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This section covers:
When to use unit symbols
There is no ‘rule’ about when to use unit symbols (e.g. kg) or words (e.g. kilogram) for units. Here are some guidelines:
- If few units are used in the text, use words.
- Use symbols in tables and figures if space is limited.
- If the document uses many units
- write in full at first use and give the symbol in brackets; then use symbols from then on, or
- use symbols and provide a table explaining them.
- If uncertain, use words for units when the document is intended for a general audience, particularly if it contains few units.
Numbers with unit symbols
Reminder. Insert a nonbreaking space between a number and its unit.
To insert a nonbreaking space:
Use Ctrl+Shift+Space (Windows), or Option+Space (Mac).
A unit symbol has either an alphabetic basis (e.g. km, kg) or a nonalphabetic basis (e.g. $, %).
Always use arabic numerals (not words) for numbers used with a symbol:
9 kg not nine kg
Do not use a symbol without a number:
Weight was measured in kilograms. not Weight was measured in kg.
Separate an alphabetic symbol from a number with a nonbreaking space. An alphabetic symbol is always singular – do not add an s to the end – and is not followed by a full stop unless it occurs at the end of a sentence:
1 g 18 mm 7 L 2 km 300 m2 pH 7
A nonalphabetic symbol is usually closed up to the number:
$10 20% <20 50× [magnification] 90° angle
except if it is a mathematical operator (in which case a nonbreaking space is used each side of the operator; see Symbols in mathematics), degree Celsius or the SI unit ohm (in which cases the number and unit are separated by a nonbreaking space):
P < 0.5 25 °C 10 Ω
However, expressions that use < or > and a measurement or number should generally only be used in parentheses or in tables; in running text, replace the symbol with a word:
Overnight temperatures for July were less than 2 °C.
Average night-time temperatures varied (<2 °C in June, >10 °C in January).
Did you know? In Microsoft Word, a nonbreaking space appears in the text like a ‘degree’ symbol: °.
Singular or plural units
Unit symbols are always singular (i.e. they are not followed by s) and in running text are treated as singular:
10 mL was added. not 10 mL were added.
During the year, $20 million was donated. not During the year, $20 million were donated.
If the name of the unit is written out, the plural form takes an s:
0.5 kilogram 1 gram 18 millimetres 7 litres 2 kilometres 1 degree Celsius 2 degrees Celsius
but it is treated as singular in running text:
In all, 10 millilitres was added.
Numbers and units at start of sentence
If a number and its associated symbol start a sentence, write them out in words:
Ten millilitres of solution was added.
not
Ten mL of solution was added.
or
10 mL of solution was added.
Twenty per cent of people with ...
not
Twenty % of people with ...
Series of numbers
In a series of numbers with alphabetic unit symbols, place the unit after the last number:
... at intervals of 5, 10, 15 and 20 m
In a series of numbers with nonalphabetic unit symbols, place the unit after each number:
either 5%, 10% or 20%
in multiples of $5, $20 or $50 notes
Providing a key to units used
For documents intended for a mixed audience (e.g. scientists, policy makers and the general public) or specifically for the general public, it is often useful to provide a key to the units used in the document. This can be in the preliminary pages of the document as a standalone box, or as a separate table within the list of shortened forms or glossary.
See Lists of shortened forms for further information.