Writing SI units and symbols

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Quite a few of us do not write the SI units correctly. If you are a Physics or Chemisty student, and still remember what you studied in school, you’ll know the importance of getting your units and symbols right.

Important in SI:

  1. The short forms for SI units (such as mm for millimeter) are called symbols, not abbreviations.

  2. SI symbols never end with a period unless they are the last word in a sentence.

    • RIGHT: 20 mm, 10 kg

    • WRONG: 20 mm., 10 kg.

  3. SI symbols should be preceded by digits and a space must separate the digits from the symbol.

    • RIGHT: It was 300 mm wide. The millimeter width was given.

    • WRONG: It was 300mm wide. The mm width was given.

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

    • RIGHT: 1 mm, 500 mm, 1 kg, 36 kg

    • WRONG: 500 mms, 36 kgs

    • BUT: It is correct to pluralize written-out metric unit names: 25 kilograms, 250 milliliters

  5. The symbol for a compound unit that is a quotient of two units is indicated by a solidus or by a negative exponent.

    • RIGHT: km/h or km·h-1 (for kilometers per hour)

    • WRONG: kmph or kph (do not use p as a symbol for “per”.)

    • BUT: It is correct to say or write “kilometers per hour”.

  6. The meaning of an SI symbol can be changed if you substitute a capital letter for a lower case letter.

    • RIGHT: mm (for millimeter, which means 1/1000 of a meter)

    • WRONG: MM or Mm (M is the prefix for mega, which means one million; a megameter is a million meters)





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