+01
kilogram-force per square meter (kgf / m2) ......... pascal (Pa) .............................................. 9.806 65 E+00
kilogram-force per square millimeter(kgf / mm2) ................................................... pascal (Pa) .............................................. 9.806 65 E+06
kilogram-force per square millimeter(kgf / mm2) ................................................... megapascal (MPa) .................................. 9.806 65 E+00
kilogram-force second squared per meter(kgf · s2 / m) .................................................. kilogram (kg) ......................................... 9.806 65 E+00
kilometer per hour (km / h) ................................. meter per second (m / s) ....................... 2.777 778 E−01
kilopond (kilogram-force) (kp) ............................ newton (N) ............................................. 9.806 65 E+00
kilowatt hour (kW · h) ......................................... joule (J) ........................................................... 3.6 E+06
kilowatt hour (kW · h) ......................................... megajoule (MJ) ............................................... 3.6 E+00
kip (1 kip =1 000 lbf) .......................................... newton (N) ........................................... 4.448 222 E+03
kip (1 kip = 1000 lbf) .......................................... kilonewton (kN) ................................... 4.448 222 E+00
kip per square inch (ksi) (kip / in2) ...................... pascal (Pa) ............................................ 6.894 757 E+06
kip per square inch (ksi) (kip / in2) ...................... kilopascal (kPa) .................................... 6.894 757 E+03
knot (nautical mile per hour) ............................... meter per second (m / s) ....................... 5.144 444 E−01
lambert17 .............................................................. candela per square meter (cd / m2) ...... 3.183 099
langley (calth / cm2) ............................................. joule per square meter (J / m2) ................... 4.184
light year (l.y.)18 .................................................. meter (m) .............................................. 9.460 73
liter (L)19 .............................................................. cubic meter (m3) ............................................. 1.0
lumen per square foot (lm / ft2) ........................... lux (lx) ................................................. 1.076 391

E+03
E+04
E+15
E−03
E+01

maxwell (Mx) ...................................................... weber (Wb) .................................................... 1.0
mho ...................................................................... siemens (S) ..................................................... 1.0
microinch ............................................................. meter (m) ...................................................... 2.54
microinch ............................................................. micrometer (μm) ........................................... 2.54
micron (μ) ............................................................ meter (m) ........................................................ 1.0
micron (μ) ............................................................ micrometer (μm) ............................................ 1.0
mil (0.001 in) ....................................................... meter (m) ...................................................... 2.54
mil (0.001 in) ....................................................... millimeter (mm) ............................................ 2.54

E−08
E+00
E−08
E−02
E−06
E+00
E−05
E−02

17

The exact conversion factor is 104/π.
This conversion factor is based on 1 d = 86 400 s; and 1 Julian century = 36 525 d. (See The Astronomical Almanac for the Year
1995, page K6, U.S. Government Printing Office, Washington, DC, 1994).
19
In 1964 the General Conference on Weights and Measures reestablished the name “liter” as a special name for the cubic decimeter.
Between 1901 and 1964 the liter was slightly larger (1.000 028 dm3); when one uses high-accuracy volume data of that time, this factmust be kept in mind.
18

51

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply bymil (angle) ........................................................... radian (rad) ........................................... 9.817 477 E−04
mil (angle) ........................................................... degree ( º ) ................................................... 5.625 E−02
mile (mi) .............................................................. meter (m) ............................................. 1.609 344 E+03
mile (mi) .............................................................. kilometer (km) ..................................... 1.609 344 E+00
mile (based on U.S. survey foot) (mi)7 ................ meter (m) ............................................. 1.609 347 E+03
mile (based on U.S. survey foot) (mi)7 ................ kilometer (km) ..................................... 1.609 347 E+00
mile, nautical 20 .................................................... meter (m) .................................................... 1.852 E+03
mile per gallon (U.S.) (mpg) (mi / gal)................. meter per cubic meter (m / m3) ............ 4.251 437 E+05
mile per gallon (U.S.) (mpg) (mi / gal) ................ kilometer per liter (km / L) .................. 4.251 437 E−01
mile per gallon (U.S.) (mpg) (mi / gal) 21 ............. liter per 100 kilometer (L / 100 km) .... divide 235.215 bynumber of miles
per gallon
mile per hour (mi / h) ........................................... meter per second (m / s) .......................... . 4.4704 E−01
mile per hour (mi / h) ........................................... kilometer per hour (km / h) .................. 1.609 344 E+00
mile per minute (mi / min) ................................... meter per second (m / s) ......................... 2.682 24 E+01
mile per second (mi / s) ....................................... meter per second (m / s) ....................... 1.609 344 E+03
millibar (mbar) .................................................... pascal (Pa) ....................................................... 1.0 E+02
millibar (mbar) .................................................... kilopascal (kPa) ............................................... 1.0 E−01
millimeter of mercury, conventional (mmHg)12 .. pascal (Pa) ............................................ 1.333 224 E+02
millimeter of water, conventional (mmH2O)12 .... pascal (Pa) .............................................. 9.806 65 E+00
minute (angle) (') ................................................. radian (rad) ........................................... 2.908 882 E−04
minute (min) ........................................................ second (s) ........................................................ 6.0 E+01
minute (sidereal) .................................................. second (s) ............................................. 5.983 617 E+01
oersted (Oe) ......................................................... ampere per meter (A / m) ..................... 7.957 747 E+01
ohm centimeter (Ω · cm) ...................................... ohm meter (Ω·m) ............................................ 1.0 E−02
ohm circular-mil per foot ..................................... ohm meter (Ω·m) ................................. 1.662 426 E−09
ohm circular-mil per foot ..................................... ohm square millimeter per meter(Ω·mm2 / m) ...................................... 1.662 426 E−03
ounce (avoirdupois) (oz) ...................................... kilogram (kg) ....................................... 2.834 952 E−02
ounce (avoirdupois) (oz)....................................... gram (g) ............................................... 2.834 952 E+01
ounce (troy or apothecary) (oz) ........................... kilogram (kg) ....................................... 3.110 348 E−02
ounce (troy or apothecary) (oz) ........................... gram (g) ............................................... 3.110 348 E+01
ounce [Canadian and U.K. fluid (Imperial)](fl oz)............................................................. cubic meter (m3) .................................. 2.841 306 E−05
ounce [Canadian and U.K. fluid (Imperial)](fl oz) ............................................................ milliliter (mL) ...................................... 2.841 306 E+01
ounce (U.S. fluid) (fl oz) ..................................... cubic meter (m3) ................................... 2.957 353 E−05
ounce (U.S. fluid) (fl oz) ..................................... milliliter (mL) ...................................... 2.957 353 E+01
ounce (avoirdupois)-force (ozf) ........................... newton (N) ........................................... 2.780 139 E−01
ounce (avoirdupois)-force inch (ozf · in) ............. newton meter (N·m) ............................. 7.061 552 E−03
ounce (avoirdupois)-force inch (ozf · in) ............. millinewton meter (mN·m) .................. 7.061 552 E+00
ounce (avoirdupois) per cubic inch (oz / in3) ....... kilogram per cubic meter (kg / m3) ...... 1.729 994 E+03
ounce (avoirdupois) per gallon [Canadian and
U.K. (Imperial)] (oz / gal) ............................. kilogram per cubic meter (kg / m3) ...... 6.236 023 E+00
ounce (avoirdupois) per gallon [Canadian and
U.K. (Imperial)] (oz / gal) ............................ gram per liter (g / L) ............................. 6.236 023 E+00
ounce (avoirdupois) per gallon(U.S.)(oz / gal) .... kilogram per cubic meter (kg / m3) ...... 7.489 152 E+00
ounce (avoirdupois) per gallon(U.S.)(oz / gal) .... gram per liter (g / L) ............................. 7.489 152 E+00
ounce (avoirdupois) per square foot (oz / ft2) ...... kilogram per square meter (kg / m2) .... 3.051 517 E−01
20

The value of this unit, 1 nautical mile = 1852 m, was adopted by the First International Extraordinary Hydrographic Conference,
Monaco, 1929, under the name “International nautical mile.”
21
See Sec. B.5.

52

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply byounce (avoirdupois) per square inch (oz / in2) ..... kilogram per square meter (kg / m2) .... 4.394 185 E+01
ounce (avoirdupois) per square yard(oz / yd2) ..... kilogram per square meter (kg / m2) .... 3.390 575 E−02
parsec (pc) ........................................................... meter (m) ............................................. 3.085 678 E+16
peck (U.S.) (pk) ................................................... cubic meter (m3) ................................... 8.809 768 E−03
peck (U.S.) (pk) ................................................... liter (L) ................................................. 8.809 768 E+00
pennyweight (dwt) ............................................... kilogram (kg) ....................................... 1.555 174 E−03
pennyweight (dwt) ............................................... gram (g) ............................................... 1.555 174 E+00
perm (0 ºC) .......................................................... kilogram per pascal second square meter[kg / (Pa·s·m2)] .................................... 5.721 35 E−11
perm (23 ºC) ........................................................ kilogram per pascal second square meter[kg / (Pa·s·m2)] .................................... 5.745 25 E−11
perm inch (0 ºC) .................................................. kilogram per pascal second meter[kg / (Pa·s·m)] ...................................... 1.453 22 E−12
perm inch (23 ºC) ................................................ kilogram per pascal second meter[kg / (Pa·s·m)] ...................................... 1.459 29 E−12
phot (ph) .............................................................. lux (lx) ............................................................ 1.0 E+04
pica (computer) (1/6 in) ....................................... meter (m) ............................................. 4.233 333 E−03
pica (computer) (1/6 in) ....................................... millimeter (mm) ................................... 4.233 333 E+00
pica (printer’s) ..................................................... meter (m) ............................................. 4.217 518 E−03
pica (printer’s) ..................................................... millimeter (mm) .................................... 4.217 518 E+00
pint (U.S. dry) (dry pt).......................................... cubic meter (m3) ................................... 5.506 105 E−04
pint (U.S. dry) (dry pt) ......................................... liter (L) ................................................. 5.506 105 E−01
pint (U.S. liquid) (liq pt) ...................................... cubic meter (m3) ................................... 4.731 765 E−04
pint (U.S. liquid) (liq pt) ...................................... liter (L) ............................................... . 4.731 765 E−01
point (computer) (1/72 in) ................................... meter (m) ............................................. 3.527 778 E−04
point (computer) (1/72 in) ................................... millimeter (mm) ................................... 3.527 778 E−01
point (printer’s) .................................................... meter (m) ............................................ . 3.514 598 E−04
point (printer’s) .................................................... millimeter (mm) ................................... 3.514 598 E−01
poise (P) ............................................................... pascal second (Pa·s) ........................................ 1.0 E−01
pound (avoirdupois) (lb)22 ................................... kilogram (kg) ....................................... 4.535 924 E−01
pound (troy or apothecary) (lb) ........................... kilogram (kg) ....................................... 3.732 417 E−01
poundal ................................................................ newton (N) ........................................... 1.382 550 E−01
poundal per square foot ....................................... pascal (Pa) ............................................ 1.488 164 E+00
poundal second per square foot ........................... pascal second (Pa·s) ............................. 1.488 164 E+00
pound foot squared (lb · ft2) ................................. kilogram meter squared (kg ·m2) .......... 4.214 011 E−02
pound-force (lbf)23 ............................................... newton (N) ........................................... 4.448 222 E+00
pound-force foot (lbf · ft) ..................................... newton meter (N · m) ........................... 1.355 818 E+00
pound-force foot per inch (lbf · ft / in) ................. newton meter per meter (N · m / m) ..... 5.337 866 E+01
pound-force inch (lbf · in) ................................... newton meter (N · m) .......................... 1.129 848 E−01
pound-force inch per inch (lbf · in / in) ............... newton meter per meter (N · m / m) ..... 4.448 222 E+00
pound-force per foot (lbf / ft) .............................. newton per meter (N / m) .................... . 1.459 390 E+01
pound-force per inch (lbf/ in) .............................. newton per meter (N / m) ..................... 1.751 268 E+02
pound-force per pound(lbf/lb) (thrust to mass ratio) ........................ newton per kilogram (N / kg) ................. 9.806 65 E+00
pound-force per square foot (lbf/ft2) .................... pascal (Pa) ............................................ 4.788 026 E+01
pound-force per square inch (psi) (lbf/in2) .......... pascal (Pa) ............................................ 6.894 757 E+03
pound-force per square inch (psi) (lbf/in2) .......... kilopascal (kPa) .................................... 6.894 757 E+00
22
The exact conversion factor is 4.535 923 7 E−01. All units in Secs. B.8 and B.9 that contain the pound refer to the avoirdupois
pound.
23
The exact conversion factor is 4.448 221 615 260 5 E+00 since the standard value of the acceleration due to gravity,
gn = 9.806 65 m/s2 exactly, is used to define the kilogram-force: 1 kgf = 9.806 65 E+00 N exactly.

