AlllOM t,Dftb3D

uniiea oiaics Department of Commerce

Technology Administration

National Institute of Standards and Technology

NIST Special Publication 811

1995 Edition

Guide for the Use of the International System

of Units (SI)

Barry N. Taylor

# Wy <i^-Vy VV'.
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NIST Special Publication 811

1995 Edition

Guide for the Use of the International System

of Units (SI)

Barry N. Taylor

Physics Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-0001

(Supersedes NIST Special Publication 811, September 1991)

April 1995

U.S. Department of Commerce

Ronald H. Brown, Secretary

Technology Administration

Mary L. Good, Under Secretary for Technology

National Institute of Standards and Technology

Arati Prabhakar, Director
National Institute of Standards

and Technology

Special Publication 81

1995 Edition

(Supersedes NIST Special

Publication 811, September 1991)

Natl. Inst. Stand. Technol.

Spec. Publ. 811

1995 Ed.

84 pages (April 1 995)

CODEN: NSPUE2

U.S. Government Printing Office

Washington: 1995

For sale by the Superintendent of

Documents

U.S. Government Printing Office

Washington, DC 20402
Guide for the Use of the International System of Units (SI)

Preface

The International System of Units, universally abbreviated SI (from the French Le

Systeme International d'Unites), 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 international commerce.

The Omnibus Trade and Competitiveness Act of August 1988 [Public Lav^' (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 United

States 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

United States trade and commerce . .

.

and requires that

each 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 other business-related activities, except to the extent that such use is impractical or is likely to cause significant 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 the voluntary aspect of the conversion to the SI for Federal agencies and

gives in detail the policy for that conversion. 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 and departments.*

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 second edition of the

Guide replaces the first edition prepared by Arthur O. McCoubrey and published in 1991.

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 technical publications at NBS.

It is quite natural for NIST to publish documents on the use of the SI. First,

NIST coordinates the Federal Government policy on the conversion to the SI by Federal

agencies and on the use of the SI by United States industry and the public. Second,

NIST provides official United States representation in the various international bodies es- tablished by the Meter Convention (Convention du Metre, 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 (nearly 50 countries are now members of the Convention).

• Executive Order 12770 was published in the Federal Register, Vol. 56, No. 145, p. 35801, July 29, 1991;

15 CFR 1 170 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. Both Executive Order 12770 and 15 CFR 1 170 are reprinted in Ref. [1]. (See

Appendix D — Bibliography, which begins on p. 72.)

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

One body created by the Meter Convention is the General Conference on Weights and

Measures (CGPM, Conference Generale des Poids et Mesures), a formal diplomatic organization.** The International 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 and technology.

This 1995 edition of the Guide corrects a number of misprints in the 1991 edition, incorporates a significant amount of additional material intended to answer frequently asked

questions concerning the SI and SI usage, and updates the bibliography. The added material

includes a check list in Chapter 11, which is reproduced immediately after this Preface for

easy reference, for reviewing the consistency of NIST manuscripts with the SI. Some changes

in format have also been made in an attempt to improve the ease of use of the Guide.

In keeping with United States and NIST practice (see Sec. C.3), this edition of the

Guide continues to use the dot as the decimal marker rather than the comma, the spellings

"meter," "liter," and "deka" rather than "metre," "litre," and "deca," and the name "metric

ton" rather than tonne.

I should like to take this opportunity to thank James B. McCracken of the NIST Metric

Program for his highly capable assistance in the early stages of the preparation of this Guide

.

** See Ref. [2] or [3] 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 and Mesures), the International Committee for Weights and Measures (CIPM, Comite International des Poids et Mesures), and the CGPM.

The BIPM, vv'hich is located in Sevres, 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. [2] and [3] also give the

various CGPM and CIPM resolutions on which it is based. With the exception of Table 8, Tables 1 to 11 of this Guide and their accompanying text are taken or are adapted from these references.

March 1995

Barry N. Taylor

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

Check List for Reviewing Manuscripts

The following check list, which constitutes Chapter 11 of this Guide and is adapted from

Ref. [22], is intended to help NIST authors review the conformity of their manuscripts 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) O Only units of the SI and those units recognized for use with the SI are used to express the values of quantities. 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) O Abbreviations such as sec (for either s or second), cc (for either cm^ 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 prefixes are used.

(See Sec. 6.1.8.)

(3) O The combinations of letters "ppm," "ppb," and "ppt," and the terms part per million, part per billion, 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 iiL/L

or 2.0 X lO"*^ V, 4.3 nm/m or 4.3 x 10'' /, 7 ps/s or 7 x 10"'' t, where V, I, and

t are, respectively, the quantity symbols for volume, length, and time. (See Sec.

7.10.3.)

(4) [H Unit symbols (or names) are not modified by the addition of subscripts or other

information. The following forms, for example, are used instead. (See Sees. 7.4

and 7.10.2.)

Kmax= 1000V but not: ]/- 1000V„,ax

a mass fraction of 10 % but not: 10 % (m/m) or 10 % (by weight)

(5) CD Statements such as "the length /i exceeds the length I2 by 0.2 %" are avoided be- cause it is recognized that the symbol % represents simply the number 0.01. Instead, forms such as "/i = /2(1 + 0.2 %)" or "A = 0.2 %" are used, where A is defined by the relation A - (li - /2)//2. (See Sec. 7.10.2.)

(6) Information is not mixed with unit symbols (or names). For example, the form

"the water content is 20 mL/kg" is used and not "20 mL HaO/kg" or "20 mL of

water/kg." (See Sec. 7.5.)

(7) n 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. (See Sec. 7.7.)

35 cm X 48 cm but not: 35 x 48cm

1 MHz to 10 MHz or (1 to 10) MHz but not: 1 MHz - 10 MHz or 1 to 10 MHz

20 °C to 30 °C or (20 to 30) °C but not : 20 °C - 30 °C or 20 to 30 °C

123g ± 2g or (123 ± 2)g but not: 123 ±2g

70 % ± 5 % or (70 ± 5) % but not: 70 ± 5 %

240 X (1 ± 10%) V but not: 240 V ± 10% (one cannot add

240 V and 10 %)

(8) Cn Unit symbols and unit names are not mixed and mathematical operations are not

applied to unit names. For example, only forms such as kg/m^ kg • m or kilogram per cubic meter are used and not forms such as kilogram/m\ kg/cubic

meter, kilogram/cubic meter, kg per m^, or kilogram per meter\ (See Sees. 6.1.7,

9.5, and 9.8.)

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

(9) n Values of quantities are expressed in acceptable units using Arabic numerals and

the symbols for the units. (See Sec. 7.6.)

m = 5 kg but not : m = five kilograms or m = five kg

the current was 15 A but not : the current was 15 amperes.

(10) O There is a space between the numerical value and unit symbol, even when the

value is used in an adjectival sense, except in the case of superscript units for plane

angle. (See Sec. 7.2.)

a 25 kg sphere but not: a 25-kg sphere an angle of 2°3'4" but not: an angle of 2 °3 '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) O The digits of numerical values having more than four digits on either side of the

decimal marker are separated into groups of three using a thin, fixed space

counting from both the left and right of the decimal marker. For example,

15 739.012 53 is highly preferred to 15739.01253. Commas are not used to separate

digits into groups of three. (See Sec. 10.5.3.)

(12) O Equations between quantities are used in preference to equations between numerical values, and symbols representing numerical values are different from symbols representing the corresponding 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) im Standardized quantity symbols such as those given in Refs. [6] and [7] are used,

for example, R for resistance and Ai 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. [6: ISO 31-11] are used, for example,

"tanx" and not "tgx." More specifically, the base of "log" in equations is specified when required by writing logo a; (meaning log to the base aofx),lhx (meaning

log2J:), \nx (meaning \ogex), or lg;c (meaning logiox). (See Sees. 10.1.1 and

10.1.2.)

(14) CH Unit symbols are in roman type, and quantity symbols are in italic type with superscripts and subscripts in roman or italic type as appropriate. (See Sec. 10.2 and

Sees. 10.2.1 to 10.2.4.)

(15) n 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, weight is 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) n 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) D The obsolete term normality and the symbol N, and the obsolete term molarity

and the symbol m, are not used, but the quantity amount-of-substance concentration of B (more commonly called concentration of B), and its symbol Cb and SI unit

mol/m^ (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 6b or ma and SI unit mol/kg (or a related unit of the SI), are used

instead. (See Sees. 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 Three Classes of SI Units and the SI Prefixes 3

4.1 SI base units 4

4.2 SI derived units 4

4.2.1 SI derived units with special names and symbols 4

4.2.1.1 Degree Celsius 5

4.2.2 Use of SI derived units with special names and symbols 6

4.3 SI supplementary units 7

4.4 Decimal multiples and submultiples of SI units: SI prefixes 7

5 Units Outside the SI 8

5.1 Units accepted for use with the SI 8

5.1.1 Hour, degree, liter, and the like 8

5.1.2 Neper, bel, shannon, and the like 9

5.1.3 Electronvolt and unified atomic mass unit 9

5.1.4 Natural and atomic units 9

5.2 Units temporarily accepted for use with the SI 10

5.3 Units not accepted for use with the SI 10

5.3.1 COS units 11

5.3.2 Other unacceptable units 11

5.4 The terms "units of the SI" 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 12

6.1.6 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 19

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 20

7.10.2 %, percentage by, fraction 20

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 Time and rotational frequency 23

8.2 Volume 23

8.3 Weight 24

8.4 Relative atomic mass and relative molecular mass 24

8.5 Temperature interval and temperature difference 25

8.6 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

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

8.7 Lx)garithmic quantities and units: level, neper, bel 28

8.8 Viscosity 30

8.9 Massic, volumic, areic, lineic 30

9 Rules and Style Conventions for Spelling Unit Names 31

9.1 Capitalization 31

9.2 Plurals 31

9.3 Spelling unit names with prefixes 31

9.4 Spelling unit names obtained by multiplication 31

9.5 Spelling unit names obtained by division 31

9.6 Spelling unit names raised to powers 32

9.7 Other spelling conventions 32

9.8 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 32

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 - italic 34

10.2.2 Units — roman 34

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 36

10.5.1 Typeface for numbers 36

10.5.2 Decimal sign or marker 36

10.5.3 Grouping digits 37

10.5.4 Multiplying numbers 37

11 Check List for Reviewing Manuscripts 38

Appendbc A. Definitions of the SI Base Units and the Radian and Steradian 40

A.l Introduction 40

A.2 Meter 40

A.3 Kilogram 40

A.4 Second 40

A.5 Ampere 40

A.6 Kelvin 40

A.7 Mole 40

A.8 Candela 40

A.9 Radian 40

A.IO Steradian 40

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

Appendix B. Conversion Factors 41

B.l Introduction 41

B.2 Notation 41

B.3 Use of conversion factors 41

B.4 Organization of entries and style 42

B.5 Factor for converting motor vehicle efficiency 43

B.6 U.S. survey foot and mile 43

B.7 Rules for rounding numbers and converted numerical values of

quantities 44

B.7.1 Rounding numbers 44

B.7.2 Rounding converted numerical values of quantities 45

B.8 Factors for units listed alphabetically 46

B.9 Factors for units listed by kind of quantity or field of science 57

Appendix C. Comments on the References of Appendix D — Bibliography 69

C. l Official interpretation of the SI for the United States:

55 FR 52242-52245 69

C.2 Defining document for the SI: BIPM SI Brochure 69

C.3 United States version of defining document for the SI: NIST SP 330 . . . . 69

C.4 ISO 1000 69

C.5 ISO 31 69

C.6 IEC27 70

C.7 ANSI/IEEE Std 268 70

C.8 Federal Register notices 70

C.9 Federal Standard 376B 71

C.IO 1986 CODATA values of the fundamental constants 71

C.ll Uncertainty in measurement 71

Appendix D. Bibliography 72

X
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 Confer- ence on Weights and Measures (CGPM — see Preface). Universally abbreviated SI (from

the French Le Syst^me International d'Unites), it is the modern metric system of measure- ment used throughout the world. This Guide has been prepared by the National Institute of

Standards and Technology (NIST) to assist members of the NIST staff, as well as others who

may have need of such assistance, in the use of the SI in their work, 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 and introduces 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 their manuscripts 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 three current

classes of SI units: base, derived, and supplementary; those derived units that have special

names and symbols, including the degree Celsius; and the SI prefbces that are used to form

decimal multiples and submultiples of SI units.

Chapter 5 discusses units that are outside the SI and indicates those that may be used

with it and those that may not. It also gives (see Sec. 5.4) precise definitions of the terms

"units of the SI" 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 clarifying comments 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 technical documents and is intended to assist NIST authors prepare manuscripts that are consistent with accepted typesetting practice.

Chapter 11 gives the check list that is reproduced immediately after the Preface.

Appendw A gives the definitions of the SI base units and the radian and steradian, while

Appendix B gives conversion factors for converting values of quantities expressed in units

that are mainly unacceptable for use with the SI to values expressed mainly in units of the

SI. Appendix B also includes a simplified discussion of rounding numbers and rounding

converted numerical values of quantities.

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

Appendix C discusses in some detail most of the references included in Appendix D —

Bibliography, which concludes the Guide.

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.' When the field of application or the special needs of users of NIST publications require the use of other units, the values of quantities shall first be expressed in acceptable units, where it is to be understood that acceptable units include the units of the SI 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 "units of the SI"

and "acceptable units" as used in this Guide, see Sec. 5.4.) Exceptions to this policy require

the prior approval of 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 the related 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 expressed in 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 done by placing scale marks on and labeling the

left-hand ordinate and bottom absiccsa in acceptable units, and placing 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 same ordinate and abscissa as is used for the acceptable units.

2.2 Descriptive information

Descriptive information characterizes arrangements, environments, the generalized

dimensions of objects, 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.

' 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 Sees. 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

Building 245, Room C229

Gaithersburg, MD 20899-0001

Telephone: (301) 975-4220

Fax: (301) 869-7682

3.3 Metric Program

Questions concerning Federal Government use of the SI and Federal Government policy on the use of the SI by U.S. industry and the public may be directed to:

Metric Program

Technology Services

National Institute of Standards and Technology

Building 411, Room A146

Gaithersburg, MD 20899-0001

Telephone: (301) 975-3690

Fax: (301) 948-1416

4 The Three Classes of SI Units and the SI Prefixes

SI units are currently divided into three classes: — base units, — derived units, — supplementary units,

which together form what is called "the coherent system of SI units." ^ The SI also includes

prefixes to form decimal multiples and submultiples of SI units.

2 According to Ref. [6: ISO 31-0], a system of units is coherent with respect to a system of quantities and equations if the system of units is chosen in such a way that the equations between numerical values have exactly the same form (including the numerical factors) as the corresponding equations between the quantities (see Sees. 7.11 and 7.14). In such a coherent system, of which the SI is an example, no numerical factor other than the number

1 ever occurs in the expressions for the derived units in terms of the base units. It should also be noted that the

class of supplementary units is likely to be abolished as a separate class in the SI — see Sec. 4.3.

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 is founded; 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 a temperature interval or a temperature difference

(see Sec. 8.5).

Table 1. SI base units

SI base unit

Base quantity Name Symbol

length meter m

mass kilogram kg

time second s electric current ampere A

thermodynamic temperature kelvin K

amount of substance mole mol

luminous intensity candela cd

4.2 SI derived units

Derived units are expressed algebraically in terms of base units or other derived units

(including the radian and steradian which are the two supplementary units — see Sec. 4.3).

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 the kilogram per mole, symbol kg/mol. Additional examples of derived units expressed in terms of SI base units are given in Table 2. (The rules and style conventions for printing and using SI unit symbols are given in Sees.

6.1.1 to 6.1.8.)

Table 2. Examples of SI derived units expressed in terms of SI base units

SI derived unit

Derived quantity Name Symbol

area square meter m^

volume cubic meter m^

speed, velocity meter per second m/s

acceleration meter per second squared m/s^ wave number reciprocal meter m-'

mass density (density) kilogram per cubic meter kg/m'

specific volume cubic meter per kilogram mVkg

current density ampere per square meter A/m^

magnetic field strength ampere per meter A/m

amount-of-substance concentration

(concentration) mole per cubic meter mol/m'

luminance candela per square meter cd/m^

4.2.1 SI derived units with special names and symbols

Certain SI derived units have special names and symbols; these are given in Tables 3a

and 3b. As discussed in Sec. 4.3, the radian and steradian, which are the two supplementary

units, are included in Table 3a.

4
Guide for the Use of the International System of Units (SI)

Table 3a. SI derived units with special names and symbols, including the radian and steradian

SI derived unit

Derived quantity

Expression Expression

Special name Special symbol in terms in terms

of other

SI units

of SI base

units

plane angle radian rad m • m"' = 1 solid angle steradian sr m^-m-^ = 1 frequency hertz Hz s-i

force newton N m • kg • s"^

pressure, stress pascal Pa N/m2 m~* • kg' s~^

energy, work, quantity

of heat joule J N-m m^ • kg • s"^

power, radiant flux watt W J/s m^ • kg • s"'

electric charge.

quantity of electricity coulomb C s-A

electric potential.

potential difference,

electromotive force volt V W/A m^-kg-s"^- A"'

capacitance farad F c/v m"^ • kg"' • s* • A^

electric resistance ohm n V/A m^ • kg • s"-' • A"^

electric conductance Siemens s A/V m-^-kg-'-s'-A^

magnetic flux weber Wb V-s m^ • kg • s"^ • A"'

magnetic flux density tesla T Wb/m^

inductance henry H Wb/A m^ • kg • • A-2

Celsius temperature^") degree Celsius °C K

luminous flux lumen Im cd • sr cd • sr^*") illuminance lux be Im/m^ m-2-cd-srW

(") See Sees. 4.2.1.1, 6.2.8, and 7.2. The steradian (sr) is not an SI base unit. However, in photometry the steradian (sr) is maintained in expressions

for units (see Sec. 4.3).

Table 3b. SI derived units with special names and symbols admitted for reasons of safeguarding human health

SI derived unit

Derived quantity Special nameSpecial

symbol

Expression in terms

of other SI units

Expression in terms

of SI base units

activity (of a radionuclide) becquerel Bq S-'

absorbed dose,

specific energy

(imparted), kerma gray Gy J/kg ra2-s-2

dose equivalent, ambient dose

equivalent, directional dose

equivalent, personal dose

equivalent, equivalent dose sievert Sv J/kg m^-s-^

The derived quantities to be expressed in the gray and the sievert have been revised in accordance with the recommendations of the International Commision on Radiation Units and Measurements (ICRU); see

Ref. [19].

4.2.1.1 Degree Celsius In addition to the quantity thermodynamic temperature (symbol

T), expressed in the unit Icelvin, use is also made of the quantity Celsius temperature

(symbol t) defined by the equation

t = T-To , where To = 273.15 K by definition. To express Celsius temperature, the unit degree Celsius,

symbol °C, which is equal in magnitude to the unit kelvin, is used; in this case, "degree Celsius" is a special name used in place of "kelvin." An interval or difference of Celsius temperature can, however, be expressed in the unit kelvin as well as in the unit degree Celsius

(see Sec. 8.5). (Note that the thermodynamic temperature To is exactly 0.01 K below the

thermodynamic temperature of the triple point of water (see Sec. A.6).)

5
Guide for the Use of the International System of Units (SI)

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 special names and symbols (including the radian and steradian) are given in Table 4.

