Unit Symbols are not abbreviations

In case you (or a colleague😉) haven’t heard, unit symbols are never pluralized and always use the same letter case in all uses. This mostly comes up in the United States, as there’s a history of people using, and accepting the use of “lbs” to represent a pounds weight measurement. However, this is incorrect.

If you look at a scale, it will show lb, and not lbs.

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 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.

and the US Metric Association

Symbols always are written in the singular form (even when more than one is meant).

• Correct: 1 mm, 500 mm, 1 kg, 36 kg
• Incorrect: 500 mms, 36 kgs
• But, it is correct to pluralize unit names: 25 kilograms, 250 millilitres

Wikipedia also shows no adjustment to the symbol for pluralization. Although there is no explicit instruction, there isn’t a single “lbs” on the page.

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