53

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply bypound-force second per square foot(lbf · s/ft2) ..................................................... pascal second (Pa · s) ........................... 4.788 026
pound-force second per square inch
(lbf · s/in2) ..................................................... pascal second (Pa · s) ........................... 6.894 757
pound inch squared (lb · in2) ................................ kilogram meter squared (kg·m2) ........... 2.926 397
pound per cubic foot (lb / ft3) .............................. kilogram per cubic meter (kg / m3) ...... 1.601 846
pound per cubic inch (lb / in3) ............................. kilogram per cubic meter (kg / m3) ...... 2.767 990
pound per cubic yard (lb /yd3) ............................ kilogram per cubic meter (kg / m3) ...... 5.932 764
pound per foot (lb / ft) ......................................... kilogram per meter (kg / m) ................. 1.488 164
pound per foot hour [lb/(ft · h)] ........................... pascal second (Pa · s) ............................ 4.133 789
pound per foot second [lb / (ft · s)] ...................... pascal second (Pa · s) ............................ 1.488 164
pound per gallon [Canadian and
U.K. (Imperial)] (lb / gal) ............................. kilogram per cubic meter (kg / m3) ...... 9.977 637
pound per gallon [Canadian and
U.K. (Imperial)] (lb / gal) ............................. kilogram per liter (kg / L) .................... 9.977 637
pound per gallon (U.S.) (lb / gal) ......................... kilogram per cubic meter (kg / m3) ...... 1.198 264
pound per gallon (U.S.) (lb / gal) ......................... kilogram per liter (kg / L) .................... 1.198 264
pound per horsepower hour [lb / (hp · h)] ............ kilogram per joule (kg / J) .................... 1.689 659
pound per hour (lb / h) ......................................... kilogram per second (kg / s) ................. 1.259 979
pound per inch (lb / in) ........................................ kilogram per meter (kg / m) ................. 1.785 797
pound per minute (lb / min) ................................. kilogram per second (kg / s) ................. 7.559 873
pound per second (lb / s) ..................................... kilogram per second (kg / s) ................. 4.535 924
pound per square foot (lb / ft2) ............................. kilogram per square meter (kg / m2) .... 4.882 428
pound per square inch (not pound-force)(lb / in2) ........................................................ kilogram per square meter (kg / m2) .... 7.030 696
pound per yard (lb / yd) ....................................... kilogram per meter (kg / m) ................. 4.960 546
psi (pound-force per square inch) (lbf / in2) ........ pascal (Pa) ............................................ 6.894 757
psi (pound-force per square inch) (lbf / in2) ........ kilopascal (kPa) .................................... 6.894 757
quad (1015 BtuIT)9 ................................................. joule (J) ................................................ 1.055 056
quart (U.S. dry) (dry qt) ....................................... cubic meter (m3) ................................... 1.101 221
quart (U.S. dry) (dry qt) ....................................... liter (L) ................................................. 1.101 221
quart (U.S. liquid) (liq qt) .................................... cubic meter (m3) ................................... 9.463 529
quart (U.S. liquid) (liq qt) .................................... liter (L) ................................................. 9.463 529

E+01
E+03
E−04
E+01
E+04
E−01
E+00
E−04
E+00
E+01
E−02
E+02
E−01
E−07
E−04
E+01
E−03
E−01
E+00
E+02
E−01
E+03
E+00
E+18
E−03
E+00
E−04
E−01

rad (absorbed dose) (rad) .................................... gray (Gy) ......................................................... 1.0 E−02
rem (rem) ............................................................. sievert (Sv) ...................................................... 1.0 E−02
revolution (r) ........................................................ radian (rad) ........................................... 6.283 185 E+00
revolution per minute (rpm) (r / min) ................... radian per second (rad / s) .................... 1.047 198 E−01
rhe ....................................................................... reciprocal pascal second (Pa·s)!1 .....................1.0 E+01
rod (based on U.S. survey foot) (rd)7.................... meter (m) ............................................. 5.029 210 E+00
roentgen (R) ........................................................ coulomb per kilogram (C / kg)....................... 2.58 E−04
rpm (revolution per minute) (r / min) .................. radian per second (rad / s) .................... 1.047 198 E−01
second (angle) (") ................................................ radian (rad) ........................................... 4.848 137 E−06
second (sidereal) .................................................. second (s) ............................................. 9.972 696 E−01
shake..................................................................... second (s) ........................................................ 1.0 E−08
shake..................................................................... nanosecond (ns) .............................................. 1.0 E+01
slug (slug) ............................................................. kilogram (kg) ....................................... 1.459 390 E+01
slug per cubic foot (slug / ft3) .............................. kilogram per cubic meter (kg / m3) ...... 5.153 788 E+02
slug per foot second [slug / (ft · s)]....................... pascal second (Pa·s) ............................. 4.788 026 E+01
square foot (ft2) .................................................... square meter (m2) ................................. 9.290 304 E−02
square foot per hour (ft2 / h) ................................ square meter per second (m2 / s) ............ 2.580 64 E−05

54

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply bysquare foot per second (ft2 / s) ............................. square meter per second (m2 / s) .......... 9.290 304 E−02
square inch (in2) ................................................... square meter (m2) ....................................... 6.4516 E−04
square inch (in2) ................................................... square centimeter (cm2) ............................ 6.4516 E+00
square mile (mi2) ................................................. square meter (m2) ................................. 2.589 988 E+06
square mile (mi2) ................................................. square kilometer (km2) ......................... 2.589 988 E+00
square mile(based on U.S. survey foot) (mi2)7................. square meter (m2) ................................. 2.589 998 E+06
square mile(based on U.S. survey foot) (mi2)7................. square kilometer (km2) ......................... 2.589 998 E+00
square yard (yd2) .................................................. square meter (m2) ................................. 8.361 274 E−01
statampere ............................................................ ampere (A) ........................................... 3.335 641 E−10
statcoulomb ......................................................... coulomb (C) ......................................... 3.335 641 E−10
statfarad ............................................................... farad (F) ............................................... 1.112 650 E−12
stathenry .............................................................. henry (H) .............................................. 8.987 552 E+11
statmho ................................................................ siemens (S) ........................................... 1.112 650 E−12
statohm ................................................................. ohm (Ω) ................................................ 8.987 552 E+11
statvolt ................................................................. volt (V) ................................................. 2.997 925 E+02
stere (st) ............................................................... cubic meter (m3) .............................................. 1.0 E+00
stilb (sb)................................................................ candela per square meter (cd / m2) .................. 1.0 E+04
stokes (St) ............................................................ meter squared per second (m2 / s) ................... 1.0 E−04
tablespoon............................................................. cubic meter (m3) .................................... 1.478 676 E−05
tablespoon............................................................. milliliter (mL) ...................................... 1.478 676 E+01
teaspoon ............................................................... cubic meter (m3) ................................... 4.928 922 E−06
teaspoon ............................................................... milliliter (mL) ...................................... 4.928 922 E+00
tex ........................................................................ kilogram per meter (kg / m) ............................ 1.0 E−06
therm (EC) 24 ........................................................ joule (J) .................................................. 1.055 06 E+08
therm (U.S.)24 ...................................................... joule (J) ................................................ 1.054 804 E+08
ton, assay (AT) .................................................... kilogram (kg) ....................................... 2.916 667 E−02
ton, assay (AT) .................................................... gram (g) ............................................... 2.916 667 E+01
ton-force (2000 lbf) ............................................. newton (N) ........................................... 8.896 443 E+03
ton-force (2000 lbf) ............................................. kilonewton (kN) ................................... 8.896 443 E+00
ton, long (2240 lb) ............................................... kilogram (kg) ....................................... 1.016 047 E+03
ton, long, per cubic yard ...................................... kilogram per cubic meter (kg / m3) ...... 1.328 939 E+03
ton, metric (t)........................................................ kilogram (kg) .................................................. 1.0 E+03
tonne (called “metric ton” in U.S.) (t) ................. kilogram (kg) .................................................. 1.0 E+03
ton of refrigeration (12 000 BtuIT / h) .................. watt (W) ............................................... 3.516 853 E+03
ton of TNT (energy equivalent) 25 ........................ joule (J) ....................................................... 4.184 E+09
ton, register .......................................................... cubic meter (m3) ................................... 2.831 685 E+00
ton, short (2000 lb) .............................................. kilogram (kg) ....................................... 9.071 847 E+02
ton, short, per cubic yard ..................................... kilogram per cubic meter (kg / m3) ...... 1.186 553 E+03
ton, short, per hour ............................................... kilogram per second (kg / s) ................. 2.519 958 E−01
torr (Torr) ............................................................ pascal (Pa) ............................................ 1.333 224 E+02
unit pole................................................................ weber (Wb) .......................................... 1.256 637 E−07
watt hour (W · h) ................................................. joule (J) ........................................................... 3.6 E+03
watt per square centimeter (W / cm2) .................. watt per square meter (W / m2) ........................ 1.0 E+04
24

The therm (EC) is legally defined in the Council Directive of 20 December 1979, Council of the European Communities (now theEuropean Union, EU). The therm (U.S.) is legally defined in the Federal Register of July 27, 1968. Although the therm (EC), which isbased on the International Table Btu, is frequently used by engineers in the United States, the therm (U.S.) is the legal unit used by the
U.S. natural gas industry.
25
Defined (not measured) value.

55

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply bywatt per square inch (W / in2) .............................. watt per square meter (W / m2) ............ 1.550 003 E+03
watt second (W · s) .............................................. joule (J) ........................................................... 1.0 E+00
yard (yd) .............................................................. meter (m) .................................................... 9.144 E−01
year (365 days) .................................................... second (s) .................................................. 3.1536 E+07
year (sidereal) ...................................................... second (s) ............................................. 3.155 815 E+07
year (tropical) ...................................................... second (s) ............................................. 3.155 693 E+07

56

Guide for the Use of the International System of Units (SI)B.9 Factors for units listed by kind of quantity or field of science
Caution: The units listed in column 1 are in general not to be used in NIST publications, with the exceptionof those few in italic type.