Table 4. Examples of SI derived units expressed with the aid of SI derived units having special names and symbols

SI derived unit

Expression

Derived quantity Name Symbol in terms of

SI base units

angular velocity radian per second rad/s m • m"' • s"' = s~'

angular acceleration radian per second squared rad/s^ m • m~' • s"^ = s"^

dynamic viscosity pascal second Pa-s m~' • kg • s"' moment of force newton meter N-m m^'kg's"^

surface tension newton per meter N/m kg-s-2

heat flux density, irradiance watt per square meter W/m^ kg-s ^

radiant intensity watt per steradian W/sr m^-kg-s~^-sr-' <">

radiance watt per square meter steradian W/(m^ • sr) kg-s~^-sr"'<''>

hpat canacitv pntrnnv loiilp ripr Icplvin J/K 111 specific heat capacity, joule per kilogram

specific entropy kelvin J/(kg-K) m^-s-^-K-'

specific energy joule per kilogram J/kg m^ • s~^

thermal conductivity watt per meter kelvin W/(m-K)

J/m'

m-kg-s-3-K-'

energy density joule per cubic meter m"' • kg • s"^

electric field strength volt per meter V/m m • kg • s"^ • A"'

electric charge density coulomb per cubic meter C/m' m"' • s • A

electric flux density coulomb per square meter C/m^ m"^'S" A

permittivity farad per meter F/m m-'-kg-'-s-'-A^

permeability henry per meter H/m m • kg • s"^ • A~^

molar energy joule per mole J/mol m^-kg-s"^'mol"'

molar entropy, molar

heat capacity joule per mole kelvin J/(mol • K) m^-kg-s-^-K-'-mol-'

exposure (x and y rays) coulomb per kilogram C/kg kg"' • sA

absorbed dose rate gray per second Gy/s m^-s-'

^"^ The steradian (sr) is not an SI base unit. However, in radiometiy the steradian (sr) is maintained in expressions

for units (see Sec. 4.3).

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 understood than its SI base-unit equivalent, m^ • kg • s~^ • K~' • mol~'.

Nevertheless, it should always be recognized that the 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 3a) is expressed in terms of the volt second ( V • s) instead

of the weber (Wb).

Tables 3a, 3b, and 4 also show that the values of several different quantities are ex- pressed 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 the name 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 and derived 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~*), and the SI unit of moment of force is specified as the newton meter

(N • m) rather than the joule (J).

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

Similarly, in the field of ionizing radiation, the SI unit of activity is designated as the

becquerel (Bq) rather than the reciprocal second (s"'), and the SI units of absorbed dose

and dose equivalent are designated as the gray (Gy) and the sievert (Sv), respectively, rather

than the joule per kilogram (J/kg).

4.3 SI supplementary units

As previously stated, there are two units in this class: the radian, symbol rad, the SI unit

of the quantity plane angle; and the steradian, symbol sr, the SI unit of the quantity solid

angle. Definitions of these units are given in Appendix A.

The SI supplementary units are now interpreted as so-called dimensionless derived

units (see Sec. 7.14) for which the CGPM allows the freedom of using or not using them in

expressions for SI derived units. ^ Thus the radian and steradian are not given in a separate

table but have been included in Table 3a together with other derived units with special

names and symbols (see Sec. 4.2.1). This interpretation of the supplementary units implies

that plane angle and solid angle are considered derived quantities of dimension one

(so-called dimensionless quantities — see Sec. 7.14), each of which has the unit one, symbol 1

as its coherent SI unit. However, in practice, when one expresses the values of derived quantities involving plane angle or solid angle, it often aids understanding if the special names

(or symbols) "radian" (rad) or "steradian" (sr) are used in place of the number 1. For

example, although values of the derived quantity angular velocity (plane angle divided by

time) may be expressed in the unit s"', such values are usually expressed in the unit rad/s.

Because the radian and steradian are now viewed as so-called dimensionless derived

units, the Consultative Committee for Units (CCU, Comite Consultatif des Unites) of the

CIPM (see footnote, p. iv), as a result of a 1993 request it received from ISO/TC 12 (see

Ref. [22]), recommended to the CIPM that it request the CGPM to abolish the class of

supplementary units as a separate class in the SI. The CIPM accepted the CCU recommendation, and if the abolishment is approved by the CGPM as is likely (the question will be on

the agenda of the 20th CGPM, October 1995), the SI will consist of only two classes of units:

base units and derived units, with the radian and steradian subsumed into the class of

derived units of the SI. (The option of using or not using them in expressions for SI derived

units, as is convenient, would remain unchanged.)

4.4 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 SI units. They allow very large or very small numerical values (see Sec. 7.1) to be avoided.

A prefix attaches directly to the name of a unit, and a prefix symbol attaches directly to the

symbol for a unit. For example, one kilometer, symbol 1 km, is equal to one thousand meters,

symbol 1000 m or 10^ m. When prefixes are attached to SI units, the units so formed are

called "multiples and submultiples of SI units" in order to distinguish them from the coherent system of SI units. (See footnote 2 for a brief discussion of coherence. The rules and style

conventions for printing and using SI prefixes are given in Sees. 6.2.1 to 6.2.8. The special

rule for forming decimal multiples and submultiples of the unit of mass is given in Sec. 6.2.7.)

Note: Alternative definitions of the SI prefixes and their symbols are not permitted. For

example, it is unacceptable to use kilo (k) to represent 2'° = 1024, mega (M) to represent 2^° = 1 048 576, or giga (G) to represent 2^ = 1 073 741 824.

' See Ref. [2] or [3]. This interpretation was given in 1980 by the CIPM (see footnote, p. iv). It was deemed

necessary because Resolution 12 of the 11th CGPM, which established the SI in 1960 [2, 3], did not specify the nature of the supplemental^ units. The interpretation is based on two principal considerations: that plane angle is

generally expressed as the ratio of two lengths and solid angle as the ratio of an area and the square of a length,

and are thus quantities of dimension one (so-called dimensionless quantities); and that treating the radian and

steradian as SI base units — a possibility not disallowed by Resolution 12 — could compromise the internal coher- ence of the SI based on only seven base units. (See Ref. [6: ISO 31-0] and also Sec. 7.14 for a discussion of the concept of dimension, and footnote 2 for a brief discussion of coherence.)

7
Guide for the Use of the International System of Units (SI)

Table 5. SI prefixes

pactor Prefix Svmbol Factor Prefix

in24 _ /in3\8

ia21 _ /in3\7 xw V / 10'" = (10')"

10'5 = (10')*

10'2 = (10')*

10' = (10')'

yotts VI in-' deci a

ZCttfl z 10-2 centi C exa E 10-' = (10')-' milli m

peta P 10-" = (10')-2 micro M- tera T 10-' = (10')-'

10-'2 = (10')-*

10-'* = (10')-*

nano n

giga G pico P

10* = (10')2 mega M femto f 10' = (10')' kilo k 10-'" = (10')-" atto a

102 hecto h 10-2' = (10')-'

10-2* = (103) -« zepto z

10' deka da yocto y

5 Units Outside the SI

Units that are outside the SI may be divided into three categories: — those units that are accepted for use with the SI; — those units that are temporarily accepted for use with the SI; and — those units that are not accepted for use with the SI and thus in the view 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.

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 combination of units of this table with SI units to form derived units

should be restricted to special cases in order not to lose 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 it may be necessary on occasion to use time-related units other than those given in Table 6; in particular, circumstances may require that

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. Units accepted for use with the SI

Name Symbol Value in SI units

minute

hour

day

degree

minute

second .

liter metric ton

time

plane angle

mm

h

1,

1 min = 60 s

1 h = 60 min = 3600 s Id = 24 h = 86 400 s 1° = (ir/180)rad

1' = (1/60)° = (ir/lO 800) rad

1" = (1/60)' = (-ir/648 000) rad IL

It = 10' kg

(") See also Sec. 7.2. The alternative symbol for the liter, L, was adopted by the CGPM in order to avoid the risk of confusion between

the letter 1 and the number 1 (see Ref. [2] or [3]). Thus, although both 1 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. [1] and [8]). The script letter € is not an approved symbol for the liter. This is the name to be used for this unit in the United States (see Refs. [1] and [8]); it is also used in some other

English-speaking countries. However, this unit is called "tonne" in Ref. [2] and is the name used in many countries.

8
Guide for the Use of the International System of Units (SI)

5.1.2 Neper, bel, shannon, and the like

There are a few highly specialized units not listed in Table 6 that are given by the

International Organization for Standardization (ISO) or the International Electrotechnical

Commission (lEC) and which in the view of this Guide are also acceptable for use with the

SI. They include the neper (Np), bel (B), octave, phon, and sone, and units used in information technology, including the baud (Bd), bit (bit), erlang (E), hartley (Hart), and shannon

(Sh)." It is the position of this Guide that the only such additional units NIST authors may

use with the SI are those given in either the International Standards on quantities and units

of ISO (Ref. [6]) or of lEC (Ref. [7]).

5.1.3 Electronvolt and unified atomic mass unit

The CIPM, and thus this Guide, also finds it necessary to accept for use with the SI the

two units given in Table 7 [2, 3]. 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.)

Note : In some fields the unified atomic mass unit is called the dalton, symbol Da; however,

this name and symbol are not accepted by the CGPM, CIPM, ISO, or lEC for use

with the SI. Similarly, 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.

Table 7. Units accepted for use with the SI whose values in SI units are obtained experimentally

Name Symbol Definition

electronvolt eV

unified atomic mass unit u

(*)

The electronvolt is the kinetic energy acquired by an electron in passing through a potential difference of 1 V

in vacuum; 1 eV = 1.602 177 33 x 10"'^ J with a combined standard uncertainty of 0.000 000 49 x 10-"* J

[20, 21]. The unified atomic mass unit is equal to 1/12 of the mass of an atom of the nuclide '"'C; 1 u = 1.660 540 2 X

10-" kg with a combined standard uncertainty of 0.000 001 0 x 10"" kg [20, 21].

5.1.4 Natural and atomic units

In some cases, particularly in basic science, the values of quantities are expressed in

terms of fundamental constants of nature or so-called natural units. The use of these units

with the SI is, in the view of this Guide , permissible when it is necessary for the most effective

communication of information. In such cases, the specific natural units that are used must

be identified. This requirement applies even to the system of units customarily called

"atomic units" used in theoretical atomic physics and chemistry, inasmuch as there are several different systems that have the appellation "atomic units." Examples of physical quantities used as natural units are given in Table 8.

This Guide also takes the position that while theoretical results intended primarily for

other theorists may be left in natural units, if they are also intended for experimentalists,

they must also be given in acceptable units. NIST measurement results must always be given

in such units first.

*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 Sec. 0.5

of Ref. [6: ISO 31-2], Ref. [7: IEC27-3], and Sec. 8.7 of this Guide. The question of the byte (B) is under international consideration.

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

Table 8. Examples of physical quantities sometimes used as natural units

Kind of quantity Physical quantity used as a unit Symbol

action Planck constant divided by Itt electric charge elementary charge e energy Hartree energy £h

length Bohr radius ao

lengin i^ompion waveiengin ^eieciron) Ac

magnetic flux magnetic flux quantum

magnetic moment Bohr magneton MB

magnetic moment nuclear magneton Mn

mass electron rest mass WJe mass proton rest mass nip

speed speed of electromagnetic waves in vacuum c

5.2 Units temporarily accepted for use with the SI

Because of existing practice in certain fields or countries, in 1978 the CIPM considered

that it was permissible for the units given in Table 9 to continue to be used with the SI until

the CIPM considers that their use is no longer necessary [2, 3]. However, these units must

not be introduced where they are not presently used. Further, this Guide strongly discourages the continued use of these units by NIST authors except for the nautical mile, knot, are, and hectare; and except for the curie, roentgen, rad, and rem until the year 2000 (the

cessation date suggested by the Committee for Interagency Radiation Research and Policy

Coordination or CIRRPC, a United States Government interagency group).

Table 9. Units temporarily accepted for use with the SI

Name Symbol Value in SI units

nautical mile 1 nautical mile = 1852 m

knot

A

1 nautical mile per hour = (1852/3600) m/s

Sngstrom 1 A = O.lnm = 10-'" m

are('') a 1 a = 1 dam^ = 10^ m^

hectare^'') ha 1 ha = 1 hm^ = 10" m^

barn b lb = lOOfm^ = 10-^ m^

bar bar 1 bar = 0.1 MPa = 100 kPa = 1000 hPa = 10' Pa

gal Gal 1 Gal = 1 cm/s^ = IQ-^ m/s^

curie Ci 1 Ci = 3.7 X 10'" Bq

1 R = 2.58x10-" C/kg

1 rad = 1 cGy = 10-^ Gy

roentgen R

rad rad^*^) rem rem 1 rem = 1 cSv = 10-^ Sv

See Sec. 5.2 for the position of this Guide regarding the continued use of these units. This unit and its symbol are used to express agrarian areas.

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

5.3 Units not accepted for use with the SI

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

5 In 1993 the CCU (see Sec. 4.3) was requested by ISO/TC 12 (see Ref. [22]) to consider asking the CIPM to

deprecate the use of the units of Table 9 except for the nautical mile and knot, and possibly the are and hectare. The CCU discussed this request at its February 1995 meeting.

10
Guide for the Use of the International System of Units (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. Further, no other units of the various CGS

systems of units, which includes the CGS Electrostatic (ESU), CGS Electromagnetic

(EMU), and CGS Gaussian systems, are accepted for use with the SI except such units as the centimeter, gram, and second that are also defined in the SI.

Table 10. Examples of CGS units with special names (not accepted for use with the SI)

Name Symbol Value in SI units

erg erg 1 erg = 10-' J dyne dyn 1 dyn = 10-' N

pwise^") P 1 P = 1 dyn • s/cm^ = 0.1 Pa • s stokes^*) St 1 St = IcmVs = lO-'mVs

gauss^'^) Gs, G 1 Gs corresponds to IQ-* T

oersted^*^) Oe 1 Oe corresponds to (1000/4Tr) A/m

1 Mx corresponds to IQ-" Wb

1 sb = 1 cd/cm^ = lO" cd/m^

maxwell^*^^ Mx

stilb sb phot ph 1 ph = 10* be

("^The poise (P) is the CGS unit for viscosity (also called dynamic viscosity). The SI unit is the pascal second

(Pa-s).

The stokes (St) is the CGS unit for kinematic viscosity. The SI unit is the meter squared per second (m^/s).

('^^ This unit is part of the so-called electromagnetic three-dimensional CGS system and cannot strictly speaking be

compared to the corresponding unit of the SI, which has four dimensions when only mechanical and electric quantities are considered.

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 special names for multiples or submultiples of SI units, such as mho for Siemens (S) and micron for micrometer (|xm). Table 11

gives a few examples of some of these other unacceptable units.

Table II. Examples of other unacceptable units

Name Symbol Value in SI units

fermi fermi 1 fermi = 1 fm = IQ-" m

metric carat metric carat 1 metric carat = 200 mg = 2x 10"* kg

ton- Torr 1 Torr = (101 325/760) Pa

standard atmosphere atm 1 atm = 101 325 Pa

kilogram-force kgf 1 kgf = 9.806 65 N

micron \>- 1 tJL = 1 M,m = IQ-'^m

calorie (various) cal,h (thermochemical) 1 cal.h = 4.184 J

X unit XU 1 XU = 0.1002 pm = 1.002 X 10 -'^m

stere St 1 St = 1 m^

gamma 7 1 7 = InT = lO-^T

gamma (mass) y 1 7 = 1 >ig = 10-" kg

lambda (volume) X 1 \ = 1 M,L = 10-" L = 10-" m'

5.4 The terms "units of the SI" and "acceptable units"

Consistent with accepted practice [2, 3], this Guide uses the term "units of the SI" to mean the SI units, that is, the SI base units, SI derived units, and SI supplementary units;

and multiples and submultiples of these units formed by using the SI prefixes. The term

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

"acceptable units," which is introduced in this Guide for convenience, is used to mean the

units of the SI plus (a) those units accepted for use with the SI (see Tables 6 and 7 and

Sees. 5.1.1, 5.1.2, and 5.1.3); (b) those units temporarily accepted for use with the SI (see

Table 9 and Sec. 5.2); and (c) appropriate multiples and submultiples of such accepted and

temporarily accepted units. Because natural and atomic units are not widely recognized for use with the SI, they are not included in the term. However, such units may be used to the

extent discussed in Sec. 5.1.4.

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 Sees. 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

Examples : m (meter) s (second) V (volt)

Pa (pascal) Im (lumen) Wb (weber)

6.1.3 Plurals

Unit symbols are unaltered in the plural.

Example : / = 75 cm but not : / = 75 cms

Note : / is the quantity symbol for length. (The rules and style conventions for expressing

the values of quantities 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 half-high (that is, centered) dot or a space. However, this Guide, as does Ref. [8],

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 "Ms the symbol

for the meter per second while ms"Ms the symbol for the reciprocal millisecond

(10^ s"' - see Sec. 6.2.3).

12
Guide for the Use of the International System of Units (SI)

2 Reference [6: 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 the common 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 to avoid possible

confusion; and that 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 (oblique stroke, /), a horizontal line, or negative exponents.

Example: m/s, or m-s"'

However, to avoid ambiguity, the solidus must not be repeated on the same line unless

parentheses are used.

Examples: m/s^ or m-s"^ but not: m/s/s

m*kg/(s^'A) or m • kg • s"'' • A"' but not: m-kg/s^/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 Sees. 9.5 and 9.8.)

Example: C/kg, C • kg"', or coulomb per but not: coulomb/kg; coulomb per kg;

kilogram C/kilogram; coulomb • kg"';

C per kg; 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 mm^ or square millimeter), cc (for either cm^ 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 (for either u or unified atomic mass unit), or mps (for either m/s or meter per second). Although the values of quantities are normally

expressed using symbols for numbers and symbols for units (see Sec. 7.6), if for some 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 symbols are printed in roman (upright) type regardless of the type used in the

surrounding text, and are attached to unit symbols without a space between the prefix symbol

and the unit symbol. This last rule also applies to prefixes attached to unit names.

Examples: mL (milliliter) pm (picometer) GCl (gigaohm) THz (terahertz)

13
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 in upper-case letters while all other prefix symbols are printed in lower- case letters (see Table 5). Prefixes are normally printed in lower-case 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 cm^ = 2.3(cm)' = 2.3(10 "^m)' = 2.3 x 10 "'^m^

Icm"* = l(cm)-' = l(lO-^m)-' = lO^m"'

SOOOjjLS-i = 5000(jjLs)-i = 5000(10-* s)-i = 5000 x 10** s"' = 5 x 10'

s"'

1 V/cm = (1 V)/(10 m) = 10^ V/m

Prefixes are also inseparable from the unit names to which they are attached. Thus, for ex- ample, millimeter, micropascal, and meganewton are single words.

6.2.4 Unacceptability of compound prefixes

Compound prefix symbols, that is, prefbc symbols formed by the juxtaposition of two or more prefix symbols, are not permitted. This rule also applies to compound prefixes.

Example: nm (nanometer) but not: m|xm (millimicrometer)

6.2.5 Use of multiple prefixes

In a derived unit formed by division, the use of a prefix symbol (or a prefbc) in both the

numerator and the denominator may cause confusion. Thus, for example, lOkV/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) may 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 the unit one. In a similar vein, prefixes 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 x 10Vm^ but not : the number density of

Pb atoms is 5 M/m^

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 10^ 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, the unit symbol for gram, and the names of such multiples

and submultiples are formed by attaching SI prefbces to the name "gram."

Example: 10"* kg = 1 mg (1 milligram) but not: 10"* kg = 1 jjikg (1 microkilogram)

14
Guide for the Use of the International System of Units (SI)

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 "C and prefixes may be used with the

unit name "degree Celsius." For example, 12 m°C (12 millidegrees Celsius) is acceptable.

However, to avoid confusion, prefix symbols (and prefixes) are not used with the timerelated unit symbols (names) min (minute), h (hour), d (day); nor with the angle-related

symbols (names) ° (degree), ' (minute), and " (second) (see Table 6).

Prefix symbols (and prefixes) may be used with the unit symbols (names) L (liter),

t (metric ton), eV (electronvolt), and u (unified atomic mass unit) (see Tables 6 and 7).

However, although submultiples of the liter such as mL (milliliter) and dL (deciliter) are in

common use, multiples of the liter such 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 the kilogram (kg), are not. Examples of the use of prefix symbols with eV and u are 80 MeV (80 megaelectronvolts) 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 the number multiplying the unit is the numerical value of the quantity expressed in that

unit.

More formally, the value of quantity^ can be written as A = {A}[A ], where {A } is the

numerical value of ^4 when the value of A is expressed in the unit [A ]. The numerical value can therefore be written as {yl } = 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 '7/°C" instead of "/ (°C)" or "Temperature (°C)."

