Principles for using unit symbols

When to use unit symbols

There is no ‘rule’ about when to use unit symbols (eg kg) or words (eg 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.
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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 (eg km, kg) or a nonalphabetic basis (eg $, %).

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 m    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 Mathematical operators), degree Celsius or the SI unit ohm (in which cases the number and unit are separated by a nonbreaking space):

< 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: °.

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Singular or plural units

Unit symbols are always singular (ie 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.

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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.
Ten mL of solution was added.   
10 mL of solution was added.

Twenty per cent of people with ...   
Twenty % of people with ...

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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

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Providing a key to units used

For documents intended for a mixed audience (eg 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.

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