Factors in boldface are exact
To convert fromtoMultiply by

ACCELERATION

acceleration of free fall, standard (gn) .................. meter per second squared (m / s2) .......... 9.806 65
foot per second squared (ft / s2) ........................... meter per second squared (m / s2) ............... 3.048
gal (Gal)................................................................ meter per second squared (m / s2) ................... 1.0
inch per second squared (in / s2) .......................... meter per second squared (m / s2) ................. 2.54

E+00
E−01
E−02
E−02

ANGLE
degree ( º ) ............................................................ radian (rad) ........................................... 1.745 329 E−02
gon (also called grade) (gon) ............................... radian (rad) ........................................... 1.570 796 E−02
gon (also called grade) (gon) ............................... degree ( º )........................................................ 9.0 E−01
mil ....................................................................... radian (rad) ........................................... 9.817 477 E−04
mil ....................................................................... degree ( º ) ................................................... 5.625 E−02
minute (')............................................................... radian (rad) ........................................... 2.908 882 E−04
revolution (r) ........................................................ radian (rad) .......................................... 6.283 185 E+00
second (") ............................................................. radian (rad) ........................................... 4.848 137 E−06

AREA AND SECOND MOMENT OF AREA

acre (based on U.S. survey foot)........................... square meter (m2) ................................. 4.046 873 E+03
are (a) ................................................................... square meter (m2) ............................................ 1.0 E+02
barn (b) ................................................................. square meter (m2) ............................................ 1.0 E−28
circular mil ........................................................... square meter (m2) ................................. 5.067 075 E−10
circular mil ........................................................... square millimeter (mm2) ....................... 5.067 075 E−04
foot to the fourth power (ft4)16 ............................. meter to the fourth power (m4) ............. 8.630 975 E−03
hectare (ha) .......................................................... square meter (m2) ............................................ 1.0 E+04
inch to the fourth power (in4)16 ............................ meter to the fourth power (m4) ............. 4.162 314 E−07
square foot (ft2) .................................................... square meter (m2) ................................. 9.290 304 E−02
square inch (in2) ................................................... square meter (m2) ...................................... 6.4516 E−04
square inch (in2) ................................................... square centimeter (cm2) ............................ 6.4516 E+00
square mile (mi2) ................................................. square meter (m2) ................................. 2.589 988 E+06
square mile (mi2) ................................................. square kilometer (km2) ......................... 2.589 988 E+00
square mile(based on U.S. survey foot) (mi2)7 ................ square meter (m2) ................................. 2.589 998 E+06
square mile(based on U.S. survey foot) (mi2)7 ................ square kilometer (km2) ......................... 2.589 998 E+00
square yard (yd2) .................................................. square meter (m2) ................................. 8.361 274 E−01

CAPACITY (see VOLUME)
DENSITY (that is, MASS DENSITY— see MASS DIVIDED BY VOLUME)
ELECTRICITY and MAGNETISM
abampere ............................................................. ampere (A) ....................................................... 1.0
abcoulomb ........................................................... coulomb (C) .................................................... 1.0
abfarad ................................................................. farad (F) .......................................................... 1.0
abhenry ................................................................ henry (H) ......................................................... 1.0
abmho .................................................................. siemens (S) ...................................................... 1.0

57

E+01
E+01
E+09
E−09
E+09

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply byabohm .................................................................. ohm (Ω) ........................................................... 1.0 E−09
abvolt ................................................................... volt (V)............................................................. 1.0 E−08
ampere hour (A · h) ............................................. coulomb (C) .................................................... 3.6 E+03
biot (Bi) ............................................................... ampere (A) ...................................................... 1.0 E+01
EMU of capacitance (abfarad) ............................. farad (F) ...........................................................1.0 E+09
EMU of current (abampere) ................................ ampere (A) ...................................................... 1.0 E+01
EMU of electric potential (abvolt) ....................... volt (V) ............................................................ 1.0 E−08
EMU of inductance (abhenry) ............................. henry (H ........................................................... 1.0 E−09
EMU of resistance (abohm).................................. ohm (Ω) ........................................................... 1.0 E−09
ESU of capacitance (statfarad) ............................ farad (F) ............................................... 1.112 650 E−12
ESU of current (statampere) ................................ ampere (A) ........................................... 3.335 641 E−10
ESU of electric potential (statvolt) ...................... volt (V) ................................................. 2.997 925 E+02
ESU of inductance (stathenry).............................. henry (H) .............................................. 8.987 552 E+11
ESU of resistance (statohm) ................................. ohm (Ω) ............................................... 8.987 552 E+11
faraday (based on carbon 12) ............................... coulomb (C) ......................................... 9.648 534 E+04
franklin (Fr) ......................................................... coulomb (C) ......................................... 3.335 641 E−10
gamma (γ) ............................................................ tesla (T) ........................................................... 1.0 E−09
gauss (Gs, G) ........................................................ tesla (T) ........................................................... 1.0 E−04
gilbert (Gi) ........................................................... ampere (A) ........................................... 7.957 747 E−01
maxwell (Mx) ...................................................... weber (Wb) ..................................................... 1.0 E−08
mho ................................................................... siemens (S) ...................................................... 1.0 E+00
oersted (Oe) ......................................................... ampere per meter (A / m) ..................... 7.957 747 E+01
ohm centimeter (Ω · cm) ...................................... ohm meter (Ω · m) ........................................... 1.0 E−02
ohm circular-mil per foot ..................................... ohm meter (Ω · m) ................................ 1.662 426 E−09
ohm circular-mil per foot ..................................... ohm square millimeter per meter(Ω · mm2 / m) ........................................ 1.662 426 E−03
statampere ............................................................ ampere (A) ............................................ 3.335 641 E−10
statcoulomb ......................................................... coulomb (C) .......................................... 3.335 641 E−10
statfarad ............................................................... farad (F) ................................................ 1.112 650 E−12
stathenry .............................................................. henry (H)............................................... 8.987 552 E+11
statmho ................................................................ siemens (S)............................................ 1.112 650 E−12
statohm ................................................................ ohm (Ω)................................................. 8.987 552 E+11
statvolt ................................................................. volt (V).................................................. 2.997 925 E+02
unit pole ............................................................... weber (Wb) ........................................... 1.256 637 E−07

ENERGY (includes WORK)

British thermal unitIT (BtuIT)9 .............................. joule (J) ................................................. 1.055 056 E+03
British thermal unitth (Btuth)9 ............................... joule (J) ................................................. 1.054 350 E+03
British thermal unit (mean) (Btu) ........................ joule (J) ................................................... 1.055 87 E+03
British thermal unit (39 ºF) (Btu) ........................ joule (J) ................................................... 1.059 67 E+03
British thermal unit (59 ºF) (Btu) ........................ joule (J) ................................................... 1.054 80 E+03
British thermal unit (60 ºF) (Btu) ........................ joule (J) ................................................... 1.054 68 E+03
calorieIT (calIT)10 .................................................. joule (J) ...................................................... 4.1868 E+00
calorieth (calth)10 .................................................... joule (J) ........................................................ 4.184 E+00
calorie (mean) (cal) .............................................. joule (J) ................................................... 4.190 02 E+00
calorie (15 ºC) (cal15) ........................................... joule (J) ................................................... 4.185 80 E+00
calorie (20 ºC) (cal20) ........................................... joule (J) ................................................... 4.181 90 E+00
calorieIT, kilogram (nutrition)11 ........................... joule (J) ...................................................... 4.1868 E+03
calorieth, kilogram (nutrition)11 ............................ joule (J) ........................................................ 4.184 E+03
calorie (mean), kilogram (nutrition)11 .................. joule (J) ................................................... 4.190 02 E+03
electronvolt (eV) .................................................. joule (J) ................................................. 1.602 176 E−19
erg (erg) ............................................................... joule (J) ............................................................ 1.0 E−07
foot poundal.......................................................... joule (J) ................................................. 4.214 011 E−02

58

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply by

foot pound-force (ft · lbf) ..................................... joule (J) ................................................. 1.355 818 E+00
kilocalorieIT (kcalIT) ............................................. joule (J) ...................................................... 4.1868 E+03
kilocalorieth (kcalth) .............................................. joule (J) ........................................................ 4.184 E+03
kilocalorie (mean) (kcal) ..................................... joule (J) ................................................... 4.190 02 E+03
kilowatt hour (kW · h) ......................................... joule (J) ............................................................ 3.6 E+06
kilowatt hour (kW · h) ......................................... megajoule (MJ) ................................................ 3.6 E+00
quad (1015 BtuIT)9 ................................................ joule (J) ................................................. 1.055 056 E+18
therm (EC)24 ........................................................ joule (J) ................................................... 1.055 06 E+08
therm (U.S.)24 ...................................................... joule (J) ................................................. 1.054 804 E+08
ton of TNT (energy equivalent)25 ........................ joule (J) ....................................................... 4.184 E+09
watt hour (W · h) ................................................. joule (J) ............................................................ 3.6 E+03
watt second (W · s) .............................................. joule (J) ............................................................ 1.0 E+00

ENERGY DIVIDED BY AREA TIME
erg per square centimeter second[erg / (cm2 · s)] .............................................. watt per square meter (W / m2) ........................ 1.0 E−03
watt per square centimeter (W / cm2) .................. watt per square meter (W / m2) ........................ 1.0 E+04
watt per square inch (W / in2) .............................. watt per square meter (W / m2) ............. 1.550 003 E+03

FLOW (see MASS DIVIDED BY TIME or VOLUME DIVIDED BY TIME)
FORCE
dyne (dyn) ............................................................ newton (N) ....................................................... 1.0 E−05
kilogram-force (kgf) ............................................ newton (N) .............................................. 9.806 65 E+00
kilopond (kilogram-force) (kp) ............................ newton (N) ............................................. 9.806 65 E+00
kip (1 kip = 1000 lbf) .......................................... newton (N) ........................................... 4.448 222 E+03
kip (1 kip = 1000 lbf) .......................................... kilonewton (kN).................................... 4.448 222 E+00
ounce (avoirdupois)-force (ozf) ........................... newton (N) ........................................... 2.780 139 E−01
poundal ................................................................ newton (N) ........................................... 1.382 550 E−01
pound-force (lbf)23 ............................................... newton (N) ........................................... 4.448 222 E+00
pound-force per pound(lbf / lb) (thrust to mass ratio) ...................... newton per kilogram (N / kg) ................. 9.806 65 E+00
ton-force (2000 lbf) ............................................. newton (N) ........................................... 8.896 443 E+03
ton-force (2000 lbf) ............................................. kilonewton (kN) ................................... 8.896 443 E+00

FORCE DIVIDED BY AREA (see PRESSURE)FORCE DIVIDED BY LENGTH
pound-force per foot (lbf / ft) .............................. newton per meter (N / m) ..................... 1.459 390 E+01
pound-force per inch (lbf / in) ............................. newton per meter (N / m) ..................... 1.751 268 E+02

HEAT
Available Energy
British thermal unitIT per cubic foot (BtuIT / ft3) .. joule per cubic meter (J / m3) ............... 3.725 895 E+04
British thermal unitth per cubic foot (Btuth / ft3) ... joule per cubic meter (J / m3) ............... 3.723 403 E+04
British thermal unitIT per pound (BtuIT / lb) ........ joule per kilogram (J / kg) ........................... 2.326 E+03
British thermal unitth per pound (Btuth / lb) ......... joule per kilogram (J / kg) .................... 2.324 444 E+03
calorieIT per gram (calIT / g) ................................. joule per kilogram (J / kg) ......................... 4.1868 E+03
calorieth per gram (calth / g) ................................. joule per kilogram (J / kg) ........................... 4.184 E+03

59

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply by

Coefficient of Heat Transfer
British thermal unitIT per hour square foot degree Fahrenheit[BtuIT / (h · ft2 · ºF)] ...................................... watt per square meter kelvin[W / (m2 · K)] ....................................... 5.678 263
British thermal unitth per hour square foot degree Fahrenheit[Btuth / (h · ft2 · ºF)] ...................................... watt per square meter kelvin[W / (m2 · K)] ....................................... 5.674 466
British thermal unitIT per second square foot degree Fahrenheit[BtuIT / (s · ft2 · ºF)] ...................................... watt per square meter kelvin[W / (m2 · K)] ....................................... 2.044 175
British thermal unitth per second square foot degree Fahrenheit[Btuth / (s · ft2 · ºF)] ....................................... watt per square meter Kelvin[W / (m2 · K)] ....................................... 2.042 808