Similarly, an axis or column heading can be labeled "£/(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 7/(10^ K), where T is thermodynamic temperature and K is the unit 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 7/(10^ K) = 3.2 or T = 3200 K. Notice the

lack of ambiguity in this form of labeling compared with "Temperature (10^ K)."

3 An expression such as ln(/?/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.

2 An alternative way of writing c /(m/s) is {c}m/s, meaning the numerical value

of c when c is expressed in the unit m/s.

15
Guide for the Use of the International System of Units (SI)

7.2 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 and a 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 plane angle: ', and ", respectively (see Table 6), in which case no space is left between

the numerical value and the unit symbol.

Example: a = 30°22'8"

Note : a is a quantity symbol for plane angle.

This rule means that:

(a) The symbol °C for the degree Celsius is preceded by a space when one expresses the

values of Celsius temperatures.

Example: f = 30.2°C but not: f = 30.2°C or r=30.2°C

(b) Even when the value of a quantity is used in an adjectival sense, a space is left between

the numerical value and the unit symbol. (This rule recognizes that unit symbols are not like

ordinary words or abbreviations 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 — see Sees.

7.6 and 7.10.3.)

Examples: aim end gauge but not: a 1-m end gauge

a lOkn resistor but not: a lO-kH resistor

However, if there is any ambiguity, the words should be rearranged accordingly. For example, the statement "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: I = 10.234 m but not: I = 10 m 23 cm 4 mm

Note : 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

22.20° rather than 22° 12', except in fields such as cartography and astronomy.

7.4 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 in order to provide information about the quantity or its conditions of measurement. Instead, the letters or other symbols should be attached to the quantity.

Example: Fmax= 1000V but not: K = 1000Vmax

Note : F is a quantity symbol for potential difference. !

I

16
Guide for the Use of the International System of Units (SI)

7.5 Unacceptability of mixing information with units

When one gives the value of a quantity, any information concerning the quantity or its conditions of measurement must be presented in such a way as not to be associated with the

unit. This means that quantities 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 x 10'°/cm^ but not: the sensitivity is 5 x 10*"

NO3 molecules/cm^

the neutron emission rate is 5 x 10 7s but not: the emission rate is 5 X lO'^n/s

the number density of O2 atoms is 3 x 10'*/cm^ but not: the density is 3 x 10'* O2

atoms/cm^

the resistance per square is 100 fl but not: the resistance is 100 fl/

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 as broad an audience as possible, including readers

with limited knowledge of English. Thus, to promote the comprehension of quantitative information in general and its broad understandability in particular, values of 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 a but not: the sample was annealed at a temperature

temperature of 955 K for 12 h of 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 "the length of the laser is five m."

17
Guide for the Use of the International System of Units (SI)

3 Occasionally, a value is used in a descriptive or literary manner and it is fitting to use the spelled-out name of the unit rather than its symbol. Thus this Guide

considers acceptable statements such 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 for their

camera."

4 The United States Government Printing Office Style Manual (Ref. [4], pp. 165-171)

gives the rule that symbols for numbers are always to be used when one expresses

(a) the value of a quantity in terms of a unit of measurement, (b) time (including

dates), and (c) an amount of money. This publication should be consulted for the

rules governing the choice between the use of symbols for numbers and the

spelled-out names of numbers when numbers are dealt with in general.

7.7 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, to avoid possible confusion, this Guide takes the position that values of quantities must

be written so that it is completely 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 of values 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

51mm X 51mm X 25mm but not: 51 x 51 x 25mm

225 nm to 2400 nm or (225 to 2400) nm but not: 225 to 2400 nm

0 °C to 100 °C or (0 to 100) °C but not: 0 °C - 100 °C

0 V to 5 V or (0 to 5) V but not: 0 - 5 V

(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

129s - 3s = 126s or (129 - 3) s = 126s but not: 129- 3s = 126s

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 not abbreviations).

Examples

there are 10* mm in 1 km but not: there are many mm in a km

it is sold by the cubic meter but not: it is sold by the m^

t/°C, E/(V/m),p/MPa, and the like are perfectly acceptable (see Sec. 7.1)

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

7.9 Choosing SI prefixes

The selection of the appropriate decimal multiple or submultiple of a unit for expressing

the value of a 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, and — 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 / = 1200 m, it is not possible to tell whether the last two zeroes are significant or only indicate the magnitude of the numerical value of /. However, in the expression

/ = 1.200 km, which uses the SI prefix symbol for 10^ (kilo, symbol k), the two zeroes are assumed to be significant because if they were not, the value of / would have been written

/ = 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 x lO'^Hz may be written as 33 x 10* Hz = 33 MHz

0.009 52 g may be written as 9.52 x 10"^ g = 9.52 mg

2703 W may be written as 2.703 x 10^ W = 2.703 kW

5.8 X 10"^ m may be written as 58 x 10"' 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 x 3 mm x 0.02 mm" rather than "the size of

the sample is 1 cm x 3 mm x 20 jxm."

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 x 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. 4.3).

19
Guide for the Use of the International System of Units (SI)

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: fir = 1.2 x 10"* but not: /ir = 1.2 |x

Note : iJLc is the quantity symbol for relative permeability.

7.10.2 %, percentage by, fraction

In keeping with Ref. [6: ISO 31-0], this Guide takes the position that it is acceptable to use the internationally recognized symbol % (percent) for the number 0.01 with the SI and

thus to express the values of quantities of dimension one (see Sec. 7.14) with its aid. When

it is used, a space is left between the symbol % and the number by which it is multiplied [6:

ISO 31-0]. Further, in keeping with Sec. 7.6, the symbol % should be used, not the name

"percent."

Example

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

Note: 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 by mass," "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)." The preferred 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 "(ps = 0.35," or "<pb = 35 %" (^b 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-substance fraction is 15 %," or "xb = 0.15," or "xb = 15 %." Mass fraction, volume fraction, and amount-of-substance fraction of B may also be expressed as in the

following examples: wb = 3 g/kg; (Pb - 6.7 mL/L; xb = 185 |xmol/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 i? i and R2 differ by 0.05 %," or "where the

resistance Ri exceeds the resistance R2 by 0.05 %." Instead, one should write, for example,

"where Ri = Riil + 0.05 %)," or define a quantity A via the relation A = (Ri - /?2)//?2

and write "where A = 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 kCl reference standard in 1994 was 0.002 %."

7.10.3 ppm, ppb, and ppt

In keeping with Ref. [6: ISO 31-0], this Guide takes the position that the languagedependent 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 of quantities. Forms such as those given

in the following examples should be used instead.

20
Guide for the Use of the International System of Units (SI)

Examples :

a stability of 0.5 (jjLA/A)/min but not: a stability of 0.5 ppm/min

a shift of 1.1 nm/m but not: a shift of 1.1 ppb

a frequency change of 0.35 x 10 ' / 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 U, = 3 iiCl/fl

or

the expanded uncertainty of the resistance R is U = 3 x 10 ^R

or

the relative expanded uncertainty of the resistance 7? is i7r = 3 x 10 *

but not:

the relative expanded uncertainty of the resistance R is Ur = 3 ppm

Because the names of numbers 10' and larger are not uniform worldwide, it is best that

they be avoided entirely (in most countries, 1 billion = 1 x 10 not 1 x 10' 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, is that it is inappropriate to use

abbreviations that are language dependent together with internationally recognized signs

and symbols, such as MPa, In, 10", and %, to express the values of quantities and in equations 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 a law or a regulation. Under these circumstances. Sees. 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 10^ 10\ and 10^, respectively.

7.11 Quantity equations and numerical-value equations

A quantity equation expresses a relation among quantities. An example is / = vt, where

/ is the distance a particle in uniform motion with velocity v travels in the time t.

Because a quantity equation such as / = vt is independent of the units used to express

the values of the quantities that compose the equation, and because /, v, and / represent

quantities and not numerical values of quantities, it is incorrect to associate the equation

with a statement such as "where / is in meters, v is in meters per second, and t is in seconds."

On the other hand, a numerical value equation expresses a relation among numerical

values of quantities and therefore does depend on the units used to express the values of the

quantities. For example, {/}„, = 3.6"' {v}kin/h {^}s expresses the relation among the numerical

values of /, v, and t only when the values of /, v, 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.)

21
Guide for the Use of the International System of Units (SI)

An alternative way of writing the above numerical value equation, and one that is preferred because of its simplicity and generality, is / /m = 3.6"' [v/(km/h)](f/s). NIST authors

should consider using this preferred form instead of the more traditional form "/ = 3.6'^ vt,

where / is in meters, v is in kilometers per hour, 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, "/* = 3.6"' v* r*, where /* is the numerical value of

the distance / travelled by a particle in uniform motion when / is expressed in meters, v* is the numerical value of the velocity v of the particle when v is expressed in kilometers per

hour, and t* is the numerical value of the time of travel t of the particle when t is expressed

in seconds." Clearly, as is done 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, it should 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 an electrolytic solution:

£g/eV = 1.425 - 1.337x: + 0.nOx\ O^x ^ 0.15,

where ;c is an appropriately defined amount-of-substance fraction (see Sec. 8.6.2).

K/(S/cm) = 0.065 135 + 1.7140 x 10-'(r/°C) + 6.4141 x 10-'(r/°C)' -

4.5028 X 10"*(r/°C)^ 0 °C ^ / ^ 50 °C, 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 units of the SI.

7.12 Proper names of quotient quantities

Derived quantities formed from other quantities by division are written using the words

"divided by" rather than the words "per unit" in order to avoid the appearance of associating a particular unit with the derived quantity.

Example : pressure is force divided by area but not : pressure is force per unit area

7.13 Distinction between an object and its attribute

To avoid confusion, when discussing quantities or reporting their values, one should

distinguish between 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

(/), mass (m), time (t), electric current (/), thermodynamic temperature (7), amount of substance (n ), and luminous intensity (h) by an equation of the form

^ j

I K

* = 1

22
Guide for the Use of the International System of Units (SI)

where the exponents a, P, y, . . . are numbers and the factors a* are also numbers. The

dimension of Q is defined to be

dimg = L^M^TM^e^NfJ" ,

where L, M, T, I, 9, 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 a, fi,y, . . . are called "dimensional exponents." The SI derived

unit of is m" • kg^ • s^ • A* • • moK* 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 / in a time f . Its velocity is v =l/t and its kinetic energy is Et = =

l^mt ~^/2. The dimension of Ev is dim Ek = L^MT~^ and the dimensional exponents are 2, 1, and -2. The SI derived unit of Ek is then m^ • kg • s"^ which is given the special name "joule" and special symbol J.

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 for such 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 = m^lm,

where /tzb is the mass of B and m is the mass of the mixture (see Sec. 8.6.10).

The dimension of is dim we = M'M = 1; all of the dimensional exponents

of Wb are zero, and its derived unit is kg' • kg"' = 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 technical calculations. When time relates to calendar cycles, the minute (min), hour (h), and

day (d) may be necessary. For example, the kilometer per hour (km/h) is the usual unit for

expressing vehicular speeds. Although there is no universaally accepted symbol for the year,

Ref. [6: ISO 31-1] suggests the symbol a.

The rotational frequency /j of a rotating body is defined to be the number of revolutions

it makes in a time interval divided by that time interval [6: ISO 31-5]. The SI unit of this

quantity is thus the reciprocal second (s~'). However, as pointed out in Ref. [6: ISO 31-5],

the designations "revolutions per second" (r/s) and "revolutions per minute" (r/min) are widely used as units for rotational frequency in specifications on rotating machinery.

8.2 Volume

The SI unit of volume is the cubic meter (m^) 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 (dm^) but the CGPM recommends that the liter not be used to give the results

of high accuracy measurements of volumes [2, 3]. 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 (6)].

23
Guide for the Use of the International System of Units (SI)

8.3 Weight

In science and technology, the weight of a body in a particular reference frame is defined as the force that gives the body an acceleration equal to the local acceleration of free

fall in that reference frame [6: ISO 31-3]. Thus the SI unit of the quantity weight defined

in this way is the newton (N). When the reference frame is a celestial object, Earth for ex- ample, the weight of a body is commonly called the local force of gravity on the body.

Example : 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.

Note : The local force of gravity on a body, that is, its weight, consists of the resultant of

all the gravitational forces acting on the body and the local centrifugal force due to the rotation of the celestial object. The effect of atmospheric buoyancy is usually

excluded, and thus the weight of a body 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 a synonym for mass. Thus the SI unit of the quantity weight used in this sense is the

kilogram (kg) and the verb "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 Net wt. 227 g

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, it should 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 have been replaced by the equivalent but preferred terms relative atomic mass, symbol

Ar, and relative molecular mass, symbol Mr, respectively [6: ISO 31-8], which better reflect

their definitions. Like atomic weight and 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 [6: ISO 31-8]:

Relative atomic mass (formerly atomic weight): ratio of the average mass per atom of an element to 1/12 of the mass of the atom of the nuclide '^C.

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 '^C.

Examples: Ar(Si) = 28.0855 A/r(H2) = 2.0159 ArC^C) = 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

AriX) = m(X)/[m ('^C)/12] and Mr(B) = m(B)/[m(''C)/12], where m(X) is the mass of X, m (B) is the mass of B, and m ('^C) is the mass of an atom of the

nuclide '^C. It should also be recognized that m('^C)/12 = u, the unified atomic mass unit, which is approximately equal to 1.66 x 10 ~" kg [see Table 7, footnote

2 It follows from the examples and note 1 that the respective average masses of Si,

H2,and*2Carem(Si) = ylr(Si) u, m(H2) = Mr(H2) u, andmC^C) =^r(''C)u.

24

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

3 In publications dealing with mass spectrometry, one may encounter a statement

such as "the mass-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 the symbol miz. For

example, the mass-to-charge ratio of the ion '^Ct'H?** 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 (7) by the equation t = T — To, where To = 273.15 K by definition.

This implies that the numerical value of a given temperature interval or temperature differ- ence whose value is expressed in the unit degree Celsius (°C) is equal to the numerical value

of the same interval or difference when its value is expressed in the unit kelvin (K); or in the

notation of Sec. 7.1, note 2, {A?}«c = {ATIk. Thus temperature intervals or temperature

differences may be expressed in either the degree Celsius or the kelvin using the same numerical value.

Example : The difference in temperature between the freezing point of gallium and the

triple point of water is At = 29.7546 °C = AT = 29.7546 K.

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, aib, ^B. However, it is generally preferable to place symbols for substances and their

states in parentheses immediately after the quantity symbol, for example n(H2S04). (For a

detailed discussion of the use of the SI in physical chemistry, see the book cited in Ref. [8],

note 5.)

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 (see Sec. 4.1 and Table 1).

2 In general, n{xB) = n{B)/x, where a; is a number. Thus, for example, if the

amount of substance of H2SO4 is 5 mol, the amount of substance of (1/3)H2S04

is 15 mol: n[(l/3)H2S04] = 3n(H2S04).

Example: The relative atomic mass of a fluorine atom is At(F) = 18.9984. The relative

molecular mass of a fluorine molecule may therefore be taken as Mr(F2) = 2/4 r(F) = 37.9968. The molar mass of F2 is then A/(F2) = 37.9968 x

10"^ 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 (alsoys). SI unit: one (1) (amount-of-substance fraction is a quantity

of dimension one).

Definition : ratio of the amount of substance of B to the amount of substance of the mixture:

Xb — Ws/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^")

Amount of substance

Symbol: n u

0w SI unit: mol

es0

s

0

c

Volume

Symbol: V

C

tity

i

SI unit: m'

C

R

Mass

c

Symbol: m

SI unit: kg

Quantity in numerator

Amount of substance

Symbol: n

SI unit: mol

Volume

Symbol: V

SI unit: m^

Mass

Symbol: m

SI unit: kg

amount-of-substance

fraction

n

SI unit: mol/mol = 1

molar volume

SI unit: m-'/mol

molar mass

M = ^

n

SI unit: kg /mol

amount-of-substance

concentration

rCBy

SI unit: moi/m'

volume fraction

XBVm,B

SI unit: mVm^ = 1

mass density

m

p = v

SI unit: kg/m'

molality

Ob = —niA

SI unit: mol/kg

specific volume

m

SI unit: m^/kg

mass fraction

wjb Wb = —m

SI unit: kg/kg = 1

Adapted from Canadian Metric Practice Guide (see Ref. [8], note 3; the book cited in Ref. [8], note

5, may also be consulted).

Notes

1 This quantity is commonly called "mole fraction of B" but this Guide prefers the

name "amount-of-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, n = ha + + nc +

... = X "a.

A

3 A related quantity is amount-of-substance ratio ofB (commonly called "mole ratio

of solute B"), symbol rs. It is the ratio of the amount of substance of B to the

amount of substance of the solvent substance: tb = ns/ns. For a single solute C

in a solvent substance (a one-solute solution), rc = xc/(^ - Xc). This follows

from the relations n = nc + ns, xc = nc/n, and rc = nc/ns, where the solvent

substance S can itself be a mixture.

8.6.3 Molar volume

Quantity symbol: Vm- SI unit: cubic meter per mole (m^/mol).

Definition : volume of a substance divided by its amount of substance: Vm = VIn

.

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. (One amagat is the molar volume Vm of a real gas atp = 101 325 Pa

and T = 273.15 K and is approximately equal to 22.4 x 10"^ mVmol. The name

"amagat" is also given to 1/Fm of a real gas atp = 101 325 Pa and T = 273.15 K

and in this case is approximately equal to 44.6 mol/m^.)

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) X 10-3 kg/mol ^ Mr(B) kg/kmol = g/mol, where A/r(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) x IQ-^ kg/mol = ^^(X) kg/kmol = y4r(X) g/mol,

where y4r(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/m^).

Definition: amount of substance of B divided by the volume of the mixture: cb = ns/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 even simply to concentration of B. Unfortunately,

this last form can cause confusion because there are several different "concentrations," for example, mass concentration of B, pb = m^lV; and molecular concentration of B, Cb = Nb/V, where A^b is the number of molecules of B.

2 The term normality and the symbol should no longer be used because they are

obsolete. One should avoid writing, for example, "a 0.5 A'^ solution of H2SO4"

and write instead "a solution having an amount-of-substance concentration of

c[(l/2)H2S04]) = 0.5 mol/dm^" (or 0.5 kmol/m^ or 0.5 mol/L since 1 mol/dm^ = 1 kmol/m^ = 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/dm\ kmol/m^ or mol/L. (A solution of, for example, 0.1

mol/dm^ 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: (ps- SI unit: one (1) (volume fraction is a quantity of dimension one).

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

.

,

^B Vm,B

<Pb = , 2 -^A V*,A

27
Guide for the Use of the International System of Units (SI)

where aja, xb, xc, ... are the amount-of-substance fractions of A, B, C, . .

. , Vm.A, Vm,B,

Vm,c, ... are the molar volumes of the pure substances A, B, C, . . . at the same temperature

and pressure, and where the summation is over all the substances A, B, C, . . . so that

2;ca = 1.

8.6.7 Mass density; density

Quantity symbol: p. SI unit: kilogram per cubic meter (kg/m^).

Definition: mass of a substance divided by its volume: p = 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, p = Q/V.

2 Mass density is the reciprocal of specific volume (see Sec. 8.6.9): p = 1/v.

8.6.8 Molality of solute B

Quantity symbol: bs (also me). SI unit: mole per kilogram (mol/kg).

Definition : amount of substance of solute B in a solution divided by the mass of the solvent:

bs = rtB/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: v. SI unit: cubic meter per kilogram (mVkg).

Definition : volume of a substance divided by its mass: v = V/m

.

Note: Specific volume is the reciprocal of mass density (see Sec. 8.6.7): v = 1/p.

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, bel

This section briefly introduces logarithmic quantities and units. It is based on Refs.

[6: ISO 31-2] and [7: lEC 27-3], which should be consulted for further details.

Two of the most common logarithmic quantities are level of a 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 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 = \n(F/Fo), where F/Fo is the

ratio of two amplitudes of the same kind, Fo being a reference amplitude. Level of a power

quantity is defined by the relation Lp = (1/2) \n{P/Po), where P/Po is the ratio of two

powers, Po being a reference power. (Note that if P/Po = (F/Fo)^ then Lp = Lf.) Similar

28
Guide for the Use of the International System of Units (SI)

names symbols, and definitions apply to levels based on other quantities 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 = HE/Eo).

