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Weights and Measures Connection, Vol. 11, Issue 1 January 2021 Page 1

Writing with the SI 

Byline: Elizabeth Benham 

Writing is a regular part of most jobs. But in the Science, Technology, Engineering, and Mathematics (STEM) fields, 

it’s especially important that measurement results are communicated clearly and unambiguously. Avoiding 

measurement unit writing errors is critical. Even slight measurement unit oversights undermine confidence in the 

resulting products and services, or worse...result in costly mistakes or deadly mishaps. In weights and measures, a 

metrology laboratory communicates calibrated values and uncertainties to the regulatory official using a calibration 

certificate. In manufacturing, a product label communicates the net quantity of contents to consumers. In a retail 

environment, a store shelf label communicates the unit price to shoppers. In some instances, we may never meet the 

end users of our measurement results face-to-face. However, they will certainly form an opinion about our 

organization based on their experiences with these written communications. 

The accepted convention of scientific and technical writing is to use the International System of Units (SI), commonly 

known as the metric system. Becoming more familiar with proper SI writing style will help add clarity to work 

correspondence and build confidence when communicating measurement results. 

The SI measurement system is built upon 7 base quantities, represented by 7 units and symbols: length (meter, m), 

mass (kilogram, kg), time (second, s), electric current (ampere, A), thermodynamic temperature (kelvin, K), amount 

of sub-stance (mole, mol), and luminous intensity (candela, cd). The system also includes derived units, which are 

formed for convenience of notation by combining base units algebraically with other derived units. While there are 

22 SI derived units with special names, there are additional commonly used derived quantities that do not have special 

names, such as area (square meter, m2

) and volume (cubic meter, m3). Additionally, there are some units that can be 

used with SI units. For example, the second (s) is the SI base unit of time but the units of minute (min), hour (h), and 

day (d) are also per-mitted for use. There are units that are given special names, such as the liter (L) and hectare (ha). 

Figure 1. Basic SI quantity and unit symbol with prefix format

A series of 20 prefix names and symbols are used to form the names and symbols of the decimal multiples and 

submultiples of SI units (www.nist.gov/pml/weights-and-measures/metric-si-prefixes). The scale of a quantity can 

be in-creased or decreased using powers of 10. Using SI prefixes helps an author represent units that are of an 

appropriate magnitude for their application but are particularly helpful when expressing very small or very large 

quantities. For example, a surveyor may choose kilometers (km) on a map legend, while an engineer would select 

millimeters (mm) on a mobile phone design specification diagram. It is important to remember that when a prefix is 

used, the prefix name and the unit name are combined to form a single word, and similarly the prefix symbol and the 

unit symbol are written without any space to form a single symbol (Figure 1). For example, write milligram (mg), not 

milli gram (m g). Compound prefix names or symbols are not permitted. For example, nm (nanometer) is correct, 

NOT mμm (millimicrometer).

One of the primary benefits of using the SI in written communication is that unit symbols are the same in all languages, 

while spelling of unit names and pronunciation are language dependent. Using the SI provides broad understandability 

of quantitative information across a wide audience who may read our communications. Because spelling varies among 

languages, it is essential to use SI symbols to facilitate the interpretation of written measurement results. It is important 

to note that spelling differences even exist between American and British English. For example, the American spelling 
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Weights and Measures Connection, Vol. 11, Issue 1 January 2021 Page 2

of the prefix deka (da) uses a “k”, not deca with a “c” and the element Cs is spelled cesium, not caesium, and the base 

unit of length is spelled meter, not metre. The National Institute of Standards and Technology (NIST) follows the 

American English writing practices found in Webster's Third New International Dictionary. 

It’s recommended to always consult NIST Special Publication (SP) 811, Guide for the International System of Units, 

when applying SI writing style rules and review written communications using the manuscript 

checklist (www.nist.gov/pml/special-publication-811). Here are a few writing best practices and tips. 

Formatting Best Practices for Writing Quantities. SI units should be written in an upright typeface with a space between the numeric quantity value and the unit symbol (see Figure 1). Unit symbols are a “shorthand” way of 

representing a unit name. They should not be confused with abbreviations, or a shortened form of a word or phrase. 

Abbreviations are commonly used in non-SI measurements (e.g., U.S. customary units). A unit symbol represents 

both the singular and plural tense, so an “s” should not be added at the end of a unit symbol to represent more than 

one (e.g., kg, not kgs). If the spelled-out name of a unit is used in a sentence, the normal rules of English grammar 

are applied. For example, “35-millimeter film.” Punctuation is not used after a unit symbol. For example, “the bottle 

has a 123 mm radius,” not “the bottle has a 123 mm. radius.” 