E+00
E+00
E+04
E+04

Density of Heat
British thermal unitIT per square foot(BtuIT / ft2) .................................................... joule per square meter (J / m2) ............. 1.135 653 E+04
British thermal unitth per square foot(Btuth / ft2) .................................................... joule per square meter (J / m2) .............. 1.134 893 E+04
calorieth per square centimeter (calth / cm2) ......... joule per square meter (J / m2) ..................... 4.184 E+04
langley (calth / cm2) .............................................. joule per square meter (J / m2) ..................... 4.184 E+04
Density of Heat Flow Rate
British thermal unitIT per square foot hour
[BtuIT / (ft2 · h)] ........................................... watt per square meter (W / m2) ............. 3.154 591 E+00
British thermal unitth per square foot hour
[Btuth / (ft2 · h)] ............................................ watt per square meter (W / m2) ............. 3.152 481 E+00
British thermal unitth per square foot minute[Btuth / (ft2 · min)] ......................................... watt per square meter (W / m2) ............. 1.891 489 E+02
British thermal unitIT per square foot second[BtuIT / (ft2 · s)] ............................................. watt per square meter (W / m2) ............. 1.135 653 E+04
British thermal unitth per square foot second[Btuth / (ft2 · s)] ............................................ watt per square meter (W / m2) ............ 1.134 893 E+04
British thermal unitth per square inch second[Btuth / (in2 · s)] ............................................. watt per square meter (W / m2) ............ 1.634 246 E+06
calorieth per square centimeter minute[calth / (cm2 · min)] ....................................... watt per square meter (W / m2) ............ 6.973 333 E+02
calorieth per square centimeter second[calth / (cm2 · s)] ............................................ watt per square meter (W / m2) ................... 4.184 E+04
Fuel Consumption
gallon (U.S.) per horsepower hour[gal / (hp · h)] ................................................ cubic meter per joule (m3 / J) ............... 1.410 089 E−09
gallon (U.S.) per horsepower hour[gal / (hp · h)] ................................................ liter per joule (L / J) ............................. 1.410 089 E−06
mile per gallon (U.S.) (mpg) (mi / gal) ................ meter per cubic meter (m / m3) ............ 4.251 437 E+05
mile per gallon (U.S.) (mpg) (mi / gal) ................ kilometer per liter (km / L) .................. 4.251 437 E−01
mile per gallon (U.S.) (mpg) (mi / gal)21 ............. liter per 100 kilometer (L / 100 km) .... divide 235.215 bynumber of miles
per gallon
pound per horsepower hour [lb / (hp · h)] ............ kilogram per joule (kg / J) .................... 1.689 659 E−07

60

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply by

Heat Capacity and Entropy
British thermal unitIT per degree Fahrenheit(BtuIT / ºF) ..................................................... joule per kelvin (J / K) ......................... 1.899 101
British thermal unitth per degree Fahrenheit(Btuth / ºF) ..................................................... joule per kelvin (J / K) ......................... 1.897 830
British thermal unitIT per degree Rankine(BtuIT / ºR) .................................................... joule per kelvin (J / K) ......................... 1.899 101
British thermal unitth per degree Rankine(Btuth / ºR) .................................................... joule per kelvin (J / K) ......................... 1.897 830

E+03
E+03
E+03
E+03

Heat Flow Rate
British thermal unitIT per hour (BtuIT / h) ............ watt (W) ............................................... 2.930 711 E−01
British thermal unitth per hour (Btuth / h) .............. watt (W) ............................................... 2.928 751 E−01
British thermal unitth per minute (Btuth / min) ..... watt (W) ............................................... 1.757 250 E+01
British thermal unitIT per second (BtuIT / s).......... watt (W) ............................................... 1.055 056 E+03
British thermal unitth per second (Btuth / s)........... watt (W) ............................................... 1.054 350 E+03
calorieth per minute (calth / min) .......................... watt (W) ............................................... 6.973 333 E−02
calorieth per second (calth / s) ............................... watt (W) ...................................................... 4.184 E+00
kilocalorieth per minute (kcalth / min) .................. watt (W) ............................................... 6.973 333 E+01
kilocalorieth per second (kcalth / s) ....................... watt (W) ...................................................... 4.184 E+03
ton of refrigeration (12 000 BtuIT / h) .................. watt (W) ............................................... 3.516 853 E+03
Specific Heat Capacity and Specific Entropy
British thermal unitIT per pound degree Fahrenheit[BtuIT / (lb · ºF)] ............................................ joule per kilogram kelvin [J / (kg · K)] ..... 4.1868
British thermal unitth per pound degree Fahrenheit[Btuth / (lb · ºF)] ............................................ joule per kilogram kelvin [J / (kg · K)] ....... 4.184
British thermal unitIT per pound degree Rankine[BtuIT / (lb · ºR)] ........................................... joule per kilogram kelvin [J / (kg · K)] ..... 4.1868
British thermal unitth per pound degree Rankine[Btuth / (lb · ºR)] ............................................ joule per kilogram kelvin [J / (kg · K)] ....... 4.184
calorieIT per gram degree Celsius[calIT / (g · ºC)] .............................................. joule per kilogram kelvin [J / (kg · K)] ..... 4.1868
calorieth per gram degree Celsius[calth / (g · ºC)] .............................................. joule per kilogram kelvin [J / (kg · K)] ....... 4.184
calorieIT per gram kelvin [calIT / (g · K)] ............. joule per kilogram kelvin [J / (kg · K)] ..... 4.1868
calorieth per gram kelvin [calth / (g · K)] .............. joule per kilogram kelvin [J / (kg · K)] ....... 4.184

E+03
E+03
E+03
E+03
E+03
E+03
E+03
E+03

Thermal Conductivity
British thermal unitIT foot per hour square foot degree Fahrenheit[BtuIT · ft / (h · ft2 · ºF)] ................................. watt per meter kelvin [W / (m · K)] ..... 1.730 735 E+00
British thermal unitth foot per hour square foot degree Fahrenheit[Btuth · ft / (h · ft2 · ºF)] .................................. watt per meter kelvin [W / (m · K)] ..... 1.729 577 E+00
British thermal unitIT inch per hour square foot degree Fahrenheit[BtuIT · in / (h · ft2 · ºF)] ................................ watt per meter kelvin [W / (m · K)] ..... 1.442 279 E−01
British thermal unitth inch per hour square foot degree Fahrenheit[Btuth · in / (h · ft2 · ºF)] ................................ watt per meter kelvin [W / (m · K)] ..... 1.441 314 E−01
British thermal unitIT inch per second square foot degree Fahrenheit[BtuIT · in / (s · ft2 · ºF)] ................................ watt per meter kelvin [W / (m · K)] ..... 5.192 204 E+02
British thermal unitth inch per second square foot degree Fahrenheit[Btuth · in / (s · ft2 · ºF)] ................................. watt per meter kelvin [W / (m · K)] ..... 5.188 732 E+02
calorieth per centimeter second degree Celsius[calth / (cm · s · ºC)] ...................................... watt per meter kelvin [W / (m · K)] ............ 4.184 E+02

61

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply by

Thermal Diffusivity
square foot per hour (ft2 / h) ................................ square meter per second (m2 / s) ............ 2.580 64 E−05
Thermal Insulanceclo ....................................................................... square meter kelvin per watt (m2 · K / W) .... 1.55 E−01
degree Fahrenheit hour square foot per British
thermal unitIT (ºF · h · ft2 / BtuIT) ........................ square meter kelvin per watt
(m2 · K / W) ........................................ . 1.761 102 E−01
degree Fahrenheit hour square foot per British
thermal unitth (ºF · h · ft2 / Btuth) .................. square meter kelvin per watt
(m2 · K / W) .......................................... 1.762 280 E−01
Thermal Resistancedegree Fahrenheit hour per British thermal unitIT(ºF · h / BtuIT) ............................................... kelvin per watt (K / W) ........................ 1.895 634
degree Fahrenheit hour per British thermal unitth(ºF · h / Btuth) ................................................ kelvin per watt (K / W) ........................ 1.896 903
degree Fahrenheit second per British thermal unitIT(ºF · s / BtuIT) ................................................ kelvin per watt (K / W) ........................ 5.265 651
degree Fahrenheit second per British thermal unitth(ºF · s / Btuth) ................................................ kelvin per watt (K / W) ........................ 5.269 175

E+00
E+00
E−04
E−04

Thermal Resistivity
degree Fahrenheit hour square foot per British thermal unitIT inch[ºF · h · ft2 / (BtuIT · in)] ................................ meter kelvin per watt (m · K / W) ........ 6.933 472 E+00
degree Fahrenheit hour square foot per British thermal unitth inch[ºF · h · ft2 / (Btuth · in)] ................................ meter kelvin per watt (m · K / W) ........ 6.938 112 E+04

LENGTHångström (Å) ........................................................ meter (m) ........................................................ 1.0 E−10ångström (Å) ........................................................ nanometer (nm) ............................................... 1.0 E−01
astronomical unit (ua) ......................................... meter (m) ............................................. 1.495 979 E+11
chain (based on U.S. survey foot) (ch)7 ............... meter (m) ............................................. 2.011 684 E+01
fathom (based on U.S. survey foot)7 .................... meter (m) ............................................. 1.828 804 E+00
fermi .................................................................... meter (m) ........................................................ 1.0 E−15
fermi .................................................................... femtometer (fm) .............................................. 1.0 E+00
foot (ft) ................................................................ meter (m) .................................................... 3.048 E−01
foot (U.S. survey) (ft)7 ......................................... meter (m) ............................................. 3.048 006 E−01
inch (in) ............................................................... meter (m) ...................................................... 2.54 E−02
inch (in) ............................................................... centimeter (cm) ............................................. 2.54 E+00
kayser (K) ............................................................ reciprocal meter (m!1) ........................................ 1 E+02
light year (l.y.)18 .................................................. meter (m) ............................................... 9.460 73 E+15
microinch ............................................................. meter (m) ...................................................... 2.54 E−08
microinch ............................................................. micrometer (μm) ........................................... 2.54 E−02
micron (μ) ............................................................ meter (m) ........................................................ 1.0 E−06
micron (μ) ............................................................ micrometer (μm) ............................................. 1.0 E+00
mil (0.001 in) ....................................................... meter (m) ...................................................... 2.54 E−05
mil (0.001 in) ....................................................... millimeter (mm) ............................................ 2.54 E−02
mile (mi) .............................................................. meter (m) ..............................................1.609 344 E+03
mile (mi) .............................................................. kilometer (km) ......................................1.609 344 E+00
mile (based on U.S. survey foot) (mi)7 ................ meter (m) ............................................. 1.609 347 E+03
mile (based on U.S. survey foot) (mi)7 ................ kilometer (km) ..................................... 1.609 347 E+00

62

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply by

mile, nautical20 .................................................... meter (m) .................................................... 1.852 E+03
parsec (pc) ........................................................... meter (m) ............................................. 3.085 678 E+16
pica (computer) (1/6 in) ....................................... meter (m) ............................................. 4.233 333 E−03
pica (computer) (1/6 in) ....................................... millimeter (mm) ................................... 4.233 333 E+00
pica (printer’s) ..................................................... meter (m) ............................................. 4.217 518 E−03
pica (printer’s) ..................................................... millimeter (mm) .................................... 4.217 518 E+00
point (computer) (1/72 in) ................................... meter (m) ............................................. 3.527 778 E−04
point (computer) (1/72 in) ................................... millimeter (mm) ................................... 3.527 778 E−01
point (printer’s) .................................................... meter (m) ............................................. 3.514 598 E−04
point (printer’s) .................................................... millimeter (mm) .................................... 3.514 598 E−01
rod (based on U.S. survey foot) (rd)7.................... meter (m) ............................................ . 5.029 210 E+00
yard (yd) .............................................................. meter (m) .................................................... 9.144 E−01