The difference between two levels of a field quantity (called "field level difference")

having the same reference amplitude FoisALf = Lf, — Lf2 = ln(Fi/Fo) — ln(F2/Fo) =

ln(Fi/F2), and is independent of Fo. This is also the case for the difference between two

levels of a power quantity (called "power level difference") having the same reference power

Po: ALp = Lp, - Lp, = ln(Pi/Fo) - ln(F2/Fo) = ln(Fi/F2).

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 the radian (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/Fo = e, that is, when

ln(F/Fo) = 1. Equivalently, 1 Np is the level of a power quantity when P IPo = e^ that is, when (1/2) ln(F/Fo) = 1. These definitions imply that the numerical value of Lf when Lf is expressed in the unit neper is {Lf}Np = ln(F/Fo), and that the numerical value of Lp when

Lp is expressed in the unit neper is {L/>}np = (1/2) ln(F/Po); that is

Lp = ln(F/Fo) Np

Lp = (l/2)ln(F/Fo) Np .

One bel (1 B) is the level of a field quantity when F/Fo = V^. that is, when

2 lg(F/Fo) = 1 (note that \gx - \ogwX — see Sec. 10.1.2). Equivalently, 1 B is the level of

a power quantity when F/Fo = 10, that is, when lg(F/Fo) = 1. These definitions imply that

the numerical value of Lf when Lf is expressed in the unit bel is {Lf}b = 2 \g(F/Fo) and that

the numerical value of Lp when Lp is expressed in the unit bel is {L/>}b = lg(F/Fo); that is

Lf = 2 lg(F/Fo) B = 20 lg(F/Fo) dB

Lp = lg(F/Fo) B = 10 lg(F/Fo) 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/Fo)Np = 21g(F/Fo)B, which

implies

In 10

1 B = Np exactly

« 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. [7: lEC 27-3], this may be done in one of two ways: L^(rexref) or Lx/x,^„ where

X is the quantity symbol for the quantity whose level is being reported, for example, electric

field strength E or sound pressure p; and jCref is the value of the reference quantity, for

example, 1 piV/m for Eo, and 20 ^.Pa for po. Thus

L£(re 1 |xV/m) = - 0.58 Np or Lf/d^v/m) = - 0.58 Np

29
Guide for the Use of the International System of Units (SI)

means that the level of a certain electric field strength is 0.58 Np below the reference electric

field strength Eo = 1 p-V/m. Similarly

Lp (re 20 (xPa) = 25 dB or Lp/2o^Pa = 25 dB

means that the level of a certain sound pressure is 25 dB above the reference pressure

po = 20 n,Pa.

Notes :

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 |xV/m); 25 dB (20 |xPa).

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. [7: lEC 27-3] preclude, for example, the use of the symbol dBm

to indicate a reference 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.

8.8 Viscosity

The proper SI units for expressing values of viscosity 77 (also called dynamic viscosity)

and values of kinematic viscosity v are, respectively, the pascal second (Pa • s) and the meter

squared per second (mVs) (and their decimal multiples and submultiples as appropriate).

The CGS units commonly used to express values of these quantities, the poise (P) and

the stokes (St), respectively [and their decimal submultiples the centipoise (cP) and the

centistoke (cSt)], are not to be used; see Sec. 5.3.1 and Table 10, which gives the relations

IP = 0.1 Pa -s and 1 St = lO'^mVs.

8.9 Massic, volumic, areic, lineic

Reference [6: 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 "lineique." They are convenient and NIST authors

may wish to use them. They are equivalent, respectively, to "specific," "density,"

"surface . . . density," and "linear . . . density," as explained below.

(a) The adjective massic, or the adjective specific, is used to modify the name of a quantity

to indicate the quotient of that quantity and its associated mass.

Examples: massic volume or specific volume: v = Vim

massic 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, to indicate the quotient of that quantity and its associated volume.

Examples: volumic mass or (mass) density: p = m/V

volumic number or number density: n = N/V

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: pA = m/A

areic charge or surface charge density: a = Q/A

30
Guide for the Use of the International System of Units (SI)

(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 : Hneic mass or linear (mass) density: pi = mil

lineic electric current or linear electric current density: A = I lb

9 Rules 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 all units start with a lower-case letter, except at the beginning of a sentence or in

capitalized material such as a title.

In keeping with this rule, the correct spelling of the name of the unit °C is "degree

Celsius" (the unit "degree" begins with a lower case "d" and the modifier "Celsius" begins

with an upper-case "C" because it is the name of a person).

9.2 Plurals

Plural unit names are used when they are required by the rules of English grammar.

They are normally formed regularly, for example, "henries" is the plural of henry. According

to Ref. [8], the following plurals are irregular: Singular — lux, hertz, Siemens; Plural — lux,

hertz, Siemens. (See also Sec. 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 the prefix and unit name (see Sec. 6.2.3).

Examples: milligram but not: milli-gram kilopascal but not: kilo-pascal

Reference [8] points out that there are three cases where the final vowel of an SI prefix

is commonly omitted: megohm {not megaohm), kilohm {not kiloohm), and hectare {not

hectoare). In all other cases where the unit name begins with a vowel, both the final vowel

of the prefix and the vowel of the unit 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. [8] and this Guide , or a hyphen is used to separate

the names of the individual units.

Example: pascal second or pascal-second

9.5 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 used and not a solidus. (See also Sees. 6.1.7 and 9.8.)

Example: ampere per meter (A/m) but not: ampere/meter

31
Guide for the Use of the International System of Units (SI)

9.6 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/s^)

The modifiers "square" or "cubic" may, however, be placed before the unit name in the case of area or volume.

Examples: square centimeter (cm^) cubic millimeter (mm^)

ampere per square meter (A/m^) kilogram per cubic meter (kg/m^)

9.7 Other spelling conventions

A derived unit is usually singular in English, for example, the value 3 m^ • K/W is usually

spelled out as "three square meter kelvin per watt," and the value 3 C • m^/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 is less than one, the unit

is always singular when spelled out; for example, 0.5 kPa is spelled out as "five-tenths

kilopascal."

Note: 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 of quantities, not the spelled-out names of numbers and units.

Reference [4] also requires that a symbol for a number be used whenever the value

of a quantity is expressed in terms of a unit of measurement.

9.8 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 Sees. 6.1.7 and 9.5.)

Example: joule per kilogram or J/kg or J -kg"' but not: joule/kilogram or

joule • kilogram ~

10 More on Printing and Using Symbols and Numbers in Scientific and

Technical Documents*

By following the guidance given in this chapter, NIST authors can prepare manuscripts

that are consistent with accepted typesetting practice.

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 which 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 for the chemical el- ements, certain mathematical symbols, and modifying superscripts and subscripts on

quantity symbols.

^ This chapter is adapted in part from Refs. [5], [6: ISO 31-0], and [6: ISO 31-11].

32
Guide for the Use of the International System of Units (SI)

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 physical sciences and technology. Many of these

are given in Refs. [6] and [7], and it is likely that symbols for the quantities 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. [6] and [7] to the fullest

extent possible.'

n (solid angle) Zm (mechanical impedance)

Lp (level of a power quantity) 4r (relative mass excess)

p (pressure) CTtot (total cross-section)

Kt (isothermal compressibility) Eu (Euler number)

E (electric field strength) Tn (Neel temperature)

10.1.2 Standardized mathematical signs and symbols

As is the case for quantity symbols, most of the mathematical signs and symbols used

in the physical sciences and technology are standardized. They may be found in Ref.

[6: ISO 31-11] and should be used by NIST authors to the fullest possible extent.'

Examples : a (conjunction sign, p /\ q means p and q )

(a ^ b, a is not equal to b) ^ {a b, a is by definition equal to b)

(a ~ b, a is approximately equal to b) ~ (a — b, a is proportional to b)

arcsin x (arc sine ofx)

logaX (logarithm to the base aofx)

Ibx (\hx = \0g2x)

\nx {\nx = \ogeX)

\gx (\gx = logiox)

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 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 .

^ In addition to Refs. [6] and [7], quantity symbols may 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. [6: ISO 31-11], mathematical signs and symbols are also given in ANSI/IEEE Std 260.3-1993, Mathematical

Signs and Symbols for Use in Physical Sciences and Technology. (See Ref. [8], note 1.)

33
Guide for the Use of the International System of Units (SI)

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 variables and quantities : 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 / in XiXi that represents a number (see Sees. 10.2.1, 10.2.3, and 10.2.4). An index that

represents a number is also called a "running number" [6: ISO 31-0].

Notes

1 The above rules also imply, for example, that |x, the symbol for the SI prefix

micro (10"*), that H, the symbol for the SI derived unit ohm, and that F, the

symbol for the SI derived unit farad, are in roman type; but they are in italic type

if they represent quantities (/a, O, and F are the recommended 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 examples of the proper typefaces for these three major

categories.

10.2.1 Quantities — italic

Symbols for quantities are italic, as are symbols for functions in general, for example, /(a:):

r = 3 s t time, s second T = 22 K T temperature, K kelvin

r = llcm r radius, cm centimeter A=633 nm A 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 Avogadro constant, A Avogadro R molar gas constant

6d Debye temperature, D Debye Z atomic number

e elementary charge me m mass, e electron

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

m y = ^ XiZi x^ = ay^ + bz^

1=1

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

symbols for matrices are italic:

A'B=C (vectors) T (tensors) A = ^^^) (matrices)

Symbols used as subscripts and superscripts are italic if they represent quantities:

Cp p pressure qm m mass an O solid angle

10.2.2 Units — roman

The symbols for units and SI prefixes are roman:

m meter g gram L liter cm centimeter |xg microgram mL milliliter

34

1
I

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

10.2.3 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,

it) and symbols representing explicitly defined functions or well defined operators (for example, r(x) or div):

Chemical elements:

Ar argon B boron C carbon

Mathematical constants, functions, and operators:

e base of natural logarithms X;c, X sum of

expx exp exponential of \ogaX loga logarithm to the base a of

dx/dt d 1st derivative of sinj: sin sine of

Symbols used as subscripts and superscripts are roman if descriptive:

Eo"' ir irrational Ek k kinetic

Vm m molar, 1 liquid phase /xb B Bohr

10.2.4 Sample equations showing correct type

qi qi F = F = ma

<PB = :cbK',b/X;caF:.a = RT' d(ln k)/dT

A

E = mc^ pB = Ab lini(xB/?/AB)

p-*0

10.3 Greek alphabet in roman and italic type

Table 13 shows the proper form, in both roman and

lower-case letters of the Greek alphabet.

Table 13. Greek alphabet in roman and italic type

alpha A OL A a

beta B B

gamma r 7 r y

delta A 5 A s

epsilon E e, € E e, €

zeta z Z

eta H H V

theta e e, ^^"^ 0 9, d*"'

iota I L I kappa K

(a) K, y. K

lambda A k A A

mu M M A* nu N V N V

xi ^

omicron O o O o

pi n IT, XH n 77, VJ rho p P. q"" p

sigma 2 a X a

tau T T T T

upsilon Y V Y V

phi * <P

chi X X X X

psi

omega ft n to

ISO (see Ref. [6: ISO 31-0]) gives these two letters in the reverse order.

pV = nRT

c, = A-V[exp(c2/Ar)-l]

^= -grad V

italic type, of the upper-case and

35
Guide for the Use of the International System of Units (SI)

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 the surrounding 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: ^Si.

The number of atoms in a molecule of a particular nuclide is shown in the right subscript

position: 'H2.

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 of which are as follows:

State of ionization: Ba ^ Co(N02)6"-" or Co(N02)i- or [Co(N02)6]'"

Electronic excited state: Ne*, CO*

Nuclear excited state: '^N* or '^NTM

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 in lightface (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 bold roman 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 [4, 8]. For numbers less than one, a zero is written before the decimal marker. For ex- ample, 0.25 s is the correct form, not .25 s.

36
Guide for the Use of the International System of Units (SI)

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 decimal marker except when uniformity in a table is desired.

Examples: 76 483 522 but not: 76,483,522

43 279.168 29 but not: 43,279.168 29

8012 or 8 012 but not: 8,012

0.491722 3 is highly preferred to: QA9\llli

0.5947 or 0.594 7 but not: 0.59 47

8012.5947 or 8 012.594 7 but not: 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.

10.5.4 Multiplying numbers

When the dot is used as the decimal marker as in the United States, the preferred sign

for the multiplication of numbers or values of quantities is a cross (that is, multiplication

sign) ( X ), not a half-high (that is, centered) dot (

• ).

Examples: 25 x 60.5 but not: 25 - 60.5

53m/s X 10.2 s but not: 53 m/s- 10.2 s

15 X 72 kg but not: 15 • 72 kg

Notes

:

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 in parentheses) may be indicated in one of the following ways: ab,

a b, O'b, a X b.

37
Guide for the Use of the International System of Units (SI)

11 Check List for Reviewing Manuscripts

The following check list, adapted from Ref. [22], is intended to help NIST authors re- view the conformity of their manuscripts with proper SI usage and the basic principles con- cerning quantities and units. For easy reference, this check list also appears immediately

after the Preface.

(1) CH Only units of the SI and those units recognized for use with the SI are used to express the values of quantities. 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) O Abbreviations such as sec (for either s or second), cc (for either cm^ or cubic

centimeter), or mps (for either m/s or meter per second), are avoided and only

standard unit symbols, SI prefbc symbols, unit names, and SI prefbces are used.

(See Sec. 6.1.8.)

(3) The combinations of letters "ppm," "ppb," and "ppt," and the terms part per million, part per billion, 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 |xL/L

or 2.0 X 10~* V, 4.3 nm/m or 4.3 x 10"' /, 7 ps/s or 7 x 10"'^ t, where V, I, and

t are, respectively, the quantity symbols for volume, length, and time. (See Sec.

7.10.3.)

(4) n Unit symbols (or names) are not modified by the addition of subscripts or other

information. The following forms, for example, are used instead. (See Sees. 7.4

and 7.10.2.)

K„ax = 1000 V but not: V = 1000 V„ax

a mass fraction of 10 % but not: 10 % (m/m) or 10 % (by weight)

(5) EH Statements such as "the length /i exceeds the length 1 2 by 0.2 %" are avoided be- cause it is recognized that the symbol % represents simply the number 0.01. Instead, forms such as "/i = hil + 0.2 %)" or "A = 0.2 %" are used, where A is defined by the relation A = (li - Iz)/^- (See Sec. 7.10.2.)

(6) Information is not mixed with unit symbols (or names). For example, the form

"the water content is 20 mL/kg" is used and not "20 mL H20/kg" or "20 mL of

water/kg." (See Sec. 7.5.)

(7) n 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. (See Sec. 7.7.)

35 cm X 48 cm but not: 35 x 48 cm

1 MHz to 10 MHz or (1 to 10) MHz but not : 1 MHz - 10 MHz or 1 to 10 MHz

20 °C to 30 °C or (20 to 30) °C but not : 20 °C - 30 "C or 20 to 30 °C

123g ± 2g or (123 ± 2)g but not: 123 ± 2g

70 % ± 5 % or (70 ± 5) % but not: 70 ± 5 %

240 X (1 ± 10%)V but not: 240 V ±10% (one cannot add

240 V and 10 %)

(8) O Unit symbols and unit names are not mixed and mathematical operations are not

applied to unit names. For example, only forms such as kg/m^ kg-m"-', or kilogram per cubic meter are used and not forms such as kilogram/m^ kg/cubic

meter, kilogram/cubic meter, kg per m\ or kilogram per meter^ (See Sees. 6.1.7,

9.5, and 9.8.)

38
Guide for the Use of the International System of Units (SI)

(9) D Values of quantities are expressed in acceptable units using Arabic numerals and

the symbols for the units. (See Sec. 7.6.)

m = 5 kg but not : m = five kilograms or m = five kg

the current was 15 A but not : the current was 15 amperes.

(10) [H There is a space between the numerical value and unit symbol, even when the

value is used in an adjectival sense, except in the case of superscript units for plane

angle. (See Sec. 7.2.)

a 25 kg sphere but not : a 25-kg sphere an angle of 2°3'4" but not: an angle of 2 °3 '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) CH The digits of numerical values having more than four digits on either side of the

decimal marker are separated into groups of three using a thin, fixed space

counting from both the left and right of the decimal marker. For example,

15 739.012 53 is highly preferred to 15739.01253. Commas are not used to separate

digits into groups of three. (See Sec. 10.5.3.)

(12) mi Equations between quantities are used in preference to equations between numerical values, and symbols representing numerical values are different from symbols representing the corresponding 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) n Standardized quantity symbols such as those given in Refs. [6] and [7] are used,

for example, 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. [6: ISO 31-11] are used, for example,

"tan;c" and not "tgx." More specifically, the base of "log" in equations is specified when required by writing logo a: (meaning log to the base a ofx), \hx (meaning

\0g2x), \nx (meaning loge^c), or \gx (meaning logiooc). (See Sees. 10.1.1 and

10.1.2.)

(14) Unit symbols are in roman type, and quantity symbols are in italic type with superscripts and subscripts in roman or italic type as appropriate. (See Sec. 10.2 and

Sees. 10.2.1 to 10.2.4.)

(15) CD 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, weight is usually a synonym for mass, for which the SI unit is the

kilogram.) (See Sec. 8.3.)

(16) [H A quotient quantity, for example, mass density, is written "mass divided by volume" rather than "mass per unit volume." (See Sec. 7.12.)

(17) O 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) n The obsolete term normality and the symbol A^, and the obsolete term molarity

and the symbol m, are not used, but the quantity amount-of-substance concentration of B (more commonly called concentration of B), and its symbol cb and SI unit

mol/m' (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 foe or nis and SI unit mol/kg (or a related unit of the SI), are used

instead. (See Sees. 8.6.5 and 8.6.8.)

39
Guide for the Use of the International System of Units (SI)

Appendix A. Definitions of tlie SI Base Units and tlie Radian and Steradian

A.1 Introduction

The following definitions of the SI base units are taken from Ref. [3]; the definitions

of the SI supplementary units, the radian and steradian, which are now interpreted as SI

derived units (see Sec. 4.3), are those generally accepted and are the same as those given in

Ref. [8]. It should be noted that SI derived units are uniquely defined only in terms of SI

base units; for example, 1 V = 1 m^ • kg • s~^ • A"'.

A.2 Meter (17th CGPM, 1983)

The meter is the length of the path travelled by light in vacuum during a time interval of

1/299 792 458 of a second.

A-3 Kilogram (3d CGPM, 1901)

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the

kilogram.

A4 Second (13th CGPM, 1967)

The second is the duration o/ 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.

A.5 Ampere (9th CGPM, 1948)

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 x 10 "^ newton per meter of length.

A.6 Kelvin (13th CGPM, 1967)

The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

A7 Mole (14th CGPM, 1971)

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 that 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 monochromatic radiation offrequency 540 x 10^^ hertz and that has a radiant intensity in that direction

of ( l/683j watt per steradian.

A.9 Radian

The radian is the plane angle between two radii of a circle that cut off on the circumference an arc equal in length to the radius.

A.10 Steradian

The steradian is the solid angle that, having its vertex in the center of a sphere, cuts off an

area of the surface of the sphere equal to that of a square with sides of length equal to the radius

of the sphere.

40
Guide for the Use of the International System of Units (SI)

Appendix B. Conversion Factors*

B.l 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 the unconverted 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 Sngstroms,

such as the wavelengths of visible laser radiations, are usually converted to values expressed

in nanometers, not meters. More generally, if desired, one can eliminate powers of 10 that

appear in converted values as a result of using the conversion factors (or simply factors for

brevity) of Sees. B.8 and B.9 by selecting an appropriate SI prefix (see Sec. B.3).

B.2 Notation

The factors given in Sees. B.8 and B.9 are written as a number equal to or greater than

1 and less than 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 which indicate the

power of 10 by which the number is multiplied.

