Editing numbers
Are SI units singular or plural?
The International System of Units (SI, after the French Système International d'Unités) was created by the International Bureau of Weights and Measures (BIPM) in 1875 to create a worldwide unified system of measures.
There is often confusion about whether SI units, and their derived units, are singular or plural.
The first thing to note is whether the unit abbreviation is used or whether it is spelled out.
Unit abbreviations are never pluralised, so you never add an ‘s’ after a unit abbreviation.
4 kg not 4 kgs
6 m not 4 ms
However, if the unit is spelled out, it should be pluralised if required by grammatical convention.
four kilograms
six metres
Do not mix unit abbreviations with spelled out unit names.
According to some published style guides, unit abbreviations are used with numerals and spelled out unit names are used where the number is written in words.
6 kg or six kilograms (not 6 kilograms or six kg)
3 m/s or three metres per second (not 3 metres/s or three m/s)
If a quantity is equal to or less than one, the unit is usually singular.
0.5 s or one-half of a second or five-tenths of a second
1 m or one metre
Finally, when writing about specific quantities that act as one unit, the verb is usually singular, because the quantity being discussed is treated as one thing or a single entity. This is because the one thing/single entity is acting as a mass noun, or as a collective singular count noun.
At the last step, 5 mL of water was added to the mix.
In the past year, 50,000 tonnes of waste was sent to landfill.
For further guidance on SI units, refer to the BIPM SI Brochure, the National Institute of Standards and Technology Guide for the Use of the International System of Units (SI), or any other relevant and respected style guide.