LIGHT

candela per square inch (cd / in2) ......................... candela per square meter (cd / m2) ....... 1.550 003 E+03
footcandle ............................................................ lux (lx) ................................................. 1.076 391 E+01
footlambert .......................................................... candela per square meter (cd / m2) ....... 3.426 259 E+00
lambert17 .............................................................. candela per square meter (cd / m2) ....... 3.183 099 E+03
lumen per square foot (lm / ft2) ........................... lux (lx) ................................................. 1.076 391 E+01
phot (ph) .............................................................. lux (lx) ............................................................ 1.0 E+04
stilb (sb) ............................................................... candela per square meter (cd / m2) .................. 1.0 E+04

MASS and MOMENT OF INERTIA
carat, metric ......................................................... kilogram (kg) .................................................. 2.0 E−04
carat, metric ......................................................... gram (g) .......................................................... 2.0 E−01
grain (gr) .............................................................. kilogram (kg) ....................................... 6.479 891 E−05
grain (gr) .............................................................. milligram (mg) ..................................... 6.479 891 E+01
hundredweight (long, 112 lb) .............................. kilogram (kg) ...................................... . 5.080 235 E+01
hundredweight (short, 100 lb) .............................. kilogram (kg) ....................................... 4.535 924 E+01
kilogram-force second squared per meter(kgf · s2 / m) .................................................. kilogram (kg) ......................................... 9.806 65 E+00
ounce (avoirdupois) (oz) ...................................... kilogram (kg) ....................................... 2.834 952 E−02
ounce (avoirdupois) (oz) ...................................... gram (g) ............................................... 2.834 952 E+01
ounce (troy or apothecary) (oz) ........................... kilogram (kg) ....................................... 3.110 348 E−02
ounce (troy or apothecary) (oz) ........................... gram (g) ............................................... 3.110 348 E+01
pennyweight (dwt) ............................................... kilogram (kg) ....................................... 1.555 174 E−03
pennyweight (dwt)................................................ gram (g) ............................................... 1.555 174 E+00
pound (avoirdupois) (lb)22 .................................... kilogram (kg) ....................................... 4.535 924 E−01
pound (troy or apothecary) (lb) ........................... kilogram (kg) ....................................... 3.732 417 E−01
pound foot squared (lb · ft2) ................................. kilogram meter squared (kg · m2).......... 4.214 011 E−02
pound inch squared (lb · in2) ................................ kilogram meter squared (kg · m2).......... 2.926 397 E−04
slug (slug) ............................................................ kilogram (kg) ....................................... 1.459 390 E+01
ton, assay (AT) .................................................... kilogram (kg) ........................................ 2.916 667 E−02
ton, assay (AT) ..................................................... gram (g) ............................................... 2.916 667 E+01
ton, long (2240 lb) ............................................... kilogram (kg) ....................................... 1.016 047 E+03
ton, metric (t) ....................................................... kilogram (kg) .................................................. 1.0 E+03
tonne (called “metric ton” in U.S.) (t) ................. kilogram (kg) ................................................... 1.0 E+03
ton, short (2000 lb) .............................................. kilogram (kg) ....................................... 9.071 847 E+02

63

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply by

MASS DENSITY (see MASS DIVIDED BY VOLUME)
MASS DIVIDED BY AREA

ounce (avoirdupois) per square foot (oz / ft2) ...... kilogram per square meter (kg / m2) .... 3.051 517
ounce (avoirdupois) per square inch (oz / in2) ..... kilogram per square meter (kg / m2) .... 4.394 185
ounce (avoirdupois) per square yard (oz / yd2) .... kilogram per square meter (kg / m2) .... 3.390 575
pound per square foot (lb / ft2) ............................. kilogram per square meter (kg / m2) .... 4.882 428
pound per square inch (not pound force) (lb / in2) kilogram per square meter (kg / m2) ..... 7.030 696

E−01
E+01
E−02
E+00
E+02

MASS DIVIDED BY CAPACITY (see MASS DIVIDED BY VOLUME)
MASS DIVIDED BY LENGTH
denier ................................................................... kilogram per meter (kg / m) ................. 1.111 111 E−07
denier ................................................................... gram per meter (g / m) ......................... 1.111 111 E−04
pound per foot (lb / ft) ......................................... kilogram per meter (kg / m) ................. 1.488 164 E+00
pound per inch (lb / in) ........................................ kilogram per meter (kg / m) .................. 1.785 797 E+01
pound per yard (lb / yd) ....................................... kilogram per meter (kg / m) ................. 4.960 546 E−01
tex ........................................................................ kilogram per meter (kg / m) ............................ 1.0 E−06

MASS DIVIDED BY TIME (includes FLOW)
pound per hour (lb / h) ......................................... kilogram per second (kg / s) ................. 1.259 979
pound per minute (lb / min) ................................. kilogram per second (kg / s) ................. 7.559 873
pound per second (lb / s) ..................................... kilogram per second (kg / s) ................. 4.535 924
ton, short, per hour ............................................... kilogram per second (kg / s) ................. 2.519 958

E−04
E−03
E−01
E−01

MASS DIVIDED BY VOLUME (includes MASS DENSITY and MASS
CONCENTRATION)

grain per gallon (U.S.) (gr / gal) .......................... kilogram per cubic meter (kg / m3) ...... 1.711 806 E−02
grain per gallon (U.S.) (gr / gal) . ........................ milligram per liter (mg / L) .................... 1.711 806 E+01
gram per cubic centimeter (g / cm3) .................... kilogram per cubic meter (kg / m3) ................. 1.0 E+03
ounce (avoirdupois) per cubic inch (oz / in3)........ kilogram per cubic meter (kg / m3) . .... 1.729 994 E+03
ounce (avoirdupois) per gallon [Canadian and
U.K. (Imperial)] (oz / gal) ............................ kilogram per cubic meter (kg / m3) ...... 6.236 023 E+00
ounce (avoirdupois) per gallon [Canadian and
U.K. (Imperial)] (oz / gal) . .......................... gram per liter (g / L) ............................. 6.236 023 E+00
ounce (avoirdupois) per gallon (U.S.) (oz / gal) .. kilogram per cubic meter (kg / m3) ...... 7.489 152 E+00
ounce (avoirdupois) per gallon (U.S.) (oz / gal) .. gram per liter (g / L) ............................. 7.489 152 E+00
pound per cubic foot (lb / ft3) .............................. kilogram per cubic meter (kg / m3) ...... 1.601 846 E+01
pound per cubic inch (lb / in3) ............................. kilogram per cubic meter (kg / m3) ....... 2.767 990 E+04
pound per cubic yard (lb / yd3) ............................ kilogram per cubic meter (kg / m3) ....... 5.932 764 E−01
pound per gallon [Canadian and
U.K. (Imperial)] (lb / gal) ............................. kilogram per cubic meter (kg / m3) ...... 9.977 637 E+01
pound per gallon [Canadian and
U.K. (Imperial)] (lb / gal) ............................. kilogram per liter (kg / L) .................... 9.977 637 E−02
pound per gallon (U.S.) (lb / gal).......................... kilogram per cubic meter (kg / m3) ...... 1.198 264 E+02
pound per gallon (U.S.) (lb / gal).......................... kilogram per liter (kg / L) .................... 1.198 264 E−01
slug per cubic foot (slug / ft3) .............................. kilogram per cubic meter (kg / m3) ...... 5.153 788 E+02
ton, long, per cubic yard ...................................... kilogram per cubic meter (kg / m3) ...... 1.328 939 E+03
ton, short, per cubic yard ..................................... kilogram per cubic meter (kg / m3) ...... 1.186 553 E+03

MOMENT OF FORCE or TORQUE
dyne centimeter (dyn · cm) .................................. newton meter (N · m) ....................................... 1.0 E−07

64

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply bykilogram-force meter (kgf · m) ............................ newton meter (N · m) .............................. 9.806 65 E+00
ounce (avoirdupois)-force inch (ozf · in).............. newton meter (N·m) .............................. 7.061 552 E−03
ounce (avoirdupois)-force inch (ozf · in) ............. millinewton meter (mN · m) ................ 7.061 552 E+00
pound-force foot (lbf · ft) ..................................... newton meter (N · m) ............................ 1.355 818 E+00
pound-force inch (lbf · in) ................................... newton meter (N · m) ........................... 1.129 848 E−01

MOMENT OF FORCE or TORQUE, DIVIDED BY LENGTH
pound-force foot per inch (lbf · ft / in) ................. newton meter per meter (N · m / m) ..... 5.337 866 E+01
pound-force inch per inch (lbf · in / in) ............... newton meter per meter (N · m / m) ..... 4.448 222 E+00

PERMEABILITY

darcy14 ................................................................. meter squared (m2) ............................... 9.869 233
perm (0 ºC) .......................................................... kilogram per pascal second square meter[kg / (Pa · s · m2)] ................................... 5.721 35
perm (23 ºC) ........................................................ kilogram per pascal second square meter[kg / (Pa · s · m2)] ................................... 5.745 25
perm inch (0 ºC) ................................................... kilogram per pascal second meter[kg / (Pa · s · m)] ..................................... 1.453 22
perm inch (23 ºC) ................................................ kilogram per pascal second meter[kg / (Pa · s · m)] .................................... 1.459 29

E−13
E−11
E−11
E−12
E−12

POWER
erg per second (erg / s) ......................................... watt (W) .......................................................... 1.0 E−07
foot pound-force per hour (ft · lbf / h) ................. watt (W) ............................................... 3.766 161 E−04
foot pound-force per minute (ft · lbf / min) ......... watt (W) ............................................... 2.259 697 E−02
foot pound-force per second (ft · lbf / s) .............. watt (W) ............................................... 1.355 818 E+00
horsepower (550 ft · lbf / s) ................................. watt (W) ............................................... 7.456 999 E+02
horsepower (boiler) ............................................. watt (W) .................................................. 9.809 50 E+03
horsepower (electric) ........................................... watt (W) ........................................................ 7.46 E+02
horsepower (metric) ............................................. watt (W) ............................................... 7.354 988 E+02
horsepower (U.K.) ............................................... watt (W) ..................................................... 7.4570 E+02
horsepower (water) .............................................. watt (W) ................................................. 7.460 43 E+02