Examples : 3.523 907 E - 02 means 3.523 907 x 10 = 0.035 239 07

3.386 389 E + 03 means 3.386 389 x 10^ = 3386.389

A factor in boldface is exact. All other factors have been rounded to the significant

digits given in accordance with accepted practice (see Sees. 7.9, B.7.2, and Refs. [6: ISO 31-0]

and [8]). Where less than six 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 Sees. B.8 and B.9 is to be interpreted as in these two examples:

To convert from to Multiply by

atmosphere, standard (atm) pascal (Pa) 1.013 25 E+05

cubic fcx)t per second (ft'/s) cubic meter per second (m'/s) 2.831 685 E-02

means 1 atm = 101 325 Pa (exactly);

1 ftVs = 0.028 316 85 mVs.

Thus to express, for example, the pressure p

(Pa), write

p = 11.8 atm X 101 325Pa/atm

= 11.8 standard atmospheres (atm) in pascals

and obtain the converted numerical value 11.8 x 101 325 = 1 195 635 and the converted

value p = 1.20 MPa.

Appendix B is a significantly revised version of Appendix C of the 1991 edition of this NIST Special Publication

(see Preface). Appendix C of the 1991 edition 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, TTie 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).

41
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 is necessary to express it in the corresponding unit of the first column, divide by the factor.

The factors for derived units not included in Sees. B.8 and B.9 can readily be found from

the factors given.

Examples : To find the factor for converting values in lb • ft/s to values in kg • m/s, obtain

from Sec. B.8 or B.9

lib = 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 x 0.3048 m/s = 0.138 255 0kg -m/s

and the factor is 1.382 550 E-01.

To find the factor for converting values in (avoirdupois) oz • in^ to values in kg • m^, obtain

from Sec. B.8 or B.9

loz = 2.834 952 E- 02 kg

1 in^ = 6.4516 E-04 m^

and substitute these values into the unit oz • in^ to obtain

1 oz • in^ = 0.028 349 52 kg x 0.000 645 16 m^ = 0.000 018 289 98 kg • m'

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 same units are listed alphabetically within the following alphabetized list of kinds of

quantities and fields of science:

ACCELERATION

ANGLE

AREA AND SECOND MOMENT

OF AREA

CAPACITY (see VOLUME)

DENSITY (that is, MASS DENSITY -

(see MASS DIVIDED BY VOLUME)

ELECTRICITY and MAGNETISM

ENERGY (includes WORK)

ENERGY DIVIDED BY AREA TIME

FLOW (see MASS DIVIDED BY TIME

or VOLUME DIVIDED BY TIME)

FORCE

FORCE DIVIDED BY AREA

(see PRESSURE)

FORCE DIVIDED BY LENGTH

HEAT

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 Insulance

Thermal Resistance

Thermal Resistivity

42
Guide for the Use of the International System of Units (SI)

LENGTH

LIGHT

MASS and MOMENT OF INERTIA

MASS DENSITY (see MASS DIVIDED

BY VOLUME)

MASS DIVIDED BY AREA

MASS DIVIDED BY CAPACITY

(see MASS DIVIDED BY VOLUME)

MASS DIVIDED BY LENGTH

MASS 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 LENGTH

PERMEABILITY

POWER

PRESSURE or STRESS (FORCE

DIVIDED BY AREA)

RADIOLOGY

SPEED (see VELOCITY)

STRESS (see PRESSURE)

TEMPERATURE

TEMPERATURE INTERVAL

TIME

TORQUE (see MOMENT

OF FORCE)

VELOCITY (includes SPEED)

VISCOSITY, DYNAMIC

VISCOSITY, KINEMATIC

VOLUME (includes CAPACITY)

VOLUME DIVIDED BY TIME

(includes FLOW)

WORK (see ENERGY)

In Sees. B.8 and B.9, the units in the left-hand columns are written as they are often

used customarily; 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 fuel economy 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 the inch). From 1893 until 1959, the yard was defined as being exactly equal to (3600/

3937) m, and thus the foot 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 other countries into agreement. Since then the yard has been defined as exactly equal to 0.9144 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 to bear 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 international foot = 0.999 998

U.S. survey foot exactly.

43
Guide for the Use of the International System of Units (SI)

In Sees. B.8 and B.9, the factors given are based on the international foot unless otherwise indicated. Users of this Guide may also find the following summary of exact relationships helpful, where for convenience 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.

1ft = (1200/3937)

m

1 ft = 0.3048 m

1 ft = 0.999 998 ft

1 rd, pole, or perch = 16jft

40 rd = 1 fur = 660 ft 8 fur = 1 U.S. survey mile (also called "statute mile") = 1 mi = 5280 ft

1 fathom = 6ft

1 international mile = 1 mi = 5280 ft 2721/4yf' = Ird'

160 rd^ = 1 acre = 43 560

640 acre = Imi^

B.7 Rules for rounding numbers and converted numerical values of quantities

Rules for rounding numbers are discussed in Refs. [6: ISO 31-0] and [8]; the latter ref- erence also gives rules for rounding the converted numerical values of quantities whose val- ues expressed in units that are 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, and the 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 : 6.974 951 5 rounded to 3 digits is 6.97

(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 : 6.91A 951 5 rounded to 2 digits is 7.0

6.974 951 5 rounded to 5 digits is 6.9750

(3) 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: 6.91A 951 5 rounded to 7 digits is 6.974 952

6.974 950 5 rounded to 7 digits is 6.974 950

44
Guide for the Use of the International System of Units (SI)

B.7.2 Rounding converted numerical values of quantities

The use of the factors given in Sees. B.8 and B.9 to convert values of quantities was

demonstrated in Sec. B.3. In most cases the product of the unconverted numerical value and

the factor will be a numerical value 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 the number of significant digits

that is consistent with the maximum possible rounding error of the unconverted numerical

value.

Example : To express the value / = 36 ft in meters, use the factor 3.048 E—01 from Sec.

B.8 or Sec. B.9 and write

/ = 36 ft X 0.3048 m/ft = 10.9728 m = 11.0 m.

The final result, / = 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 for simplicity) 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 three times smaller than 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 larger than the ± 1.4 % RE of the unconverted numerical value "36." This

example therefore shows that when selecting the number 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 the converted value can

often help one decide which choice to make.

Note: Consider that one had been told initially that the value / = 36 ft had been rounded

to the nearest inch. Then in this case, since / is known to within ± 1 in, the RE of

the numerical value "36" is ± 1 in/(36 ft x 12 in/ft) = ± 0.23 %. Although this is

less than the ± 0.45 % RE of the number 11.0, it is comparable to it. Therefore, the

result / = 11.0 m is still given as the converted value. (Note that the numerical value

"10.97" would give excessive unwarranted information because it has an RE that is 5 times smaller than ± 0.23 %.)

45
Guide for the Use of the International System of Units (SI)

B.8 Factors for units listed alphabetically

Caution: The units listed in column 1 are in general not to be used in NIST publications,

with the exception of those few in italic type.

Factors in boldface are exact

To convert from to Multiply by

abampere 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 (O) 1.0 E-09

abvolt volt (V) 1.0 E-08

acceleration of free fall, standard (gn) meter per second squared (m/s^) 9.806 65 E+00

acre (based on U.S. survey foot)' square meter (m^) 4.046 873 E + 03

acre foot (based on U.S. survey foot)' cubic meter (m^) 1.233 489 E + 03

ampere hour (A • h) coulomb (C) 3.6 E+03

Sngstrom (A) meter (m) 1.0 E—10

Angstrom (A) nanometer (nm) 1.0 E—01

are (a) square meter (m^) 1.0 E+02

astronomical unit (AU) 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)^° pascal (Pa) 9.806 65 E+04

atmosphere, technical (at)'° 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 (m^) 1.0 E-28

barrel [for petroleum, 42 gallons (U.S.)](bbl). . .cubic meter (m^) 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 unitir (Bturr)^^ joule (J) 1.055 056 E + 03

British thermal unit.h (Btu.h)^^ joule (J) 1.054 350 E + 03

British thermal unit (mean) (Btu) joule (J) 1.055 87 E + 03

British thermal unit (39 °F) (Btu) joule (J) 1.059 67 E + 03

British thermal unit (59 °F) (Btu) joule (J) 1.054 80 E + 03

British thermal unit (60 °F) (Btu) joule (J) 1.054 68 E + 03

British thermal unitir foot per hour square foot degree Fahrenheit

[BtUiT • ft/(h • ft^ • °F)] watt per meter kelvin [W/(m • K)] 1.730 735 E + 00

British thermal unit,h foot per hour square foot degree Fahrenheit

[Btu.h • ft/(h • ft^ • °F)] watt per meter kelvin [W/(m • K)] 1.729 577 E + 00

British thermal unitir inch per hour square foot degree Fahrenheit

[Btuir • in/(h • ft^ • °F)] watt per meter kelvin [W/(m • K)] 1.442 279 E-01

British thermal unit,h inch per hour square foot degree Fahrenheit

[Btu.h • in/(h • ft^ • °F)] watt per meter kelvin [W/(m • K)] 1.441 314 E-01

British thermal unitir inch per second square foot degree Fahrenheit

[Btu,T-in/(s-ft2-°F)] watt per meter kelvin [W/(m-K)] 5.192 204 E + 02

' See Sec. B.6.

"* One technical atmosphere equals one kilogram-force per square centimeter (1 at = 1 kgf/cm^). " 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 for

International Table used in this listing is subscript "IT". Similarily, the notation for thermochemical is subscript "th." Further, the thermochemical Btu, Btu,h, is based on the thermochemical calorie, cal,h, where calih = 4.184 J exactly.

46
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

British thermal unittb inch per second square foot degree Fahrenheit

[Btu.h • in/(s • ft^ • "F)] watt per meter keivin [W/(m • K)] 5.188 732 E + 02

British thermal unitix per cubic foot

(Btuix/ft^) joule per cubic meter (J/m^) 3.725 895 E + 04

British thermal unitth per cubic foot

(Btu,h/ft^) joule per cubic meter (J/m^) 3.723 403 E + 04

British thermal unitir per degree Fahrenheit

(Bturr/°F) joule per keivin (J/k) 1.899 101 E + 03

British thermal unit,h per degree Fahrenheit

(Btu,h/°F) joule per keivin (J/k) 1.897 830 E + 03

British thermal unitir per degree Rankine

(BtuiT/°R) joule per keivin (J/k) 1.899 101 E + 03

British thermal unitih per degree Rankine

(Btu,h/°R) joule per keivin (J/k) 1.897 830 E + 03

British thermal unitix per hour (Btuix/h) watt (W) 2.930 711 E-01

British thermal unit,h per hour (Btu,h/h) watt (W) 2.928 751 E-01

British thermal unitir per hour square foot degree Fahrenheit

[BtUiT/(h • ft^ • °F)] watt per square meter keivin [W/(m2 • K)] 5.678 263 E + 00

British thermal unit,h per hour square foot degree Fahrenheit

[Btu,h/(h • ft^ • °F)] watt per square meter keivin [W/(m2 • K)] 5.674 466 E + 00

British thermal unit.h per minute (Btu,h/min) ..watt (W) 1.757 250 E + 01

British thermal unitix per pound (Btuix/lb) joule per kilogram (J/kg) 2326 E+03

British thermal unitih per pound (Btu,h/lb) joule per kilogram (J/kg) 2.324 444 E + 03

British thermal unitix per pound degree Fahrenheit

[Btuix/(lb • °F)] joule per kilogram keivin (J/(kg • K)] 4.1868 E+03

British thermal unit,h per pound degree Fahrenheit

[Btu,h/(lb • °F)] joule per kilogram keivin [J/(kg • K)] 4.184 E+03

British thermal unitix per pound degree Rankine

[Btu,x/(lb • "R)] joule per kilogram keivin [J/(kg • K)] 4.1868 E+03

British thermal unit,h per pound degree Rankine

[Btu,h/(lb • °R)] joule per kilogram keivin [J/(kg K)] 4.184 E+03

British thermal unitix per second (Btuir/s) watt (W) 1.055 056 E + 03

British thermal unit.h per second (Btu,h/s) watt (W) 1.054 350 E + 03

British thermal unitix per second square foot degree Fahrenheit

[Btuix/(s • ft^ • °F)] watt per square meter keivin [W/(m2 • K)] 2.044 175 E + 04

British thermal unit.h per second square foot degree Fahrenheit

[Btu,h/(s • ft^ • °F)] watt per square meter keivin [W/(m2 • K)] 2.042 808 E + 04

British thermal unitix per square foot

(Btuix/ft^) joule per square meter (J/m^) 1.135 653 E + 04

British thermal unit,h per square foot

(Btu,h/ft^) joule per square meter (J/m^) 1.134 893 E + 04

British thermal unitix per square foot hour

[(Btuix/(ft^ • h)] watt per square meter (W/m^) 3.154 591 E + 00

British thermal unitth per square foot hour

[Btu,h/(ft^ • h)] watt per square meter (W/m^) 3.152 481 E + 00

British thermal unit,h per square foot minute

[Btu,h/(ft^ • min)] watt per square meter (W/m^) 1.891 489 E + 02

British thermal unitix per square foot second

[(Btuix/(ft^ • s)] watt per square meter (W/m^) 1.135 653 E + 04

British thermal unitth per square foot second

[Btuth/(ft^ • s)] watt per square meter (W/m^) 1.134 893 E + 04

British thermal unit,h per square inch second

[BtUth/(in2 • s)] watt per square meter (W/m^) 1.634 246 E + 06

47
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

bushel (U.S.) (bu) cubic meter (m') 3.523 907 E - 02

bushel (U.S.) (bu) liter (L) 3.523 907 E + 01

calorieiT (calir)^^ joule (J) 4.1868 E+00

calorie,h (cal.h)^^ joule (J) 4.184 E+00

calorie (cal) (mean) joule (J) 4.190 02 E + 00

calorie (15 °C) (calis) joule (J) 4.185 80 E + 00

calorie (20 °C) (cabo) joule (J) 4.181 90 E + 00

calorieiT, kilogram (nutrition) joule (J) 4.1868 E+03

calorieib, kilogram (nutrition) joule (J) 4.184 E+03

calorie (mean), kilogram (nutrition)^^ joule (J) 4.190 02 E + 03

calorie,h per centimeter second degree Celsius

[cal,h/(cm • s • °C)] watt per meter kelvin [W/(m • K)] 4.184 E+02

calorieiT per gram (caliy/g) joule per kilogram (J/kg) 4.1868 E+03

calorie,h per gram (caU/g) joule per kilogram (J/kg) 4.184 E+03

calorieiT per gram degree Celsius

[caliT/(g • °C)] joule per kilogram kelvin [J/(kg • K)] 4.1868 E+03

caloricih per gram degree Celsius

[cal,h/(g • °C)] 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

caloricth per gram kelvin [ca!,h/(g • K)] joule per kilogram kelvin [J/(kg • K)] 4.184 E+03

calorieih per minute (cal,h/min) watt (W) 6.973 333 E-02

calorie,h per second (cai,h/s) watt (W) 4.184 E+00

calorieih per square centimeter (calth/cm^) joule per square meter (J/m^) 4.184 E+04

calorieih per square centimeter minute

[cal,h/(cm^ • min)] watt per square meter (W/m^) 6.973 333 E + 02

calorieih per square centimeter second

[calih/(cm^ • s)] watt per square meter (W/m^) 4.184 E+04

candela per square inch (cd/in^) candela per square meter (cd/m^) 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 °C) pascal (Pa) 1.333 22 E + 03

centimeter of mercury (0 °C) kilopascal (kPa) 1.333 22 E + 00

centimeter of mercury, conventional (cmHg)^^. . .pascal (Pa) 1.333 224 E + 03

centimeter of mercury, conventional (cmHg)^^. . .kilopascal (kPa) 1.333 224 E + 00

centimeter of water (4 °C) pascal (Pa) 9.806 38 E + 01

centimeter of water, conventional (cmH20)^-' . . .pascal (Pa) 9.806 65 E+01

centipoise (cP) pascal second (Pa • s) 1.0 E—03

centistokes (cSt) meter squared per second (m^/s) 1.0 E—06

chain (based on U.S. survey foot) (ch)^ meter (m) 2.011 684 E + 01

circular mil square meter (m^) 5.067 075 E-10

circular mil square millimeter (mm^) 5.067 075 E — 04

clo square meter kelvin per watt (m^ • K/W). . . 1.55 E — 01

cord (128 ft^) cubic meter (m') 3.624 556 E + 00

cubic foot (ft') cubic meter (m') 2.831 685 E-02

cubic foot per minute (ft'/min) cubic meter per second (m'/s) 4.719 474 E-04

cubic foot per minute (ft'/min) liter per second (L/s) 4.719 474 E — 01

cubic foot per second (ft'/s) cubic meter per second (m'/s) 2.831 685 E-02

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. Conversion factors for mercury manometer pressure units are calculated using the standard value for the acceleration of gravity and the 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.

Similar comments also apply to water manometer pressure units. Conversion factors for conventional mercury and water manometer

pressure units are based on Ref. [6: ISO 31-3].

48
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

cubic inch (in') cubic meter (m^) 1.638 706 E-05

cubic inch per minute (in'/min) cubic meter per second (m'/s) 2.731 177 E-07

cubic mile (mi^) cubic meter (m^) 4.168 182 E + 09

cubic yard (yd') cubic meter (m') 7.645 549 E-01

cubic yard per minute (yd'/min) cubic meter per second (m'/s) 1.274 258 E — 02

cup (U.S.) cubic meter (m') 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

darcy^^ meter squared (m^) 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) (") radian (rad) 1.745 329 E-02

degree Celsius (temperature) ("C) kelvin (K) T/K = t/°C + 273.15

degree Celsius (temperature interval) (°C) kelvin (K) '. 1.0 E+00

degree centigrade (temperature)^^ degree Celsius (°C) t fC ~ //deg. cent.

degree centigrade (temperature interval)^'' degree Celsius (°C) 1.0 E + 00

degree Fahrenheit (temperature) (°F) degree Celsius (°C) //°C = (//°F - 32)/1.8

degree Fahrenheit (temperature) (°F) kelvin (K) T/K= {t/°F + 459.67)/ 1.8

degree Fahrenheit (temperature interval) (°F)... degree Celsius (°C) 5.555 556 E-01

degree Fahrenheit (temperature interval) (°F) . . .kelvin (K) 5.555 556 E-01

degree Fahrenheit hour per British thermal unitir (°F- h/Btu,T) kelvin per watt (K/W) 1.895 634 E + 00

degree Fahrenheit hour per British thermal unitih ("F- h/Btu,h) kelvin per watt (K/W) 1.896 903 E + 00

degree Fahrenheit hour square foot per British thermal unitrr ("F • h • ft^/BtUiT) square meter kelvin per watt (m^ • K/W) . . 1.761 102 E-01

degree Fahrenheit hour square foot per British thermal unit,h

("F • h • ft^/Btu,h) square meter kelvin per watt (m^- K/W).. 1.762280 E-01

degree Fahrenheit hour square foot per British thermal unitrr inch

["F - h • ftV(Btu,T • in)] meter kelvin per watt (m • K/W) 6.933 472 E + 00

degree Fahrenheit hour square foot per British thermal unitih inch

["F • h • ft^ / (Btu.h - in)] meter kelvin per watt (m • K/W) 6.938 112 E + 00

degree Fahrenheit second per British thermal unitrr ("F-s/BtUir) kelvin per watt (K/W) 5.265 651 E-04

degree Fahrenheit second per British thermal unit,h

degree Rankine (°R) kelvin (K) ,

denier gram per me

dyne (dyn) newton (N).

dyne per square centimeter (dyn/cm^)

5.269 175 E-04

, r/K = (r/''R)/1.8

, . 5.555 556 E-01

1.111 111 E-07

1.111 111 E-04

1.0 E-05

1.0 E-07

1.0 E-01

eleclronvolt (eV) joule (J) 1.602 177 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

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. The centigrade temperature scale is obsolete; the degree centigrade is only approximately equal to the degree Celsius.