The decimal marker in the U.S. is commonly a dot on the baseline, while a comma is frequently used outside the U.S., 

but both methods are permitted by the International Bureau of Weights and Measures (BIPM) SI Brochure for use 

with the SI (www.bipm.org/en/publications/si-brochure/). When numerical values have more than four digits on 

either side of the decimal marker, it is a formatting best practice to separate the numbers into groups of three using a 

thin nonbreaking space (Ctrl + Shift + Space) counting from both the left and right of the decimal marker. For 

example, 12 345.678 91 is highly preferred to 12345.67891. Commas are not used to separate digits into groups of 

three. For numbers less than one, a leading zero is written before the decimal marker. Leading zeros help minimize 

the risk of magnitude errors. For example, accidently interpreting 0.1 g as 1 g (a ten-fold error). 

Use formatting practices to make it clear to readers which unit symbol belongs with a numerical value or mathematical 

operation. For example, write 35 cm × 48 cm, not 35 × 48 cm. Write quantity ranges with the text “to” instead of a 

dash; a dash may be misinterpreted as a negative symbol. For example, 20 °C to 30 °C or (20 to 30) °C, but not 20 °C 

- 30 °C.

Measure Best Practice Unacceptable

Numerical Values (4 or more) 

Grouping 12 345.678 91 12345.67891

Leading Zero 0.1 g .1 g

Space Between Quantity & 

Symbol 0.25 % 0.25%

Formatting Ranges 20 °C to 30 °C or (20 to 30) °C 20 °C - 30 °C

Table 1. Tips for writing values of quantities

SI Writing Tips 

The written names of all SI units begin with a lowercase letter, except when grammar rules require capitalization, such 

as at the beginning of a sentence or capitalizing a proper noun. For example, the base unit for time is the second 

(lowercase) and s (lowercase) is the unit symbol. For those units named after a person, the unit name is still written 

in lowercase, but the unit symbol is capitalized like a proper noun. For example, the SI unit for pressure is the pascal 

(lowercase), represented by the unit symbol Pa (uppercase), in honor of the French mathematician and physicist Blaise 

Pascal. One exception involves writing “degree Celsius” where degree is lowercase and Celsius is capitalized. 

Another exception has been made for the liter (L), where an uppercase L is used for the unit symbol to avoid confusion 

between a lowercase l and the numeral 1; in some fonts (like the present) they are indistinguishable! 
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Weights and Measures Connection, Vol. 11, Issue 1 January 2021 Page 3

Capitalization also gives meaning to prefix symbols. For example, the prefix symbol M (uppercase) represents the 

prefix mega, while the symbol m (lowercase) represents the prefix milli. In this case, an accidental capitalization 

would cause a huge error in magnitude. NIST SP 811 describes several capitalization best practices 

(www.nist.gov/pml/special-publication-811). 

To quickly improve SI writing practices, be aware of these very common SI unit symbol writing mistakes (Table 2). 

These tips and more are highlighted on the Writing with Metric Units webpage (www.nist.gov/pml/weights-andmeasures/writing-metric-units). Please keep in mind that some variations of SI writing format are allowed by 

regulation or within certain industry sectors. For example, “mcg” is accepted substitute in some healthcare 

applications for “millionth” instead of using the symbol μ (mu) for the prefix micro. Weights and measures writers 

should look to the Uniform Packaging and Labeling Regulation (UPLR) in NIST Handbook (HB) 130 for accepted SI 

labeling practices (www.nist.gov/pml/weights-and-measures/publications/nist-handbooks/other-nisthandbooks/other-nist-handbooks-2-1). 

For more information, please contact Elizabeth Benham, NIST Metric Coordinator, at elizabeth.benham@nist.gov or 

TheSI@nist.gov.

Measure SI Symbol

Unacceptable Examples

Symbol Format Abbreviation

Mass

kg

KG

KG.

Kg

Kg.

Kgr

kgs

g

G

G.

g.

GR

GRM

Grms

gr

gms

Length

km

Km

KM

km.

kms

m

M

m.

mtr

mm

MM

Mm

mm.

mmtr

Volume

L

L.

l

l.

ltr

mL

ML

Ml

mL.

mls

cm3 cm^3 cc

Pressure

kPa

KPa

KPA

Kpa

mmHg mm Hg

Temperature 20 °C 20°C

20° C 20 deg C
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Weights and Measures Connection, Vol. 11, Issue 1 January 2021 Page 4

Measure SI Symbol

Unacceptable Examples

Symbol Format Abbreviation

Terminology Acceptable Obsolete

micrometer micron

degree Celsius degree centigrade