PRESSURE or STRESS (FORCE DIVIDED BY AREA)
atmosphere, standard (atm) ................................. pascal (Pa) .............................................. 1.013 25 E+05
atmosphere, standard (atm) ................................. kilopascal (kPa) ...................................... 1.013 25 E+02
atmosphere, technical (at)8 ................................... pascal (Pa) .............................................. 9.806 65 E+04
atmosphere, technical (at)8 ................................... kilopascal (kPa)....................................... 9.806 65 E+01
bar (bar) ............................................................... pascal (Pa) ....................................................... 1.0 E+05
bar (bar) ............................................................... kilopascal (kPa) ............................................... 1.0 E+02
centimeter of mercury (0 ºC)12 ............................ pascal (Pa) .............................................. 1.333 22 E+03
centimeter of mercury (0 ºC)12 ............................ kilopascal (kPa) ...................................... 1.333 22 E+00
centimeter of mercury, conventional (cmHg)12 ... pascal (Pa) ............................................ 1.333 224 E+03
centimeter of mercury, conventional (cmHg)12 ... kilopascal (kPa) .................................... 1.333 224 E+00
centimeter of water (4 ºC)12 ................................. pascal (Pa) .............................................. 9.806 38 E+01
centimeter of water, conventional (cmH2O) 12 .... pascal (Pa) .............................................. 9.806 65 E+01
dyne per square centimeter (dyn / cm2) ............... pascal (Pa) ....................................................... 1.0 E−01
foot of mercury, conventional (ftHg)12 ................ pascal (Pa) ............................................ 4.063 666 E+04
foot of mercury, conventional (ftHg)12 ................ kilopascal (kPa)...................................... 4.063 666 E+01
foot of water (39.2 ºF)12 ....................................... pascal (Pa) .............................................. 2.988 98 E+03
foot of water (39.2 ºF)12 ....................................... kilopascal (kPa)....................................... 2.988 98 E+00
foot of water, conventional (ftH2O)12 .................. pascal (Pa) ............................................ 2.989 067 E+03
foot of water, conventional (ftH2O)12 .................. kilopascal (kPa)..................................... 2.989 067 E+00
gram-force per square centimeter (gf / cm2) ........ pascal (Pa) .............................................. 9.806 65 E+01

65

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply byinch of mercury (32 ºF)12 ..................................... pascal (Pa) .............................................. 3.386 38 E+03
inch of mercury (32 ºF)12 ..................................... kilopascal (kPa) ....................................... 3.386 38 E+00
inch of mercury (60 ºF)12 ..................................... pascal (Pa) .............................................. 3.376 85 E+03
inch of mercury (60 ºF)12 ..................................... kilopascal (kPa)....................................... 3.376 85 E+00
inch of mercury, conventional (inHg)12 ............... pascal (Pa) ............................................ 3.386 389 E+03
inch of mercury, conventional (inHg)12 ............... kilopascal (kPa)..................................... 3.386 389 E+00
inch of water (39.2 ºF)12 ...................................... pascal (Pa) .............................................. 2.490 82 E+02
inch of water (60 ºF)12 ......................................... pascal (Pa) ................................................. 2.4884 E+02
inch of water, conventional (inH2O)12 ................. pascal (Pa) ............................................ 2.490 889 E+02
kilogram-force per squarecentimeter (kgf / cm2) ................................... pascal (Pa) .............................................. 9.806 65 E+04
kilogram-force per squarecentimeter (kgf / cm2) ................................... kilopascal (kPa)....................................... 9.806 65 E+01
kilogram-force per square meter (kgf / m2) .......... pascal (Pa) .............................................. 9.806 65 E+00
kilogram-force per square
millimeter (kgf / mm2)................................... pascal (Pa) .............................................. 9.806 65 E+06
kilogram-force per square
millimeter (kgf / mm2)................................... megapascal (MPa) .................................. 9.806 65 E+00
kip per square inch (ksi) (kip / in2) ...................... pascal (Pa) ............................................ 6.894 757 E+06
kip per square inch (ksi) (kip / in2) ...................... kilopascal (kPa) .................................... 6.894 757 E+03
millibar (mbar) .................................................... pascal (Pa) ....................................................... 1.0 E+02
millibar (mbar) .................................................... kilopascal (kPa)................................................ 1.0 E−01
millimeter of mercury, conventional (mmHg)12 .. pascal (Pa) ............................................ 1.333 224 E+02
millimeter of water, conventional (mmH2O)12 .... pascal (Pa) .............................................. 9.806 65 E+00
poundal per square foot ....................................... pascal (Pa) ............................................ 1.488 164 E+00
pound-force per square foot (lbf / ft2) .................. pascal (Pa) ............................................ 4.788 026 E+01
pound-force per square inch (psi) (lbf / in2) ........ pascal (Pa) ............................................ 6.894 757 E+03
pound-force per square inch (psi) (lbf / in2) ........ kilopascal (kPa) .................................... 6.894 757 E+00
psi (pound-force per square inch) (lbf / in2) ........ pascal (Pa) ............................................ 6.894 757 E+03
psi (pound-force per square inch) (lbf / in2) ........ kilopascal (kPa) .................................... 6.894 757 E+00
torr (Torr) ............................................................ pascal (Pa) ............................................ 1.333 224 E+02

RADIOLOGY
curie (Ci) ............................................................. becquerel (Bq) ................................................. 3.7
rad (absorbed dose) (rad) .................................... gray (Gy) ......................................................... 1.0
rem (rem) ............................................................. sievert (Sv) ...................................................... 1.0
roentgen (R) ........................................................ coulomb per kilogram (C / kg)....................... 2.58

E+10
E−02
E−02
E−04

SPEED (see VELOCITY)
STRESS (see PRESSURE)TEMPERATURE
degree Celsius (ºC) .............................................. kelvin (K) ..................................... T / K = t / ºC + 273.15
degree centigrade15 .............................................. degree Celsius (ºC) .......................... t / ºC ≈ t / deg. cent.
degree Fahrenheit (ºF) .......................................... degree Celsius (ºC) ................... t / ºC = (t / ºF ! 32) / 1.8
degree Fahrenheit (ºF) .......................................... kelvin (K) ........................... T / K = (t / ºF + 459.67) / 1.8
degree Rankine (ºR) ............................................. kelvin (K) ........................................ T / K = (T / ºR) / 1.8
kelvin (K) ............................................................. degree Celsius (ºC) ....................... t / ºC = T / K ! 273.15

66

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply by

TEMPERATURE INTERVAL
degree Celsius (ºC) .............................................. kelvin (K) ........................................................ 1.0 E+00
degree centigrade15 .............................................. degree Celsius (ºC) ......................................... 1.0 E+00
degree Fahrenheit (ºF) ......................................... degree Celsius (ºC) .............................. 5.555 556 E−01
degree Fahrenheit (ºF) ......................................... kelvin (K).............................................. 5.555 556 E−01
degree Rankine (ºR) ............................................. kelvin (K) ............................................. 5.555 556 E−01

TIME
day (d) ................................................................. second (s) ...................................................... 8.64 E+04
day (sidereal) ....................................................... second (s) ............................................. 8.616 409 E+04
hour (h) ................................................................ second (s) ........................................................ 3.6 E+03
hour (sidereal) ...................................................... second (s) ............................................. 3.590 170 E+03
minute (min) ........................................................ second (s) ........................................................ 6.0 E+01
minute (sidereal) .................................................. second (s) ............................................. 5.983 617 E+01
second (sidereal) .................................................. second (s) ............................................. 9.972 696 E−01
shake .................................................................... second (s) ......................................................... 1.0 E−08
shake .................................................................... nanosecond (ns) .............................................. 1.0 E+01
year (365 days) ..................................................... second (s) ................................................... 3.1536 E+07
year (sidereal) ...................................................... second (s) ............................................. 3.155 815 E+07
year (tropical) ...................................................... second (s) ............................................. . 3.155 693 E+07

TORQUE (see MOMENT OF FORCE)VELOCITY (includes SPEED)
foot per hour (ft / h) ............................................. meter per second (m / s) ....................... 8.466 667 E−05
foot per minute (ft / min) ..................................... meter per second (m / s) ................................ 5.08 E−03
foot per second (ft / s) .......................................... meter per second (m / s) .............................. 3.048 E−01
inch per second (in / s) ......................................... meter per second (m / s) ................................ 2.54 E−02
kilometer per hour (km / h) ................................. meter per second (m / s)........................ 2.777 778 E−01
knot (nautical mile per hour) ............................... meter per second (m / s) ....................... 5.144 444 E−01
mile per hour (mi / h) ........................................... meter per second (m / s) ............................ 4.4704 E−01
mile per hour (mi / h) ........................................... kilometer per hour (km / h) .................. 1.609 344 E+00
mile per minute (mi / min) ................................... meter per second (m / s) ......................... 2.682 24 E+01
mile per second (mi / s) ....................................... meter per second (m / s) ....................... 1.609 344 E+03
revolution per minute (rpm) (r / min) .................. radian per second (rad / s) .................... 1.047 198 E−01
rpm (revolution per minute) (r / min) .................. radian per second (rad / s) .................... 1.047 198 E−01

VISCOSITY, DYNAMIC
centipoise (cP) ...................................................... pascal second (Pa · s) ...................................... 1.0 E−03
poise (P) ............................................................... pascal second (Pa · s) ...................................... 1.0 E−01
poundal second per square foot ........................... pascal second (Pa · s) ........................... 1.488 164 E+00
pound-force second per square foot(lbf · s / ft2) ................................................... pascal second (Pa · s) ........................... 4.788 026 E+01
pound-force second per square inch
(lbf · s / in2) ................................................... pascal second (Pa · s) ........................... 6.894 757 E+03
pound per foot hour [lb / (ft · h)] ......................... pascal second (Pa · s) ........................... 4.133 789 E−04
pound per foot second [lb / (ft · s)] ...................... pascal second (Pa · s) ........................... 1.488 164 E+00
rhe ...................................................................... reciprocal pascal second (Pa · s)!1 ................... 1.0 E+01
slug per foot second [slug / (ft · s)] ...................... pascal second (Pa · s) ........................... 4.788 026 E+01

67

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply by

VISCOSITY, KINEMATIC

centistokes (cSt) .................................................. meter squared per second (m2 / s) ................... 1.0 E−06
square foot per second (ft2 / s) ............................. meter squared per second (m2 / s) ........ 9.290 304 E−02
stokes (St) ............................................................ meter squared per second (m2 / s) ................... 1.0 E−04

VOLUME (includes CAPACITY)