49
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

EMU of resistance (abohm) ohm (CI) 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/(cm^ • s)] watt per square meter (W/m^) 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 + 1

1

ESU of resistance (statohm) ohm (fl) 8.987 552 E + 11

faraday (based on carbon 12) coulomb (C) 9.648 531 E + 04

fathom (based on U.S. survey foot)^ 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 (m') 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)' meter (m) 3.048 006 E-01

footcandle lux (be) 1.076 391 E + 01

footlambert candela per square meter (cd/m^) 3.426 259 E + 00

foot of mercury, conventional (ftHg)^^ pascal (Pa) 4.063 666 E + 04

foot of mercury, conventional (ftHg)'^ kilopascal (kPa) 4.063 666 E + 01

foot of water (39.2 °F) pascal (Pa) 2.988 98 E + 03

foot of water (39.2 °F) kilopascal (kPa) 2.988 98 E + 00

foot of water, conventional (ftH20)^^ pascal (Pa) 2.989 067 E + 03

foot of water, conventional (ftH20)^^ 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/s^) meter per second squared (m/s^) 3.048 E—01

foot poundal joule (J) 4.214 Oil E-02

foot pound force (ft • Ibf) joule (J) 1.355 818 E + 00

foot pound force per hour (ft • Ibf/h) watt (W) 3.766 161 E-04

foot pound force per minute (ft • Ibf/min) watt (W) 2.259 697 E-02

foot pound force per second (ft • Ibf/s) watt (W) 1.355 818 E + 00

foot to the fourth power (ff*)^' meter to the fourth power (m'') 8.630 975 E-03

franklin (Fr) coulomb (C) 3.335 641 E - 10

gal (Gal) meter per second squared (m/s^) 1.0 E-02

gallon [Canadian and U.K (Imperial)] (gal) . . . .cubic meter (m^) 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 (m') 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 (m'/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 horsefxjwer hour

[gal/(hp • h)] cubic meter per joule (mVJ) 1.410 089 E-09

gallon (U.S.) per horsepower hour

[gal/(hp-h)] liter per joule (L/ J) 1.410089 E-06

gallon (U.S.) per minute (gpm)(gal/min) cubic meter per second (m'/s) 6.309 020 E-05

gallon (U.S.) per minute (gpm)(gal/min) liter per second (L/s) 6.309 020 E-02

This is a unit for the quantity second moment of area, which is sometimes called the "moment of section" or "area moment of

inertia" of a plane section about a specified axis.

50
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

gamma (-y) 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 (m^) 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 (m^) 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) (°) 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/m^) 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/cm^) pascal (Pa) 9.806 65 E+01

gram per cubic centimeter (g/cm^) kilogram per cubic meter (kg/m^) 1.0 E+03

hectare (ha) square meter (m^) 1.0 E+04

horsepower (550 ft • Ibf/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, 1121b) kilogram (kg) 5.080 235 E + 01

hundredweight (short, 100 lb) kilogram (kg) 4.535 924 E + 01

nch (in) meter (m) 2.54 E-02

nch (in) centimeter (cm) 2.54 E+00

nch of mercury (32 T) pascal (Pa) 3.386 38 E + 03

nch of mercury (32 °F) kilopascal (kPa) 3.386 38 E + 00

nch of mercury (60 °F) pascal (Pa) 3.376 85 E + 03

nch of mercury (60 "F) 1^ kilopascal (kPa) 3.376 85 E + 00

nch of mercury, conventional (inHg)^^ pascal (Pa) 3.386 389 E + 03

nch of mercury, conventional (inHg) kilopascal (kPa) 3.386 389 E + 00

nch of water (39.2 °F) " pascal (Pa) 2.490 82 E + 02

nch of water (60 °F) " pascal (Pa) 2.4884 E + 02

nch of water, conventional (inH20) pascal (Pa) 2.490 889 E + 02

nch per second (in/s) meter per second (m/s) 2.54 E—02

nch per second squared (in/s^) meter per second squared (m/s^) 2.54 E—02

nch to the fourth power (in'')'' meter to the fourth power (m") 4.162 314 E-07

kayser(K) reciprocal meter (m~') 1.0 E+02

kelvin (K) degree Celsius (°C) trC = T/K- 273.15

kilocalorierr (kcalix) joule (J) 4.1868 E+03

kiIocalorie,h (kcaU) joule (J) 4.184 E+03

kilocalorie (mean) (kcal) joule (J) 4.190 02 E + 03

kilocaloricth per minute (kcaU/min) watt (W) 6.973 333 E + 01

kilocalorie,h per second (kcaU/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

51
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

kilogram-force per square centimeter

(kgf/cm^) pascal (Pa) 9.806 65 E+04

kilogram-force per square centimeter

(kgf/cm^) kilopascal (kPa) 9.806 65 E-t-01

kilogram-force per square meter (kgf/m^) pascal (Pa) 9.806 65 E-HOO

kilogram-force per square millimeter

(kgf/mm^) pascal (Pa) 9.806 65 E-l-06

kilogram-force per square millimeter

(kgf/mm^) megapascal (MPa) 9.806 65 E+00

kilogram-force second squared per meter

(kgf • s^/m) kilogram (kg) 9.806 65 E-f-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-HOO

kilowatt hour (kW • h) joule (J) 3.6 E-l-06

kilowatt hour (kW • h) megajoule (MJ) 3.6 E-t-00

kip (1 kip = 1000 Ibf) newton (N) 4.448 222 E -I- 03

kip (1 kip = 1000 Ibf) kilonewton (kN) 4.448 222 E -I- GO

kip per square inch (ksi) (kip/in^) pascal (Pa) 6.894 757 E-l-06

kip per square inch (ksi) (kip/in^) kilopascal (kPa) 6.894 757 E + 03

knot (nautical mile per hour) meter per second (m/s) 5.144 444 E-01

lambert^* candela per square meter (cd/m^) 3.183 099 E-l-03

langley (calih/cm^) joule per square meter (J/m^) 4.184 E-l-04

light year (l.y.)l' meter (m) 9.460 73 E -t-15

liter (L)2° cubic meter (m^) 1.0 E-03

lumen per square foot (Im/ft^) lux (bf) 1.076 391 E-t-Ol

maxwell (Mx) weber (Wb) 1.0 E-08

mho Siemens (S) 1.0 E-l-00

microinch meter (m) 2.54 E—08

microinch micrometer (|xm) 2.54 E-02

micron (p.) meter (m) 1.0 E—06

micron (|ji) micrometer (jim) 1.0 E+00

mil (0.001 in) meter (m) 2.54 E-05

mil (0.001 in) millimeter (mm) 2.54 E-02

mil (angle) radian (rad) 9.817 477 E - 04

mil (angle) degree (") 5.625 E-02

mile (mi) meter (m) 1.609 344 E-l-03

mile (mi) kilometer (km) 1.609 344 E-l-00

mile (based on U.S. survey foot) (mi)' meter (m) 1.609 347 E + 03

mile (based on U.S. survey foot) (mi)' kilometer (km) 1.609 347 E-l-00

mile, nautical^^ meter (m) 1.852 E+03

mile per gallon (U.S.) (mpg) (mi/gal) meter per cubic meter (m/m') 4.25 1 437 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)^^ liter per 100 kilometer (L/lOO km). . . divide 235.215 by number

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

The exact conversion factor is 10*/-n. 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).

^" In 1964 the General Conference on Weights and Measures reestablished the name "liter" as a special name for the cubic deci- meter. Between 1901 and 1964 the liter was slightly larger (1.000 028 dm'); when one uses high-accuracy volume data of that time,

this fact must be kept in mind.

^' 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."

22 See Sec. B.5.

52
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

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 mercuiy, conventional (mmHg)^^. . .pascal (Pa) 1.333 224 E + 02

millimeter of water, conventional (mmH20)'^ . . .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 (fl • cm) ohm meter (ft m) 1.0 E-02

ohm circular-mil per foot ohm meter (ft • m)

1.662

426 E - 09

ohm circular-mil per foot ohm square millimeter per meter (ft-mmVm)

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 (m')

2.84

1 306 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 (m^)

2.957

353 E-05

ounce (U.S. fluid) (fl oz) millimeter (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) miiiinewton meter (mN • m) 7.061 552 E + 00 ounce (avoirdupois) per cubic inch (oz/in^) kilogram per cubic meter (kg/m') 1.729 994 E + 03 ounce (avoirdupois) per gallon [Canadian and

U.K. (Imperial)] (oz/gal) kilogram per cubic meter (kg/m^) 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/m') 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/ft^) . . .kilogram per square meter (kg/m^) 3.051 517 E-01

ounce (avoirdujjois) per square inch (oz/in^). . .kilogram per square meter (kg/m^) 4.394 185 E + 01

ounce (avoirdupois) per square yard(oz/yd^) . . .kilogram per square meter (kg/m^) 3.390575 E-02

parsec (pc) meter (m)

3.085

678 E + 16

peck (U.S.) (pk) cubic meter (m')

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 °C) kilogram per pascal second square meter

[kg/(Pa • s • m^)]

5.721

35 E-11

perm (23 °C) kilogram per pascal second square meter

[kg/(Pa • s • m^)]

5.745

25 E-11

perm inch (0°C) kilogram per pascal second meter [kg/(Pa-s-m)]

1.453

22 E-12

perm inch (23 °C) kilogram per pascal second meter

[kg/(Pa • s • m)]

1.459

29 E-12

53
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

phot (ph) lux (Ix) 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 (m^)

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 (m^)

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)

3.5

14 598 E-04

point (printer's) millimeter (mm)

3.514

598 E-01

jxjise (P) pascal second (Pa • s) 1.0 E—01

pound (avoirdupois) (Ib)^^ 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 • ft^) kilogram meter squared (kg-m^) 4.214 011 E-02

pound-force ( Ibf)2'* newton (N)

4.448

222 E + 00

pound-force foot (ibf - ft) newton meter (N • m)

1.355

818 E + 00

pound-force foot per inch (Ibf • ft/in) newton meter per meter (N • m/m) 5.337 866 E + 01

pound-force inch (Ibf - in) newton meter (N • m)

1.129

848 E-01

pound-force inch per inch (Ibf- in/in) newton meter per meter (N • m/m) 4.448 222 E + 00

pound-force per foot (Ibf/ft) newton per meter (N/m)

1.459

390 E + 01

pound-force per inch (Ibf/in) newton per meter (N/m) 1.75 1 268 E + 02

pound-force per p)ound

(Ibf/lb) (thrust to mass ratio) newton per kilogram (N/kg) 9.806 65 E+00

pound-force per square foot (ibf/ft^) pascal (Pa)

4.788

026 E + 01

pound-force per square inch (psi) (Ibf/in^) pascal (Pa)

6.894

757 E + 03

pound-force per square inch (psi) (Ibf/in^) kilopascal (kPa)

6.894

757 E + 00

pound-force second per square foot

(Ibf • s/ft^) pascal second (Pa • s)

4.788

026 E + 01

fxjund-force second per square inch

(Ibf • s/in^) pascal second (Pa • s)

6.894

757 E + 03

pound inch squared (lb • in^) kilogram meter squared (kg • m^) 2.926 397 E-04

pound per cubic foot (Ib/ft^) kilogram per cubic meter (kg/m^) 1.601 846 E + 01 pound per cubic inch (lb/in') kilogram per cubic meter (kg/m^) 2.767 990 E + 04

jjound per cubic yard (\b/y<P) kilogram per cubic meter (kg/m') 5.932 764 E-01

pound per foot (lb/ft) kilogram per meter (kg/m) 1.488 164 E + 00

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

pound per gallon [Canadian and

U.K. (Imperial)] (Ib/gal) kilogram per cubic meter (kg/m^) 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/m') 1.198 264 E + 02

pound per gallon (U.S.) (Ib/gal) kilogram per liter (kg/L)

1.198

264 E-01

pound per horsepower hour [lb/(hp h)] kilogram per joule (kg/J)

1.689

659 E — 07

pound per hour (Ib/h) kilogram per second (kg/s) 1.259 979 E-04

The exact conversion factor is 4.535 923 7 E-OI. All units in Sees. B.8 and B.9 that contain the pound refer to the avoirdupois

pound.

^* If the local value of the acceleration of free fall is taken as g„ = 9.806 65 m/s^ (the standard value), the exact conversion factor

is 4.448 221 615 260 5 E + 00.

54
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

pound per inch (lb/in) kilogram per meter (kg/m) 1.785 797 E + 01 pound per minute (Ib/min) kilogram per second (kg/s) 7.559 873 E — 03

pound per second (ib/s) kilogram per second (kg/s) 4.535 924 E — 01 pound per square foot (Ib/ft^) kilogram per square meter (kg/m^) 4.882 428 E + 00

pound per square inch {nol jx)und-force)

(Ib/in^) kilogram per square meter (kg/m^) 7.030 696 E + 02

pound per yard (lb/yd) kilogram per meter (kg/m) 4.960 546 E-01

psi (pound-force per square inch) (Ibf/in^) pascal (Pa)

6.894

757 E + 03

psi (pound-force per square inch) (Ibf/in^) kilopascal (kPa)

6.894

757 E-l-00

quad (10'' Bturr)^* joule (J)

1.055

056 E+ 18

quart (U.S. dry) (dry qt) cubic meter (m^) 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 (m')

9.463

529 E-04

quart (U.S. liquid) (liq qt) liter (L)

9.463

529 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-l-00

revolution per minute (rpm) (r/min) radian per second (rad/s) 1.047 198 E — 01

rhe reciprocal pascal second [(Pa-s)"'] 1.0 E-l-01

rod (based on U.S. survey foot) (rd)^ 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/ft') kilogram per cubic meter (kg/m^) 5.153 788 E + 02

slug per foot second [slug/(ft • s)] pascal second (Pa • s)

4.788

026 E + 01

square foot (ft^) square meter (m^)

9.290

304 E-02

square foot per hour (ft^/h) square meter per second (m^/s) 2.580 64 E—05

square foot per second (ft^/s) square meter per second (m^/s) 9.290 304 E-02

square inch (in^) square meter (m^) 6.4516 E—04

square inch (in^) square centimeter (cm^) 6.4516 E+00

square mile (mi^) square meter (m^)

2.589

988 E + 06

square mile (mi^) square kilometer (km^)

2.589

988 E + 00

square mile

(based on U.S. survey foot) (mi^)^ square meter (m^)

2.589

998 E + 06

square mile

(based on U.S. survey foot) (mi^f square kilometer (km^)

2.589

998 E + 00

square yard (yd^) square meter (m^)

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 + 1

1

statmho Siemens (S)

1.112650

E-12

statohm ohm (fl) 8.987 552 E + 1

1

statvolt volt (V)

2.997

925 E + 02

stere (st) cubic meter (m^) 1.0 E+00

stilb (sb) candela per square meter (cd/m^) 1.0 E+04

stokes (St) meter squared per second (mVs) 1.0 E-04

55
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

tablespoon cubic meter (m^) 1.478 676 E-05

tablespoon milliliter (mL) 1.478 676 E+01

teaspoon cubic meter (m') 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)^^ joule (J) 1.055 06 E+08

therm (U.S.)^^ 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 Ibf) newton (N) 8.896 443 E + 03

ton-force (2000 Ibf) 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/m') 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 Btu,T/h) watt (W) 3.516 853 E + 03

ton of TNT (energy equivalent)^* joule (J) 4.184 E+09

ton, register cubic meter (m^) 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/m') 1.186 553 E + 03

ton, short, per hour kilogram per second (kg/s) 2.519 958 E-01

ton- (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/cm^) watt per square meter (W/m^) 1.0 E+04

watt per square inch (W/in^) watt per square meter (W/m^) 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

I

I

I

J

f " The therm (EC) is legally defined in the Council Directive of 20 December 1979, Council of the European Communities (now [ the European Union, EU). The therm (U.S.) is legally defmed in the Federal Register of July 27, 1968. Although the therm (EC), | which is based 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. I Defined (not measured) value.

56
Guide for the Use of the International System of Units (SI)

B.9 Factors for units listed by kind of quantity or fleld of science

Caution: The units listed in column 1 are in general not to be used in NIST publications,

with the exception of those few in italic type.

Factors in boldface are exact

To convert from to Multiply by

ACCELERATION

(m/s^) .... ... 9.806 65 E+00

(m/s^) .... 3.048 E-01

(m/s^) .... .... 1.0 E-02

(m/s2) .... .... 2.54 E-02

ANGLE

degree (°) 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 (

°

) 9.0 E-01

mil radian (rad) 9.817 477 E-04

mil degree (°) 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 (m^) 4.046 873 E + 03

are (a) square meter (m^) 1.0 E+02

barn (b) square meter (m^) 1.0 E—28

circular mil square meter (m^) 5.067075 E-10

circular mil square millimeter (mm^) 5.067 075 E — 04

foot to the fourth power (ff)^'' meter to the fourth power (m*) 8.630 975 E-03

hectare (ha) square meter (m^) 1.0 E+04

inch to the fourth power (in")*' meter to the fourth power (m") 4.162 314 E-07

square foot (ft^) square meter (m^) 9.290 304 E-02

square inch (in^) square meter (m^) 6.4516 E-04

square inch (in"*) square centimeter (cm^) 6.4516 E+00

square mile (mi^) square meter (m^) 2.589 988 E + 06

square mile (mi^) square kilometer (km^) 2.589 988 E + 00

square mile

(based on U.S. survey foot) (mi^)' square meter (m^) 2.589 998 E + 06

square mile

(based on U.S. survey foot) (mi^)^ square kilometer (km^) 2.589 998 E + 00

square yard (yd^) square meter (m^) 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 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 (fl) 1.0 E-09

abvolt voh (V) 1.0 E-08

ampere hour (A-h) coulomb (C) 3.6 E+03

57
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

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 (fl) 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 (CI) 8.987 552 E + 11

faraday (based on carbon 12) coulomb (C)

9.648

531 E + 04

franklin (Fr) coulomb (C)

3.335

641 E - 10 gamma (7) 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 (n-cm) ohm meter (fl • m) 1.0 E—02

ohm circular-mil per foot ohm meter (ft • m)

1.662

426 E-09

ohm circular-mil per foot ohm square millimeter per meter (fl-mm^/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 + 1

statmho Siemens (S)

1.112650

E-12

statohm ohm (ft)

8.987

552 E + 1

statvolt volt (V)

2.997

925 E + 02

unit pole weber (Wb)

1.256

637 E-07

ENERGY (includes WORK)

British thermal unitir (Btuix)^^ . joule (J)

1.055

056 E + 03

British thermal unit.h (Btu.h)^^ joule (J)

1.054

350 E + 03

British thermal unit (mean) (Btu) joule (J)

1.055

87 E + 03

British thermal unit (39 "F) (Btu) joule (J)

1.059

67 E + 03

British thermal unit (59 °F) (Btu) joule (J)

1.054

80 E + 03

British thermal unit (60 °F) (Btu) joule (J)

1.054

68 E + 03

calorierr (calrr)^^ joule (J)

4.1868

E+00

calorie.h (caU)^^ joule (J) 4.184 E+00

calorie (mean) (cal) joule (J)

4.190

02 E + 00

calorie (15 °C) (calis) joule (J)

4.185

80 E + 00

calorie (20 "C) (caho) joule (J) 4.181 90 E + 00

calorieiT, kilogram (nutrition) joule (J)

4.1868

E+03

calorieih, kilogram (nutrition) joule (J) 4.184 E+03

calorie (mean), kilogram (nutrition)^^ joule (J)

4.190

02 E + 03

electronvolt (eV) joule (J)

1.602

177 E- 19

erg (erg) joule (J) 1.0 E-07

foot poundal joule (J) 4.214 Oil E-02

foot pound-force (ft • Ibf) joule (J)

1.355

818 E + 00

kilocalorieiT (kcalir) joule (J)

4.1868

E+03

kilocalorie.h (kcal.h) joule (J) 4.184 E+03

kilocalorie (mean) (kcal) joule (J)

