# Can units be plural?

According to the International System of Units (SI)

Unit symbols are mathematical entities and not abbreviations. Therefore, they are not followed by a period except at the end of a sentence, and one must neither use the plural nor mix unit symbols and unit names within one expression, since names are not mathematical entities.

as well as to the international standard ISO/IEC 80000 Quantities and units

Symbols for units are always written in roman (upright) type, irrespective of the type used in the rest of the text. The unit symbol shall remain unaltered in the plural and is not followed by a full stop except for normal punctuation, e.g. at the end of a sentence.

it is not acceptable to use the plural of unit symbols.

By the way, it is also not permissible to use abbreviations such as “hr” for unit symbols (“h”) or unit names (“hour”).

This is what I would tell your colleague: one of the main points of using units is that we can use them throughout physics formulas and they serve as a check for coherence (even though I am a mathematician, I cringe when I see physics' examples in math books that remove the units completely). So, say you have some quantity $r=a/b$, where $a$ is measured in kilograms and $b$ in kilograms per hour. Now you want to calculate $r$, and in a particular measurement/problem, $a$ is 2 kilogram, and $b$ is 1 kilogram per hour. Then $$r=\frac{2\,\mathrm{kg}}{1\frac{\mathrm{kg}}{\mathrm{h}}}=2\,\mathrm{h},$$ where one "cancels" the kilograms. With your colleague's suggestion, the above formula would be $$r=\frac{2\,\mathrm{kgs}}{1\frac{\,\mathrm{kg}}{\mathrm{h}}}=2\,\mathrm{h},$$ and you have the mathematical awkwardness of "cancelling" kgs with kg.

When you write that a length is $$75\,\rm metre$$ or $$75\,\rm m$$ you are really write that your length is seventy five times bigger than a length of $$1\,\rm metre$$ or $$1\,\rm m$$ ie $$75 \times (1\,\rm metre)$$ or $$75 \times (1\,\rm m)$$

I think that this illustrates that you should not use an “s” to signify a plural for the unit name and symbol.

However this is not the end of the story as the NPL in the UK recommends the following for the unit name.

For unit values more than 1 or less than -1 the plural of the unit is used and a singular unit is used for values between 1 and -1.

This is at variance of NIST in the USA which states.

Unit symbols are unaltered in the plural. proper: l = 75 cm improper: l = 75 cms.

Using a unit symbol which is never used in the plural form removes this ambiguity.

There’s a related question on English Stack Exchange with this answer:

In Standard English, this crucially depends on whether the phrase is prenominal or not. Prenominally, the phrase will not show plural marking, while elsewhere it will have the normal plural marking, as appropriate.

Compare:

• The bureau is 3 meters long.
• This is a 3-meter-long bureau. (prenominal)

• The period is 2 weeks.

• This is a 2-week period. (prenominal)

• The bill was 0.50 dollars.

• This is a 0.50-dollar bill. (prenominal)

Note also that a hyphen is normally inserted to connect the words in the adjectival phrase when the phrase is prenominal.

I suggest that a scientific text should not use the plural form of the names of a unit whereas non-scientific text and oral presentations may use the plural form of the name of the unit.