acre-foot (based on U.S. survey foot)7 ................. cubic meter (m3) ................................... 1.233 489 E+03
barrel [for petroleum, 42 gallons (U.S.)](bbl) ..... cubic meter (m3) ................................. . 1.589 873 E−01
barrel [for petroleum, 42 gallons (U.S.)](bbl) ..... liter (L) ................................................. 1.589 873 E+02
bushel (U.S.) (bu) ................................................ cubic meter (m3) ................................... 3.523 907 E−02
bushel (U.S.) (bu) ................................................ liter (L) ................................................. 3.523 907 E+01
cord (128 ft3) ....................................................... cubic meter (m3) ................................... 3.624 556 E+00
cubic foot (ft3) ...................................................... cubic meter (m3) ................................... 2.831 685 E−02
cubic inch (in3 )13 .................................................. cubic meter (m3) ................................... 1.638 706 E−05
cubic mile (mi3) ................................................... cubic meter (m3) ................................... 4.168 182 E+09
cubic yard (yd3) .................................................... cubic meter (m3) ................................... 7.645 549 E−01
cup (U.S.) ............................................................ cubic meter (m3) ................................... 2.365 882 E−04
cup (U.S.) ............................................................ liter (L) ................................................. 2.365 882 E−01
cup (U.S.) ............................................................ milliliter (mL) ...................................... 2.365 882 E+02
fluid ounce (U.S.) (fl oz) ..................................... cubic meter (m3).................................... 2.957 353 E−05
fluid ounce (U.S.) (fl oz) ..................................... milliliter (mL) ...................................... 2.957 353 E+01
gallon [Canadian and U.K. (Imperial)] (gal) ....... cubic meter (m3) ..................................... 4.546 09 E−03
gallon [Canadian and U.K. (Imperial)] (gal) ....... liter (L) ................................................... 4.546 09 E+00
gallon (U.S.) (gal) ................................................ cubic meter (m3) ................................... 3.785 412 E−03
gallon (U.S.) (gal) ................................................ liter (L) ................................................. 3.785 412 E+00
gill [Canadian and U.K. (Imperial)] (gi) .............. cubic meter (m3).................................... 1.420 653 E−04
gill [Canadian and U.K. (Imperial)] (gi) .............. liter (L) ................................................. 1.420 653 E−01
gill (U.S.) (gi) ....................................................... cubic meter (m3)..................................... 1.182 941 E−04
gill (U.S.) (gi) ....................................................... liter (L) ................................................. 1.182 941 E−01
liter (L)19 .............................................................. cubic meter (m3)............................................... 1.0 E−03
ounce [Canadian and U.K. fluid (Imperial)](fl oz) ............................................................ cubic meter (m3).................................... 2.841 306 E−05
ounce [Canadian and U.K. fluid (Imperial)](fl oz) ............................................................ milliliter (mL) ...................................... 2.841 306 E+01
ounce (U.S. fluid) (fl oz) ..................................... cubic meter (m3).................................... 2.957 353 E−05
ounce (U.S. fluid) (fl oz) ..................................... milliliter (mL) ...................................... 2.957 353 E+01
peck (U.S.) (pk) ................................................... cubic meter (m3) ................................... 8.809 768 E−03
peck (U.S.) (pk) ................................................... liter (L) ................................................. 8.809 768 E+00
pint (U.S. dry) (dry pt).......................................... cubic meter (m3) ................................... 5.506 105 E−04
pint (U.S. dry) (dry pt) ......................................... liter (L) ................................................. 5.506 105 E−01
pint (U.S. liquid) (liq pt) ...................................... cubic meter (m3).................................... 4.731 765 E−04
pint (U.S. liquid) (liq pt) ...................................... liter (L) ................................................. 4.731 765 E−01
quart (U.S. dry) (dry qt) ....................................... cubic meter (m3).................................... 1.101 221 E−03
quart (U.S. dry) (dry qt) ....................................... liter (L) ................................................. 1.101 221 E+00
quart (U.S. liquid) (liq qt) .................................... cubic meter (m3).................................... 9.463 529 E−04
quart (U.S. liquid) (liq qt) .................................... liter (L) ................................................. 9.463 529 E−01
stere (st) ................................................................ cubic meter (m3) .............................................. 1.0 E+00
tablespoon............................................................. cubic meter (m3).................................... 1.478 676 E−05
tablespoon............................................................. milliliter (mL) ...................................... 1.478 676 E+01
teaspoon ............................................................... cubic meter (m3) ................................... 4.928 922 E−06
teaspoon ............................................................... milliliter (mL) ..................................... . 4.928 922 E+00
ton, register .......................................................... cubic meter (m3) ................................... 2.831 685 E+00

68

Guide for the Use of the International System of Units (SI)To convert fromtoMultiply by

VOLUME DIVIDED BY TIME (includes FLOW)
cubic foot per minute (ft3 / min) .......................... cubic meter per second (m3 / s) ............ 4.719 474
cubic foot per minute (ft3 / min) .......................... liter per second (L / s) .......................... 4.719 474
cubic foot per second (ft3 / s) ............................... cubic meter per second (m3 / s) ............ 2.831 685
cubic inch per minute (in3 / min) .......................... cubic meter per second (m3 / s) ............ 2.731 177
cubic yard per minute (yd3 / min) ........................ cubic meter per second (m3 / s) ............ 1.274 258
gallon (U.S.) per day (gal / d) .............................. cubic meter per second (m3 / s) ............ 4.381 264
gallon (U.S.) per day (gal / d) .............................. liter per second (L / s) .......................... 4.381 264
gallon (U.S.) per minute (gpm) (gal / min ............ cubic meter per second (m3 / s) ............ 6.309 020
gallon (U.S.) per minute (gpm) (gal / min) .......... liter per second (L / s) .......................... 6.309 020
WORK (see ENERGY)

69

E−04
E−01
E−02
E−07
E−02
E−08
E−05
E−05
E−02

Guide for the Use of the International System of Units (SI)Appendix C. Comments on the References of Appendix D— Bibliography
C.1 Defining document for the SI: BIPM SI Brochure
The defining document for the International System of Units is the Brochure published by theInternational Bureau of Weights and Measures (BIPM) in French, followed by an English translation [1].
This document is revised from time to time in accordance with the decisions of the General Conference on
Weights and Measures (CGPM) and the International Committee for Weights and Measures (CIPM).
C.2 United States version of defining document for the SI: NIST SP 330
The United States edition of the English translation in the BIPM SI Brochure (see Sec. C.1) ispublished by the National Institute of Standards and Technology as NIST Special Publication 330 [2]; itdiffers from the translation in the BIPM publication in the following details:— the spelling of English-language words—for example, “meter,” “liter,” and “deka” are usedinstead of “metre,” “litre,” and “deca”—is in accordance with the United States Government
Printing Office Style Manual [3], which follows Webster’s Third New International Dictionary
rather than the Oxford Dictionary used in many English-speaking countries. This spelling also
reflects recommended United States practice (see Secs. C.1 and C.5);— editorial notes regarding the use of the SI in the United States are added.
Inasmuch as NIST Special Publication 330 reflects the interpretation of the SI for the UnitedStates by the U.S. Secretary of Commerce (see the Preface) while at the same time highly consistent with
Ref. [1] (see Sec. C.1), SP 330 is the authoritative source document on the SI for the purposes of thisGuide.
C.3 ISO and IEC
The International Organization for Standardization (ISO) and the International ElectrotechnicalCommission (IEC) both publish a series of international consensus standards to promote international
uniformity in the practical use of the SI in various fields of science and technology, and in particular tostandardize the symbols for various quantities and the units in which the values of these quantities are
expressed. These standards are in general compatible with Ref. [1] published by the BIPM (see Sec. C.1).
Currently ISO 31 is being revised jointly by technical committees ISO TC12 and IEC TC25. The
revised standards ISO/IEC 80000-1—ISO/IEC 80000-15, will supersede ISO 31-0:1992—ISO 31-13:1992[4], which constitute a series of international consensus standards published by ISO.
IEC 60027-1—IEC 60027-4 [5] constitute a series of international consensus standards publishedby the IEC to promote international uniformity in the practical use of the SI in electrical technology, and inparticular to standardize the symbols for various quantities used in electrotechnology and the units in which
the values of these quantities are expressed. These IEC standards are also compatible with Ref. [1]published by the BIPM (see Sec. C.1), and they are coordinated with the ISO standards [4].
C.4 IEEE/ASTM SI 10
SI 10-2002 “American National Standard for Use of the International System of Units (SI): The
Modern Metric System,” Ref. [6], is the product of a joint effort by Institute of Electrical and Electronics
Engineers (IEEE) and ASTM International (ASTM) to develop a single American National Standard

70

Guide for the Use of the International System of Units (SI)

Institute (ANSI) standard.26 It is based on the International System of Units as interpreted for use in the
United States (see Secs. C.1 and C.2), and has been approved by a consensus of providers and consumers
that includes interests in industrial organizations, government agencies, and scientific associations. SI 10 isrecommended as a comprehensive source of authoritative information for the practical use of the SI in the
United States. (Similar documents have also been developed by other North American technicalorganizations; see Ref. [6], note 1.)
C.5 Federal Register Notices
Important details concerning United States customary units of measurement and the interpretationof the SI for the United States are published from time to time in the Federal Register; these notices havethe status of official United States Government policy.
A Federal Register Notice of July 1, 1959, [7] states the values of conversion factors to be used intechnical and scientific fields to obtain the values of the United States yard and pound from the SI baseunits for length and mass, the meter and the kilogram. These conversion factors were adopted on the basisof an agreement of English-speaking countries to reconcile small differences in the values of the inchpound units as they were used in different parts of the world. This action would have affected the value ofthe yard or foot used for geodetic surveys in the United States. To prevent this from happening, it became
necessary to recognize on a temporary basis a small difference between United States customary units oflength for “international measure” and “survey measure.” A Federal Register Notice of July 19, 1988, [8]announced a tentative decision not to adopt the international foot of 0.3048 meters for surveying andmapping activities in the United States. A final decision to continue the use of the survey foot indefinitely
is pending the completion of an analysis of public comments on the tentative decision; this decision will
also be announced in the Federal Register.
Even if a final decision affirms the continued use of the survey foot in surveying and mapping
services of the United States, it is significant to note that the Office of Charting and Geodetic Services of
the National Ocean Service in the National Oceanic and Atmospheric Administration uses the meter
exclusively for the North American Datum [9]. The North American Datum of 1983, the most recent
definition and adjustment of this information, was announced in a Federal Register Notice of June 14, 1989
[10].
The definitions of the international foot and yard and the corresponding survey units are also
addressed in a Federal Register Notice published on February 3, 1975, [11].
A Federal Register Notice of July 27, 1968, [12] provides a list of the common customarymeasurement units used in commerce throughout the United States, together with the conversion factorsthat link them with the meter and the kilogram.
A Federal Register Notice concerning the SI [13] is a restatement of the interpretation of theInternational System for use in the United States, and it updates the corresponding information published inearlier notices.
A Federal Register Notice of January 2, 1991, [14] removes the voluntary aspect of the conversion
to the SI for Federal agencies and provides policy direction to assist Federal agencies in their transition to
the use of the metric system of measurement.
A Federal Register Notice of July 29, 1991, [15] provides Presidential authority and direction for
the use of the metric system of measurement by Federal departments and agencies in their programs.
26

The American National Standards Institute, Inc. (11 West 42nd Street, New York, NY 10036) is a private sector organization thatserves as a standards coordinating body, accredits standards developers that follow procedures sanctioned by ANSI, designates as
American National Standards those standards submitted for and receiving approval, serves as the United States Member Body of theInternational Organization for Standardization (ISO), and functions as the administrator of the United States National Committee for
the International Electrotechnical Commission (IEC).

71

Guide for the Use of the International System of Units (SI)A Federal Register Notice of July 28, 1998, [16] declares that there are now only two classes ofunits in the International System of Units: base units and derived units. The units of these two classes forma coherent set of units and are designated by the name ‘‘SI units.”
C.6 Federal Standard 376B
Federal Standard 376B [17] was developed by the Standards and Metric Practices Subcommitteeof the Metrication Operating Committee, which operates under the Interagency Council on Metric Policy.
Specified in the Federal Standardization Handbook and issued by, and available from, the General
Services Administration, Washington, DC, 20406, it is the basic Federal standard that lists preferred metricunits for use throughout the Federal Government. It gives guidance on the selection of metric units required
to comply with PL 94-168 (see Preface) as amended by PL 100-418 (see Preface), and with Executive
Order 12770 [15] (see Sec. C.5).
C.7 2006 CODATA recommended values of the fundamental constants
The set of self-consistent recommended values of the fundamental physical constants resulting fromthe 2006 Committee on Data for Science and Technology (CODATA) least-squares adjustment of theconstants, see Ref. [19], can be found at the NIST website: http://physics.nist.gov/cuu/Constants/index.html.
C.8 Uncertainty in measurement
Reference [20] cites two publications that describe the evaluation and expression of uncertainty inmeasurement based on the approach recommended by the CIPM in 1981 and which have been adopted
worldwide.

72

Guide for the Use of the International System of Units (SI)Appendix D. Bibliography[1]

Le Systéme International d’Unités (SI), The International System of Units (SI), 8th Edition (Bur. Intl.
Poids et Mesures, Sèvres, France, 2006).
Note: This publication, which is commonly called the BIPM SI Brochure, consists of the official
French text followed by an English translation.

[2]

The International System of Units (SI), Ed. by B. N. Taylor and Ambler Thompson, Natl. Inst. Stand.
Technol. Spec. Publ. 330, 2008 Edition (U.S. Government Printing Office, Washington, DC, March
2008).
Itisavailableinelectronic
form
at
no
chargeat
http://physics.nist.gov/cuu/Units/bibliography.html.
Note: This publication is the United States edition of the English translation in Ref. [1].