4.190

02 E + 03

58
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

kilowalt hour (kW • h) joule (J) 3.6 E+06

kilowatt hour (kW • h) megajoule (MJ) 3.6 E+00

quad (10'* Bturr)^^ joule (J) 1.055 056 E+18

therm (EC)^^ joule (J) 1.055 06 E+08

therm (U.S.)^ joule (J) 1.054 804 E+08

ton of TNT (energy equivalent)^^ 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/(cm^ • s)] watt per square meter (W/m^) 1.0 E-03

watt per square centimeter (W/cm^) watt per square meter (W/m^) 1.0 E+04

watt per square inch (W/in^) watt per square meter (W/m^) 1.550 003 E + 03

FLOW (see MASS DIVIDED BY TIME or VOLUME DIVIDED BY TIME)

FORCE

dyne(dyn) newton (N) 1.0 E-OS

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 Ibf) newton (N) 4.448 222 E + 03

kip (1 kip = 1000 Ibf) 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 (Ibf)24 newton (N) 4.448 222 E + 00

pound-force per pound

(Ibf/lb) (thrust to mass ratio) newton per kilogram (N/kg) 9.806 65 E+00

ton-force (2000 Ibf) newton (N) 8.896 443 E + 03

ton-force (2000 Ibf) kilonewton (kN) 8.896 443 E + 00

FORCE DIVIDED BY AREA (see PRESSURE)

FORCE DIVIDED BY LENGTH

pound-force per foot (Ibf/ft) newton per meter (N/m) 1.459 390 E + 01

pound-force per inch (Ibf/in) newton per meter (N/m) 1.751 268 E + 02

HEAT

Available Energy

British thermal unitix per cubic foot

(Btuix/ft^) joule per cubic meter (J/m^) 3.725 895 E + 04

British thermal unit,h per cubic foot

(Btu,h/ft^) joule per cubic meter (J/m^) 3.723 403 E + 04

British thermal unitiT per pound (Btuix/lb) joule per kilogram (J/kg) 2326 E+03

British thermal unit,h per pound (Btu,h/lb) joule per kilogram (J/kg) 2.324 444 E + 03

calorieiT per gram (calix/g) joule per kilogram (J/kg) 4.1868 E+03

caloricih per gram (cal,h/g) joule per kilogram (J/kg) 4.184 E+03

Coefficient of Heat Transfer

British thermal unitir per hour square foot degree Fahrenheit

[BtUiT/(h • ft^ • °F)] watt per square meter kelvin [W/(m2-K)] 5.678 263 E + 00

British thermal unit,h per hour square foot degree Fahrenheit

[BtUth/(h • ft^ • °F)] watt per square meter kelvin

[ W/(m^ • K)] 5.674 466 E + 00

British thermal unitir per second square foot degree Fahrenheit

[BtUiT/(s • ft^ • °F)] watt per square meter kelvin [W/(m^ • K)] 2.044 175 E + 04

British thermal unitih per second square foot degree Fahrenheit

[Btu,h/(s • ft^ • °F)] watt per square meter kelvin [W/(m2 • K)] 2.042 808 E + 04

59
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

Density of Heat

British thermal unitir per square foot

(Btuir/ft^) joule per square meter (J/m^) 1.135 653 E + 04

British thermal unitih per square foot

(Btu,h/ft^) joule per square meter (J/m^) 1.134 893 E + 04

calorie,h per square centimeter (cal,h/cm^) joule per square meter (J/m^) 4.184 E+04

langley (cal,h/cm^) joule per square meter (J/m^) 4,184 E+04

Density of Heat Flow Rate

British thermal unitir per square foot hour

[Btu,T/(ft^ • h)] watt per square meter (W/m^) 3.154 591 E + 00

British thermal unitih per square foot hour

[Btu,h/(ft^ • h)] watt per square meter (W/m^) 3.152 481 E + 00

British thermal unitth per square foot minute

[Btu,h/(ft^ • min)] watt per square meter (W/m^) 1.891 489 E + 02

British thermal unitix per square foot second

[BtuiT/(ft^ • s)] watt per square meter (W/m^) 1.135 653 E + 04

British thermal unit,h per square foot second

[Btu,h/(ft^ • s)] watt per square meter (W/m^) 1.134 893 E + 04

British thermal unitth per square inch second

[Btu,h/(in^ • s)] watt per square meter (W/m^) 1.634 246 E + 06

calorie,h per square centimeter minute

[cal,h/(cm^ • min)] watt per square meter (W/m^) 6.973 333 E + 02

caloricth per square centimeter second

[cal,h/(cm^ • s)] watt per square meter (W/m^) 4.184 E+04

Fuel Consumption

gallon (U.S.) per horsepower hour

[gal/(hp • h)] cubic meter per joule (m'/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/m^) 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)^^ liter per 100 kilometer (L/lOO km). . . divide 235.215 by number

of miles per gallon

pound per horsepower hour [lb/(hp • h)] kilogram per joule (kg/J) 1.689 659 E-07

Heat Capacity and Entropy

British thermal unitix per degree Fahrenheit

(BtuiT/°F) joule per kelvin (J/k)

1.899

101 E + 03

British thermal unitth per degree Fahrenheit

(BtUth/°F) joule per kelvin (J/k)

1.897

830 E + 03

British thermal unitir per degree Rankine

(Btu,T/°R) joule per kelvin (J/k)

1.899

101 E + 03

British thermal unitth per degree Rankine

(Btu,h/''R) joule per kelvin (J/k)

1.897

830 E + 03

Heat Flow Rate

British thermal unitir per hour (Bturr/h) watt (W)

2.930

711 E-01

British thermal unitth per hour (BtUth/h) watt (W)

2.928

751 E-01

British thermal unit,h per minute (Btu,h/min) . . watt (W)

1.757

250 E + 01

British thermal unitir per second (Btuir/s) watt (W)

1.055

056 E + 03

British thermal unit,h per second (BtUth/s) watt (W)

1.054

350 E + 03

caIorie,h per minute (cal,h/min) watt (W)

6.973

333 E — 02

caIorie,h per second (cal,h/s) watt (W) 4.184 E+00

kilocaloricih per minute (kcalih/min) watt (W)

6.973

333 E + 01

kiIocalorie,h per second (kcal,h/s) watt (W) 4.184 E+03

ton of refrigeration (12 000 Btuir/h) watt (W)

3.516

853 E + 03

60
! Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

Specific Heat Capacity and Specific Entropy

British thermal unitir per pound degree Fahrenheit

[Btu,T/(lb • °F)] joule per kilogram kelvin [J/(kg • K)] 4.1868 E+03

British thermal unit,h per pound degree Fahrenheit

[Btu,h/(lb • "F)] joule per kilogram kelvin [J/(kg • K)] 4,184 E+03

British thermal unitiT per pound degree Rankine

[BtuiT/(lb • °R)] joule per kilogram kelvin [J/(kg • K)] 4.1868 E+03

British thermal unit,h per pound degree Rankine

[Btu,h/(lb • °R)] joule per kilogram kelvin [J/(kg • K)] 4.184 E+03

calorierr per gram degree Celsius

[cal,T/(g • °C)] joule per kilogram kelvin [J/(kg • K)] 4.1868 E+03

calorie,h per gram degree Celsius

[cal,h/(g • "C)] joule per kilogram kelvin [J/(kg • K)] 4.184 E+03

calorieiT per gram kelvin [calrr/(g • K)] joule per kilogram kelvin [J/(kg • K)] 4.1868 E+03

calorie,h per gram kelvin [cal,h/(g • K)] joule per kilogram kelvin [J/(kg • K)] 4.184 E+03

Thermal Conductivity

Britsh thermal unitiT foot per hour square foot degree Fahrenheit

[Btu,T-ft/(h-ft2-°F)] watt per meter kelvin [W/(m-K)] 1.730 735 E + 00

Britsh thermal unit,h foot per hour square foot degree Fahrenheit

[Btu.h • ft/(h • ft^ • °F)] watt per meter kelvin [W/(m • K)] 1.729 577 E + 00

Britsh thermal unitij inch per hour square foot degree Fahrenheit

[BtUiT • in/(h • ft^ • °F)] watt per meter kelvin [W/(m • K)] 1.442 279 E-01

Britsh thermal unit,h inch per hour square foot degree Fahrenheit

[Btu.h • in/(h • ft^ • °F)] watt per meter kelvin [W/(m • K)] 1.441 314 E-01

Britsh thermal unitir inch per second square foot degree Fahrenheit

[BtUiT • in/(s • ft^ • °F)] watt per meter kelvin [W/(m-K)] 5.192 204 E + 02

Britsh thermal unit,h inch per second square foot degree Fahrenheit

[Btu.h • in/(s • ft^ • °F)] watt per meter kelvin [W/(m • K)] 5.188 732 E + 02

calorieth per centimeter second degree Celsius

[cal,h/(cm • s • °C)] watt per meter kelvin [W/(m • K)] 4.184 E+02

Thermal Diffusivity

square foot per hour (ft^/h) square meter per second (m^/s) 2.580 64 E-05

Thermal Insulance

clo square meter kelvin per watt (m^ • K/W) . . 1.55 E — 01

degree Fahrenheit hour square foot per British thermal unitir (°F- h • ft^/BtUir) square meter kelvin per watt (m^ • K/W) . . 1.761 102 E-01

degree Fahrenheit hour square foot per British thermal unitih ("F-h-ft^/Btu.h) square meter kelvin per watt (m^ • K/W) . . 1.762 280 E-01

Thermal Resistance

degree Fahrenheit hour per British thermal unitir ("F- h/BtUir) kelvin per watt (K/W)

1.895

634 E + 00

degree Fahrenheit hour per British thermal unit,h

CF-h/Btu.h) kelvin per watt (K/W)

1.896

903 E + 00

degree Fahrenheit second per British thermal unitir ("F-s/BtUir) kelvin per watt (K/W)

5.265

651 E-04

degree Fahrenheit second per British thermal unit,h

("F • s/Btu,h) kelvin per watt (K/W)

5.269

175 E-04

Thermal Resistivity

degree Fahrenheit hour square foot per British thermal unitir inch

[''F-h-ftV(Btuir-in)] meter kelvin per watt (m-K/W) 6.933 472 E + 00

degree Fahrenheit hour square foot per British thermal unitu, inch

["F • h • ftV(Btu,h • in)] meter kelvin per watt (m • K/W) 6.938 112 E + 04

61
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

LENGTH

Sngstrom (A) meter (m) 1.0 E-10

Sngstrom (A) nanometer (nm) 1.0 E-01

astronomical unit (AU) meter (m) 1.495 979 E+ 11

chain (based on U.S. survey foot) (ch)^ meter (m) 2.011 684 E + 01 fathom (based on U.S. survey foot)^ 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)' meter (m) 3.048 006 E-01

inch (in) meter (m) 2.S4 E—02

inch (in) centimeter (cm) 2.54 E+00

kayser(K) reciprocal meter (m"') 1 E+02

light year (l.y.) meter (m) 9.460 73 E + 15

microinch meter (m) 2.54 E-08

microinch micrometer (jim) 2.54 E—02

micron (p.) meter (m) 1.0 E—06

micron (jt) micrometer ((im) 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)' meter (m) 1.609 347 E + 03

mile (based on U.S. survey foot) (mi)' kilometer (km) 1.609 347 E + 00

mile, nautical^^ 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)' meter (m) 5.029 210 E + 00

yard (yd) meter (m) 9.144 E-01

LIGHT

candela per square inch (cd/in^) candela per square meter (cd/m^) 1.550 003 E + 03

footcandle lux (Ix) 1.076391 E + 01

footlambert candela per square meter (cd/m^) 3.426 259 E + 00

lambert** candela per square meter (cd/m^) 3.183 099 E + 03

lumen per square foot (Im/ft^) lux (Ix) 1.076 391 E + 01

phot (ph) lux (Ix) 1.0 E+04

stilb (sb) candela per square meter (cd/m^) 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

62
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

kilogram-force second squared per meter

(kgf • sVm) kilogram (kg)

9.806

65 E+00

ounce (avoirdufKjis) (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) (Ib)^^ kilogram (kg)

4.535

924 E-01

pound (troy or apothecary) (lb) kilogram (kg)

3.732

417 E-01

pound foot squared (lb • ft^) kilogram meter squared (kg • m^) 4.214 Oil E-02

pound inch squared (lb • in^) kilogram meter squared (kg-m^) 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

MASS DENSITY (see MASS DIVIDED BY VOLUME)

MASS DIVIDED BY AREA

ounce (avoirdupois) per square foot (oz/ft^) kilogram per square meter (kg/m^) 3.051 517 E — 01

ounce (avoirdupois) per square inch (oz/in^) . . .kilogram per square meter (kg/m^) 4.394 185 E + 01

ounce (avoirdupois) per square yard (oz/yd^). . .kilogram per square meter (kg/m^) 3.390 575 E — 02

pound per square foot (Ib/ft^) kilogram per square meter (kg/m^) 4.882 428 E + 00

pound per square inch {not pound force)

(Ib/in^) kilogram per square meter (kg/m^) 7.030 696 E + 02

MASS DIVIDED BY CAPACITY (see MASS DIVIDED BY VOLUME)

MASS DIVIDED BY LENGTH

denier kilogram per meter (kg/m) 1.111111 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 (Ib/h) kilogram per second (kg/s) 1.259 979 E-04

pound per minute (Ib/min) kilogram per second (kg/s)

7.559

873 E-03

pound per second (Ib/s) kilogram per second (kg/s)

4.535

924 E-01

ton, short, per hour kilogram per second (kg/s)

2.519

958 E-01

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

grain per gallon (U.S.) (gr/gal) kilogram per cubic meter (kg/m') 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/cm^) kilogram per cubic meter (kg/m') 1.0 E+03

ounce (avoirdupois) per cubic inch (oz/in') kilogram per cubic meter (kg/m') 1.729 994 E + 03 ounce (avoirdupois) per gallon [Canadian and

U.K. (Imperial)] (oz/gal) kilogram per cubic meter (kg/m') 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/m') 7.489 152 E + 00

ounce (avoirdupois) per gallon (U.S.) (oz/gal). . .gram per liter (g/L)

7.489

152 E + 00

63
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

pound per cubic foot (lb/ft') kilogram per cubic meter (kg/m^) 1.601 846

pound per cubic inch (lb/in') kilogram per cubic meter (kg/m') 2.767 990

pound per cubic yard (Ib/yd^) kilogram per cubic meter (kg/m') 5.932 764

pound per gallon [Canadian and

U.K. (Imperial)] (lb/gal) kilogram per cubic meter (kg/m') 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/m^) 1.198 264

pound per gallon (U.S.) (lb/gal) kilogram per liter (kg/L) 1.198 264

slug per cubic foot (slug/ft^) kilogram per cubic meter (kg/m^) 5.153 788

ton, long, per cubic yard kilogram per cubic meter (kg/m^) 1.328 939

ton, short, per cubic yard kilogram per cubic meter (kg/m^) 1.186 553

MOMENT OF FORCE or TORQUE

dyne centimeter (dyn • cm) newton meter (N m) 1.0

kilogram-force meter (kgf • m) newton meter (N m) 9.806 65 ounce (avoirdupois)-force inch (ozf • in) newton meter (N • m) 7.061 552 ounce (avoirdupois)-force inch (ozf - in) millinewton meter (mN • m) 7.061 552

pound-force foot (Ibf • ft) newton meter (N • m) 1.355 818

pound-force inch (Ibf • in) newton meter (N • m) 1.129 848

MOMENT OF FORCE or TORQUE, DIVIDED BY LENGTH

pound-force foot per inch (Ibf -ft/in) newton meter per meter (N-m/m) 5.337 866

pound-force inch per inch (Ibf* in/in) newton meter per meter (N • m/m) 4.448 222

PERMEABILITY

darcy^^ meter squared (m^) 9.869233

perm (0 °C) kilogram per pascal second square meter

[kg/ (Pa • s • m^)] 5.721 35 perm (23 °C) kilogram per pascal second square meter

[kg/(Pa • s • m^)] 5.745 25 perm inch (0 °C) kilogram per pascal second meter

[kg/ (Pa • s • m)] 1.453 22

perm inch (23 °C) kilogram per pascal second meter

[kg/(Pa • s • m)] 1.459 29

E-l-01

E-l-04 E-01

E-f-01

E-02

E-l-02 E-01

E + 02

E + 03

E + 03

E-07

E-l-00 E-03

E-l-00

E-l-00 E-01

E-l-01 E + 00

E-13

E-11

E-11

E-12

E-12

POWER

erg per second (erg/s) watt

foot pound-force per hour (ft • Ibf/h) watt

foot pound-force per minute (ft • Ibf/min) watt

foot pound-force per second (ft • Ibf/s) watt

horsepower (550 ft • Ibf/s) watt

horsepower (boiler) watt

horsepower (electric) watt

horsepower (metric) watt

horsepower (U.K.) watt

horsepower (water) watt

W) 1.0 W) 3.766 161 W) 2.259 697

W) 1.355 818

W) 7.456 999

W) 9.809 50

W) 7.46 W) 7.354 988

W) 7.4570 W) 7.460 43

PRESSURE or STRESS (FORCE DIVIDED BY AREA)

atmosphere, standard (atm) pascal (Pa) 1.013 25

atmosphere, standard (atm) kilopascal (kPa) 1.013 25

atmosphere, technical (at)^" pascal (Pa) 9.806 65

atmosphere, technical (at)^° kilopascal (kPa) 9.806 65

bar (bar) pascal (Pa) 1.0

bar (bar) kilopascal (kPa) 1.0

E-07

E-04

E-02

E-l-00

E-l-02 E + 03

E+02

E + 02

E + 02

E + 02

E+OS

E+02

E+04

E+Ol

E+OS

E+02

64
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

centimeter of mercury (0 °C) " pascal (Pa)

1.333

22 E + 03

centimeter of mercuiy (0 °C) kilopascal (kPa)

1

.333 22 E + 00

centimeter of mercuiy, conventional (cmHg)^^. . .pascal (Pa)

1.333

224 E + 03

centimeter of mercuiy, conventional (cmHg)^^. . .kilopascal (kPa)

1.333

224 E + 00

centimeter of water (4 °C) pascal (Pa)

9.806

38 E + 01

centimeter of water, conventional (cmH20)^^ . .pascal (Pa) 9.806 65 E+01

dyne per square centimeter (dyn/cm^) pascal (Pa) 1,0 E—01

foot of mercury, conventional (ftHg)*^ pascal (Pa)

4.063

666 E + 04

foot of mercury, conventional (ftHg) kilopascal (kPa)

4.063

666 E + 01

foot of water (39.2 T) pascal (Pa)

2.988

98 E + 03

foot of water (39.2 "F)^^ kilopascal (kPa)

2.988

98 E + 00

foot of water, conventional (ftH20)^3 pascal (Pa)

2.989

067 E + 03

foot of water, conventional (ftH20)^^ kilopascal (kPa)

2.989

067 E + 00

gram-force per square centimeter (gf/cm^) pascal (Pa)

9.806

65 E+01

inch of mercuiy (32 °F) pascal (Pa)

3.386

38 E + 03

inch of mercuiy (32 °F) kilopascal (kPa)

3.386

38 E + 00

inch of mercury (60 T) pascal (Pa)

3.376

85 E + 03

inch of mercury (60 °F)l^ kilopascal (kPa)

3.376

85 E + 00

inch of mercury, conventional (inHg)^^ pascal (Pa)

3.386

389 E + 03

inch of mercury, conventional (inHg)^^ kilopascal (kPa)

3.386

389 E + 00

inch of water (39.2 °F) pascal (Pa)

2.490

82 E + 02

inch of water (60 °F) pascal (Pa)

2.4884

E + 02

inch of water, conventional (inH20) pascal (Pa)

2.490

889 E + 02

kilogram-force per square

centimeter (kgf/cm^) pascal (Pa)

9.806

65 E+04

kilogram-force per square

centimeter (kgf/cm^) kilopascal (kPa)

9.806

65 E+01

kilogram-force per square meter (kgf/m^) pascal (Pa)

9.806

65 E+00

kilogram-force per square

millimeter (kgf/mm^) pascal (Pa)

9.806

65 E+06

kilogram-force per square

millimeter (kgf/mm^) megapascal (MPa)

9.806

65 E+00

kip per square inch (ksi) (kip/in^) pascal (Pa)

6.894

757 E + 06

kip per square inch (ksi) (kip/in^) 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 mercuiy, conventional (mmHg) . . . pascal (Pa)

1.333

224 E + 02

millimeter of water, conventional (mmH20)*^ pascal (Pa)

9.806

65 E+00

poundal per square foot pascal (Pa)

1.488

164 E + 00

pound-force per square foot (Ibf/ft^) pascal (Pa)

4.788

026 E + 01

pound-force per square inch (psi) (Ibf/in^) pascal (Pa)

6.894

757 E + 03

pound-force per square inch (psi) (Ibf/in^) kilopascal (kPa)

6.894

757 E + 00

psi (pound-force per square inch) (Ibf/in^) pascal (Pa)

6.894

757 E + 03

psi (pound-force per square inch) (Ibf/in^) kilopascal (kPa)

6.894

757 E + 00

torr (Torr) pascal (Pa)

1.333

224 E + 02

RADIOLOGY

curie (Ci) becquerel (Bq) 3.7 E+10

rad (absorbed dose) (rad) gray (Gy) 1.0 E—02 rem (rem) sievert (Sv) 1.0 E-02

roentgen (R) coulomb per kilogram (C/kg) 2.58 E-04

SPEED (see VELOCITY)

STRESS (see PRESSURE)

65
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

TEMPERATURE

degree Celsius (°C) kelvin (K) r/K = //°C + 273.15

degree centigrade degree Celsius (°C) //°C » </deg. cent.

degree Fahrenheit (°F) degree Celsius ("C) //°C = (//"F - 32)/1.8

degree Fahrenheit (°F) kelvin (K) T/K = (//°F + 4S9.67)/1.8

degree Rankine (°R) kelvin (K) T/K= (r/''R)/1.8

kelvin (K) degree Celsius (°C) trc =T/K- 273.15 TEMPERATURE INTERVAL

degree Celsius (°C) kelvin (K) 1.0 E+00

degree centrigrade^^ degree Celsius (°C) 1.0 E + 00

degree Fahrenheit (°F) degree Celsius (°C) 5.555 556 E-01

degree Fahrenheit (°F) kelvin (K)

5.555

556 E-01

degree Rankine (°R) 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

(Ibf • s/ft^) pascal second (Pa • s) 4.788 026 E + 01

pound-force second per square inch

(Ibf • s/in^) 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)"'] 1.0 E+01

slug per foot second [slug/(ft • s)] pascal second (Pa • s) 4.788 026 E + 01

66
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

VISCOSITY, KINEMATIC

centistokes (cSt) meter squared per second (m^/s) 1.0

square foot per second (ft^/s) meter squared per second (m^/s) 9.290 304

stokes (St) meter squared per second (m^/s) 1.0 VOLUME (includes CAPACITY)

acre-foot (based on U.S. survey foot)^ cubic meter

barrel [for petroleum, 42 gallons (U.S.)](bbl). . .cubic meter

barrel [for petroleum, 42 gallons (U.S.)](bbl). . .liter (L) . .

