The following recommendations are based on those of the United States' National Institute of Standards and Technology:
For a detailed, more rigorous treatment of written usage in SI, see Taylor, 1995 (a PDF file, 539KB).
When written out, the names of units start with a lowercase letter, except at the beginning of a sentence or in a title. The only exception is “degree Celsius.”
Symbols for units are lowercase unless they come from a person's name, in which case the first letter of the symbol is capitalized. The exception, in the United States, is L for liter. See liter. Click here for a list of the symbols in the United States.
Symbols for numerical prefixes are lowercase, except for those representing multipliers of 106 or more: mega- (M), giga- (G), tera- (T), peta- (P), exa- (E), zetta- (Z), and yotta- (Y). When spelled out, all prefixes are lowercase.
When the names of units are written out, they should be made plural when the number to which they refer is greater than 1. Fractions are always singular. Symbols are never made plural.
| correct | incorrect |
|---|---|
| 3 kilograms | 3 kilogram |
| 3 kg | 3 kgs |
| 1.46 kg | |
| 1.46 kilograms | 1.46 kilogram |
| 0.46 kilogram | 0.46 kilograms |
A period or full stop is not used after a symbol, except at the end of a sentence.
In scientific and technical writing, all numbers expressing physical quantities should be represented by numerals.
In newspapers, it is usual to spell out the numbers from one to nine and use numerals for everything else.
In ordinary magazines and books, whole numbers from one through ninety-nine, and any of these followed by “hundred,” “thousand,” “million,” etc., are written out. If the unit is represented by an abbreviation or symbol, the associated number should be written as numerals.
In the United States, the dot is used as the decimal marker and is placed on the line. In the rest of the world, the comma is used as the decimal marker. (A centered dot was formerly used in Great Britain. e. g. “0·14”.)
Digits should be separated by spaces into groups of 3, counting from the decimal marker. The use of a space, and not a comma, is necessary because in the United States, in nontechnical writing a period is used as the decimal marker and a comma is used to separate groups of three digits, while in other countries the comma is used as the decimal marker. Using a space avoids crosscultural confusion. No space should be put in a number that has only 4 digits.
| correct | incorrect |
|---|---|
| 3.141 592 | 3.141592 |
| 176 000 | 176,000 |
| 1568 | 1 568 |
A prefix should not be separated from the name of the unit by a space or hyphen.
A space should be left between a symbol and the number to which it refers, with the exception of the symbols for degree, minute, and second of angles.
| correct | incorrect |
|---|---|
| 3 kg | 3kg |
| 3° 27′ 59″ | 3 ° 27 ′ 59 ″ |
| 5 nanoseconds | 5 nano-seconds |
When the SI units are used with symbols for mathematical operations, the units should also be represented by symbols, not by their written-out names. This principle also applies to compound units like the newton meter.
| correct | incorrect |
|---|---|
| joules per mole | joules/mole |
| J/mol | |
| j·mol−1 | joules·mol−1 |
| N·m | newton·meter |
| newton meter | |
| newton-meter |
For the official position on usage, see 9th CGPM, 1948, resolution 7.
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Last revised: 21 March 2006.