[3]United States Government Printing Office Style Manual (U.S. Government Printing Office,Washington, DC, 2000).

[4]

ISO 31-0 is cited in the text in the form [4: ISO 31-0]. Currently ISO 31 is being revised jointly by
ISO TC12 and IEC TC25. The revised joint standards ISO/IEC 80000-1—ISO/IEC 80000-15 willsupersede ISO 31-0:1992—ISO 31-13. The completed revised joint standards published to date areincluded in this list, though the part numbers maybe different from the earlier designation.
Quantities and units — Part 0: General principles, ISO 31-0:1992.
Quantities and units — Part 2: Periodic and related phenomena, ISO 31-2:1992.
Quantities and units — Part 3: Space and time, ISO 80000-3 (2006).
Quantities and units — Part 4: Mechanics, ISO 80000-4 (2006).
Quantities and units — Part 4: Heat, ISO 31-4:1992.
Quantities and units — Part 5: Electricity and magnetism, ISO 31-5:1992.
Quantities and units — Part 5: Thermodynamics, ISO 80000-5 (2007).
Quantities and units — Part 6: Light and related electromagnetic radiations, ISO 31-6:1992.
Quantities and units — Part 7: Acoustics, ISO 31-7:1992.
Quantities and units — Part 8: Physical chemistry and molecular physics, ISO 31-8:1992.
Quantities and units — Part 9: Atomic and nuclear physics, ISO 31-9:1992.
Quantities and units — Part 10: Nuclear reactions and ionizing radiations, ISO 31-10:1992.
Quantities and units — Part 11: Mathematical signs and symbols for use in physical sciences andtechnology, ISO 31-11:1992.
Quantities and units — Part 12: Characteristic numbers, ISO 31-12:1992.
Quantities and units — Part 13: Solid state physics, ISO 31-13:1992.

73

Guide for the Use of the International System of Units (SI)

Note:

[5]

ISO 31-0:1992— ISO 31-13:1992 and ISO 1000:1992 are reprinted in the ISO Standards
Handbook Quantities and units (International Organization for Standardization, Geneva,
Switzerland, 1993).

The following four standards, which are cited in the text in the form [IEC 60027-X], are publishedby the International Electrotechnical Commission (IEC), Geneva, Switzerland.
Letter symbols to be used in electrical technology, Part 1: General, IEC 60027-1 (1992).
Letter symbols to be used in electrical technology, Part 2: Telecommunications and electronics, IEC
60027-2 (2005).
Note:

As pointed out in Sec. 4.3, the SI prefixes refer strictly to powers of 10. They should not be
used to indicate powers of 2 (for example, one kilobit represents 1000 bits and not 1024bits). The IEC has adopted prefixes for binary powers in the above standard. The names
and symbols for the prefixes corresponding to 210, 220, 230, 240, 250, and 260 are, respectively:kibi, Ki; mebi, Mi; gibi, Gi; tebi, Ti; pebi, Pi; and exbi, Ei. Thus, for example, one kibibyte
would be written: 1 KiB = 210 B = 1024 B, where B denotes a byte. Although theseprefixes are not part of the SI, they should be used in the field of information technology to
avoid the incorrect usage of the SI prefixes.

Letter symbols to be used in electrical technology, Part 3: Logarithmic and related quantities andtheir units, IEC 60027-3 (2003).
Letter symbols to be used in electrical technology, Part 4: Rotating electrical machines,
IEC 60027-4 (2006).
[6]

SI 10-2002 IEEE/ASTM Standard for Use of the International System of Units (SI): The Modern
Metric System. A joint ASTM-IEEE effort to develop a single ANSI standard.
Notes:
1.

A number of similar standards for metric practice are published by technical organizations.
They include:
Rules for SAE Use of SI (Metric) Units, TSB003 MAY 1999 (Society of Automotive
Engineers, Warrendale, PA, May 1999).

2.

The Canadian Standards Association, 5060 Spectrum Way, Suite 100, Mississauga,
Ontario, Canada, L4W 5N6, publishes CAN/CSA-Z234.1-00 (R2006), Canadian Metric
Practice Guide, a Canadian National Standard.

3.

The application of the SI to physical chemistry is discussed in Quantities, Units andSymbols in Physical Chemistry, Third Edition (International Union of Pure and Applied
Chemistry, RSC Publications, Cambridge, 2007).

[7]

Federal Register, Vol. 24, No. 128, p. 5348, July 1, 1959.

[8]

Federal Register, Vol. 53, No. 138, p. 27213, July 19, 1988.

[9]

Federal Register, Vol. 42, No. 57, p. 8847, March 24, 1977.

[10]

Federal Register, Vol. 54, No. 113, p. 25318, June 14, 1989.

[11]

Federal Register, Vol. 40, No. 23, p. 5954, February 3, 1975.

74

Guide for the Use of the International System of Units (SI)[12]Federal Register, Vol. 33, No. 146, p. 10755, July 27, 1968.

[13]

Federal Register, Vol. 55, No. 245, p. 52242, December 20, 1990.

[14]

Federal Register, Vol. 56, No. 1, p. 160, January 2, 1991.

[15]

Federal Register, Vol. 56, No. 145, p. 35801, July 29, 1991.

[16]

Federal Register, Vol. 63, No. 144, p. 40334, July 28, 1998.

[17]

Preferred Metric Units for General Use by the Federal Government, Federal Standard 376B
(General Services Administration, Washington, DC, 1993).

[18]

Quantities and Units in Radiation Protection Dosimetry, ICRU Report 51, 1993 (International
Commission on Radiation Units and Measurements, 7910 Woodmont Avenue, Bethesda, MD,
20814).

[19]

Values from the 2006 adjustment of the fundamental physical constants can be found at the NIST
website: http://physics.nist.gov/cuu/Constants/index.html. The definitive paper describing the 2006
adjustment has been published in two journals: P. J. Mohr, B. N. Taylor, and D. B. Newell, Rev.
Mod. Phys. 80(2), 633-730 (2008); J. Phys. Chem. Ref. Data. 37(3), 1187-1284 (2008). The paper
may be obtained electronically through a link on the above web page.

[20]

The term standard uncertainty used in the footnotes to Table 7 of this Guide, and the related terms
expanded uncertainty and relative expanded uncertainty used in some of the examples of Sec. 7.10.3,
are discussed in ISO, Guide to the Expression of Uncertainty in Measurement (International
Organization for Standardization, Geneva, Switzerland, 1995); and in B. N. Taylor and C. E. Kuyatt,
Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results, Natl. Inst.
Stand. Technol. Spec. Publ. 1297, 1994 Edition (U.S. Government Printing Office, Washington, DC,
September 1994).

75

Guide for the Use of the International System of Units (SI)This page intentionally left blank.

76

SI COHERENT DERIVED UNITS WITH SPECIAL NAMES AND SYMBOLS(Explanation of Graphic on Back Cover)
Derived units are defined as products of powers of the base units. When the product of powers includes nonumerical factor other than one, the derived units are called “coherent derived” units. The base and coherent derived
units of the SI form a coherent set, designated the set of “coherent SI units.” The word coherent is used here in thefollowing sense: when coherent units are used, equations between the numerical values of quantities take exactly
the same form as the equations between the quantities themselves. Thus, if only units from a coherent set are used,conversion factors between units are never required.
The diagram on the back page of SP811 shows graphically how the 22 SI coherent derived units with specialnames and symbols are related to the seven SI base units.
1.

In the first column, the symbols of the SI base units are shown in rectangles, with the name of the unit
shown toward the upper left of the rectangle and the name of the associated base quantity shown initalic type below the rectangle.

2.

In the second column are shown those additional coherent derived units without special names thatare necessary for the derivation of the coherent derived units with special names [the cubic meter (m3)excepted]. In the diagram, the derivation of each coherent derived unit is indicated by arrows thatbring in units in the numerator (solid lines) and units in the denominator (broken lines), asappropriate.

3.

In the third column the symbols of the 22 SI coherent derived units with special names are shown insolid circles, with the name of the unit shown toward the upper left of the circle, the name of theassociated derived quantity shown in italic type below the circle, and an expression for the derived
unit in terms of other units shown toward the upper right in parenthesis.

Two SI coherent derived units with special names and symbols, the radian, symbol rad, and the steradian,symbol sr (bottom-right of the third column of the diagram), are shown without any connections to SI base units –either direct or through other SI derived units. The reason is that in the SI, the quantities plane angle and solid angle
are defined in such a way that their dimension is one – they are so-called dimensionless quantities. This means thatthe coherent SI derived unit for each of these quantities is the number one, symbol 1. That is, because plane angle isexpressed as the ratio of two lengths, and solid angle as the ratio of an area and the square of a length, the SI
coherent derived unit for plane angle is m/m = 1, and the SI coherent derived unit for solid angle is m2/m2 = 1. To
aid understanding, the special name “radian” with symbol rad is given to the number 1 for use in expressing valuesof plane angle; and the special name “steradian” with symbol sr is given to the number 1 for use in expressing valuesof solid angle. However, one has the option of using or not using these names and symbols in expressions for otherSI derived units, as is convenient.
The unit “degree Celsius,” which is equal in magnitude to the unit “kelvin,” is used to express Celsius
temperature t. In this case, “degree Celsius’’ is a special name used in place of “kelvin.” This equality is indicatedin the diagram by the symbol K in parenthesis toward the upper right of the °C circle. The equation below“CELSIUS TEMPERATURE” relates Celsius temperature t to thermodynamic temperature T. An interval ordifference in temperature may be expressed equivalently in either kelvins or in degrees Celsius.

SI BASE UNITS

kilogram

SI coherent
derived units
without special
names

kg
MASS

m
LENGTHsecond

mole

m3

TIME

VELOCITYA

ELECTRIC CURRENT

kelvinK

THERMODYNAMICTEMPERATURE

candelaAREA
m/s

AMOUNT OF
SUBSTANCE

ampere

m2

s

mol

cd

LUMINOUS INTENSITY

Solid lines indicate multiplication, broken lines indicate division

newtonVOLUME

meter

SI COHERENT DERIVED UNITS WITH SPECIAL NAMES AND SYMBOLS

m/s2

ACCELERATION

N(kg·m/s2)

FORCE

joule

J

Pa

(N/m2)PRESSURE,
STRESS(N·m)

ENERGY, WORK,QUANTITY OF HEAT
katal

pascal(mol/s)

watt

W

(J/s)

gray

Gy

(J/kg)

ABSORBED
DOSE
becquerel

Bq(OF A RADIONUCLIDE)

weber

henry

Wb

CATALYTICACTIVITY

MAGNETICFLUXINDUCTANCE

C

volt

(A·s)

V

farad

F

Ω

CELSIUSTEMPERATURE
t/°C = T/K – 273.15

CAPACITANCERESISTANCElx

ILLUMINANCE

(1/s)

tesla

T

(Wb/m2)MAGNETICFLUX DENSITY(W/A)

degree(K)
Celsius °C(lm/m2)Hz

VOLTAGE,ELECTROMOTIVEFORCEELECTRICCHARGElux(J/kg)

FREQUENCY(Wb/A)

katcoulomb

hertz

ACTIVITY

H

Sv

DOSE
EQUIVALENT(1/s)

POWER,HEAT FLOW RATE
(V·s)

sievert

lumen

(C/V)

lm

(cd·sr)

LUMINOUS
FLUX

ohm

steradian

sr

(V/A)

siemens(1/Ω )S
CONDUCTANCE(m2/m2 = 1)

SOLID ANGLE

radianrad(m/m = 1)

PLANE ANGLE