.

bushel (U.S.) (bu) cubic meter

bushel (U.S.) (bu) liter (L)...

cord (128 ft^) cubic meter

cubic foot (ft') cubic meter

cubic inch (in' )^'* cubic meter

cubic mile (mi') cubic meter

cubic yard (yd') cubic meter

cup (U.S.) cubic meter

cup (U.S.) liter (L)

cup (U.S.) milliliter (mL)

fluid ounce (U.S.) (fl oz) cubic meter

fluid ounce (U.S.) (fl oz) milliliter (mL) 2

gallon [Canadian and U.K. (Imperial)] (gal) cubic meter

gallon [Canadian and U.K. (Imperial)] (gal) liter (L)

gallon (U.S.) (gal) cubic meter

gallon (U.S.) (gal) liter (L) . .

.

gill [Canadian and U.K. (Imperial)] (gi) cubic meter

gill [Canadian and U.K. (Imperial)] (gi) liter (L)

gill (U.S.) (gi) cubic meter

gill (U.S.) (gi) liter (L)...

liter (L)^" cubic meter

ounce [Canadian and U.K. fluid (Imperial)]

(fl oz) .cubic meter (m') 2.841 306

ounce [Canadian and U.K. fluid (Imperial)]

(fl oz) milliliter (mL) , 2

ounce (U.S. fluid) (fl oz) cubic meter (m') 2

ounce (U.S. fluid) (fl oz) milliliter (mL) 2

peck (U.S.) (pk) cubic meter

peck (U.S.)(pk) liter (L)...,

pint (U.S. dry) (dry pt) cubic meter

pint (U.S. dry) (dry pt) liter (L) . . .

,

pint (U.S. liquid) (liq pt) cubic meter

pint (U.S. liquid) (liq pt) liter (L) . . .

,

quart (U.S. dry) (dry qt) cubic meter

quart (U.S. dry) (dry qt) liter (L) . . .

,

quart (U.S. liquid) (liq qt) cubic meter

quart (U.S. liquid) (liq qt) liter (L) . . .

.

stere (st) cubic meter

tablespoon cubic meter

8,

5

5,

4,

4

1

1

9,

9.

1,

1,

tablespoon milliliter (mL) 1.

4

4

2

teasf)oon cubic meter

teaspoon milliliter (mL)

ton, register cubic meter

m').

m').

m').

m').

m').

m').

m').

m').

m')

m')

m').

m').

m').

m').

m').

m').

233 489

589 873

589 873

523 907

523 907

624 556

,831 685

.638 706

.168 182

.645 549

.365 882

.365 882

.365 882

.957 353

.957 353

,546 09

,546 09

.785 412

.785 412

.420 653

.420 653

.182 941

.182 941

,0

m').

m').

m').

m').

m').

m').

m').

m')

m')

m').

.841 306

.957 353

.957 353

809 768

809 768

506 105

506 105

731 765

731 765

101 221

101 221

463 529

463 529

0

478 676

478 676

928 922

928 922

831 685

E-06

E-02

E-04

E + 03

E-01

E + 02

E-02

E + Ol

E + 00

E-02

E-05

E + 09

E-01

E-04

E-01

E + 02

E-05

E + 01 E-03

E+00

E-03

E + 00

E-04

E-01

E-04

E-01

E-03

E-05

E + 01 E-05

E + 01

E-03

E + 00

E-04

E-01

E-04

E-01

E-03

E + 00

E-04

E-01

E+00

E-05

E + 01 E-06

E + 00

E + 00

67
Guide for the Use of the International System of Units (SI)

To convert from to Multiply by

VOLUME DIVIDED BY TIME (includes FLOW)

cubic foot per minute (ft^/min) cubic meter per second (m'/s) 4.719 474 E — 04

cubic foot per minute (ft^/min) liter per second (L/s) 4.719 474 E-01

cubic foot per second (ft^/s) cubic meter per second (m'/s) 2.831 685 E-02

cubic inch per minute (in'/min) cubic meter per second (m'/s) 2.731 177 E-07

cubic yard per minute (yd'/min) cubic meter per second (m^/s) 1.274 258 E-02

gallon (U.S.) per day (gal/d) cubic meter per second (m^/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 minute (gpm) (gal/min) cubic meter per second (m'/s) 6.309 020 E-05

gallon (U.S.) per minute (gpm) (gal/min) liter per second (L/s) 6.309 020 E-02

WORK (see ENERGY)

68
Guide for the Use of the International System of Units (SI)

Appendix C. Comments on the References of Appendix D — Bibliography

C.1 OfTicial interpretation of the SI for the United States: 55 FR 52242-52245

The official interpretation of the International System of Units for the United States,

which is the responsibility of the United States Department of Commerce, is stated in the

Federal Register, Vol. 55, No. 245, p. 52242, December 20, 1990 [15]. This Federal Register

Notice is reprinted in NIST Special Publication 814 [1], together with the Federal Register

Notice that states the metric conversion policy for Federal Agencies [16] and the Executive

Order on metric usage in Federal Government programs [17].

C.2 Defining document for the SI: BIPM SI Brochure

The defining document for the International System of Units is the Brochure published

by the International Bureau of Weights and Measures (BIPM) in French, followed by an

English translation [2]. This document is revised from time to time in accordance with the

decisions of the General Conference on Weights and Measures (CGPM).

C.3 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.2) is published by the National Institute of Standards and Technology as NIST

Special Publication 330 [3]; it differs from the translation in the BIPM publication in the

following details: — the dot is used as the decimal marker, in keeping with recommended United States

practice (see Sees. C.l and C.7); — the spelling of English-language words — for example, "meter," "liter," and "deka" are used instead of "metre," "litre," and "deca" — is in accordance with the United States

Government Printing Office Style Manual [4], 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 Sees. C.l and

C.7); — editorial notes regarding the use of the SI in the United States are added; — the index is moderately expanded.

Inasmuch as NIST Special Publication 330 is consistent with Ref. [1] (see Sec. C.l),

SP 330 is the authoritative source document on the SI for the purposes of this Guide.

C.4 ISO 1000

ISO 1000:1992 [5] is an international consensus standard published by the International

Organization for Standardization (ISO) to promote international uniformity in the technical

interpretation of the actions of the CGPM as they are published by the BIPM in Ref. [2] (see

Sec. C.2).

C.5 ISO 31-0

ISO 31-0:1992 — ISO 31-13:1992 [6] constitute a series of international consensus standards published by ISO to promote international uniformity in the practical use of the SI in

various fields of science and technology, and in particular to standardize the symbols for

various quantities and the units in which the values of these quantities are expressed. These

standards are compatible with Ref. [2] published by the BIPM (see Sec. C.2).

69
Guide for the Use of the International System of Units (SI)

C.6 IEC27

lEC 27-1— lEC 27-4 [7] constitute a series of international consensus standards published by the International Electrotechnical Commission (lEC) to promote international

uniformity in the practical use of the SI in electrical technology, and in particular to standardize the symbols for various quantities used in electrotechnology and the units in

which the values of these quantities are expressed. These lEC standards are also compatible

with Ref. [2] published by the BIPM (see Sec. C.2), and they are coordinated with the ISO

standards cited in Sec. C.5 (Ref. [6]). The lEC standards should be regarded as more authoritative than the corresponding ISO standards only in connection with electrical technology.

C.7 ANSI/IEEE Std 268

ANSI/IEEE Std 268-1992 [8] is an American National Standard for Metric Practice; it

is based on the International System of Units as interpreted for use in the United States (see

Sees. C.l and C.3). It has been approved by a consensus of providers and consumers that

includes interests in industrial organizations, government agencies, and scientific associations. This standard was developed by the Institute of Electrical and Electronics

Engineers (IEEE), and approved as an American National Standard by the American

National Standards Institute (ANSI)." ANSI/IEEE Std 268-1992 has been adopted for use by the United States Department of Defense (DoD) and serves as the basis of Ref. [18] (see

Sec. C.9); it is recommended 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 United States technical organizations; see Ref. [8], note 2.)

C.8 Federal Register notices

Important details concerning United States customary units of measurement and the

interpretation of the SI for the United States are published from time to time in the Federal

Register; these notices have the status of official United States Government policy.

A Federal Register notice of July 1, 1959 [9] states the values of conversion factors to be used in technical and scientific fields to obtain the values of the United States yard and

pound from the SI base units for length and mass, the meter and the kilogram. These

conversion factors were adopted on the basis of an agreement of English-speaking countries

to reconcile small differences in the values of the inch-pound units as they were used in

different parts of the world. This action did not affect the value of the yard or foot used for

geodetic surveys in the United States. Thus, at that time, it became necessary to recognize

on a temporary basis a small difference between United States customary units of length for

"international measure" and "survey measure." A Federal Register notice of July 19, 1988

[10] announced a tentative decision not to adopt the international foot of 0.3048 meters for

surveying and mapping 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 [11]. 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 [12].

^^The American National Standards Institute, Inc. (11 West 42nd Street, New York, NY 10036) is a private

sector organization that serves 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 Untied States Member Body of the International Organization for Standardization (ISO),

and functions as the administrator of the United States National Committee for the International Electrotechnical

Commission (lEC).

70
Guide for the Use of the International System of Units (SI)

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 [13].

A Federal Register notice of July 27, 1968 [14] provides a list of the common customary

measurement units used in commerce throughout the United States, together with the conversion factors that link them with the meter and the kilogram.

A recent Federal Register notice concerning the SI [15] is a restatement of the interpretation of the International System for use in the United States, and it updates the corre- sponding information published in earlier notices.

A Federal Register notice of January 2, 1991 [16] 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 [17] provides Presidential authority and

direction for the use of the metric system of measurement by Federal departments and

agencies in their programs.

C.9 Federal Standard 376B

Federal Standard 376B [18] was developed by the Standards and Metric Practices

Subcommittee of the Metrication Operating Committee, which operates under the Inter- agency 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 metric units 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 [17] (see Sec. C.8). The basis of Fed. Std. 376B is ANSI/IEEE Std. 268-1992

[8] (see Sec. C.7).

C.IO 1986 CODATA values of the fundamental constants

The set of self-consistent recommended values of the fundamental physical constants

resulting from the 1986 Committee on Data for Science and Technology (CODATA) leastsquares adjustment of the constants, the most up-to-date set currently available, is given in

Ref. [20]. The next CODATA adjustment of the constants is planned for completion in 1996;

some of the considerations relevant to that adjustment may be found in B. N. Taylor and

E. R. Cohen, Recommended Values of the Fundamental Physical Constants: A Status

Report, J. Res. Natl. Inst. Stand. Technol., Vol. 95, No. 5, p. 497 (September-October 1990).

C.ll Uncertainty in measurement

Reference [21] cites two publications that describe the evaluation and expression of

uncertainty in measurement based on the approach recommended by the CIPM in 1981 and

which is currently being adopted worldwide.

71
Guide for the Use of the International System of Units (SI)

Appendix D. Bibliography

[1] Interpretation ofthe SIfor the United States and Metric Conversion Policy for FederalAgencies, Ed. by B. N. Taylor, Natl. Inst. Stand. Technol. Spec. Publ. 814 (U.S. Government

Printing Office, Washington, DC, October 1991).

[2] Le Systeme International d'Unites, The International System of Units, 6th Edition (Bur.

Intl. Folds et Mesures, Sevres, France, 1991).

Note : This publication, which is commonly called the SI Brochure, consists of the

official French text followed by an English translation.

[3] The International System of Units (SI) , Ed. by B. N. Taylor, Natl. Inst. Stand. Technol.

Spec. Publ. 330, 1991 Edition (U.S. Government Printing Office, Washington, DC,

August 1991).

Note: This publication is the United States edition of the English translation in

Ref. [2].

[4] United States Government Printing Office Style Manual (U.S. Government Printing

Office, Washington, DC, 1984).

[5] SI units and recommendations for the use of their multiples and of certain other units, ISO

1000:1992 (International Organization for Standardization, Geneva, Switzerland,

1992).

Notes :

1 ISO publications are available in the United States from the sales department

of the American National Standards Institute (ANSI), 105-111 South State

Street, Hackensack, NJ 07601.

2 See the note at the end of Ref. [6].

[6] The following 14 Standards, which are cited in the text in the form [6: ISO 31- . . . ], are published by the International Organization for Standardization (ISO) Geneva,

Switzerland:

Quantities and units

Quantities and units

Quantities and units

Quantities and units

Quantities and units

Quantities and units

Quantities and units

Quantities and units

Quantities and units

Quantities and units

Quantities and units

Quantities and units

Quantities and units

Quantities and units

72

- Part 0 General principles, ISO 31-0:1992. - Part 1 Space and time, ISO 31-1:1992. - Part 2 Periodic and related phenomena, ISO 31-2:1992. - Part 3 Mechanics, ISO 31-3:1992. - Part 4 Heat, ISO 31-4:1992. - Part 5 Electricity and magnetism, ISO 31-5:1992. - Part 6 Light and related electromagnetic radiations , ISO

31-6:1992. - Part 7 Acoustics, ISO 31-7:1992. - Part 8 Physical chemistry and molecular physics , ISO

31-8:1992. - Part 9 Atomic and nuclear physics, ISO 31-9:1992. — Part 10: Nuclear reactions and ionizing radiations , ISO

31-10:1992. — Part II: Mathematical signs and symbols for use in physical

sciences and technology, ISO 31-11:1992. - Part 12: Characteristic numbers, ISO 31-12:1992. - Part 13: Solid state physics, ISO 31-13:1992.
Guide for the Use of the International System of Units (SI)

Note: 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 availability of ISO publications in the United States is discussed in Ref. [5], note 1.)

[7] The following four standards, which are cited in the text in the form [7: lEC 27- . . . ], are published by the International Electrotechnical Commission (lEC), Geneva,

Switzerland.

Note: lEC publications are available in the United States from the American

National Standards Institute — see Ref. [5], note 1.

Letter symbols to be used in electrical technology, Part 1: General, lEC 27-1 (1991).

Letter symbols to be used in electrical technology. Part 2: Telecommunications and electronics, lEC 27-2 (1972) [including lEC 27-2A (1975) and lEC 27-2B (1980), first and

second supplements to lEC 27-2 (1972)].

Letter symbols to be used in electrical technology, Part 3: Logarithmic quantities and units,

lEC 27-3 (1989).

Letter symbols to be used in electrical technology, Part 4: Symbols for quantities to be used

for rotating electrical machines, lEC 27-4 (1985).

[8] American National Standard for Metric Practice, ANSI/IEEE Std 268-1992 (Institute of

Electrical and Electronics Engineers, New York, NY, October 1992).

Notes

:

1 IEEE publications are available from the Institute of Electrical and Electronics

Engineers, Service Center, 445 Hoes Lane, P.O. Box 1331 Piscataway, NJ

08855-1331.

2 A number of similar standards for metric practice are published by United

States technical organizations. They include:

Standard Practice for Use of the International System of Units (SI) (The Modernized Metric System), E 380-93 (American Society for Testing and Materials,

Philadelphia, PA, 1993).

Note: ASTM publications are available from the Customer Service

Department, American Society for Testing and Materials, 1916

Race Street, Philadelphia, PA 19103.

Rules for SAE Use of SI (Metric) Units, SAE J916 MAY 91 (Society of

Automotive Engineers, Warrendale, PA, May 1991).

Note: SAE publications are available from the Society of Automotive

Engineers, 400 Commonwealth Drive, Warrendale, PA 15096.

3 The Canadian Standards Association, 178 Rexdale Boulevard, Rexdale

(Toronto), Ontario, Canada, M9W 1R3, publishes CAN/CSA-Z234. 1-89,

Canadian Metric Practice Guide , a Canadian National Standard. It is similar in

scope to ANSI/IEEE Std 268-1992 (Ref. [8]).

4 A joint ASTM-IEEE effort is currently underway to consolidate ANSI/IEEE

Std. 268-1992 and ASTM E 380-93 into a single ANSI standard.

5 The application of the SI to physical chemistry is discussed in Quantities, Units

and Symbols in Physical Chemistry, prepared by I. Mills, T. CvitaS, K. Homann,

N. Kallay, and K. Kuchitsu, Second Edition (International Union of Pure and

Applied Chemistry, Blackwell Scientific Publications, Oxford, 1993).

73
Guide for the Use of the International System of Units (SI)

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

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

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

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

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

[14] Federal Register, Vol. 33, No. 146, p. 10755, July 27, 1968.

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

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

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

[18] Preferred Metric Units for General Use by the Federal Government, Federal Standard

376B (General Services Administration, Washington, DC, 1993).

[19] Radiation Quantities and Units, ICRU Report 33, 1980; and Quantities and Units in

Radiation Protection and Dosimetry, ICRU Report 51, 1993 (International Commission

on Radiation Units and Measurements, 7910 Woodmont Avenue, Bethesda, MD,

20814).

[20] E. R. Cohen and B. N. Taylor, The 1986 adjustment of the fundamental physical constants. Rev. Mod. Phys., Vol. 59, No. 4, p. 1121 (October, 1987).

[21] The term combined 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, 1993); 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).

[22] A. J. Thor, Secretary, International Organization for Standardization (ISO) Technical

Committee (TC) 12, Quantities, units, symbols, conversion factors (private communication, 1993). ISO/TC 12 is responsible for the ISO International Standards cited in Refs.

[5] and [6].

74
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