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Eight style points for metric measurements

Christa Bedwin
Editors’ Association of Canada
(The Editors’ Association of Canada began using the name Editors Canada on July 1, 2015.)

2014-05-05

Certain style points for metric are different from those for the Imperial measurement system. Though some style guides may continue to follow the historical style for decades to come, here we would like to set the record straight and clarify the correct styles, according to international metric standards.

  1. Put a space before a metric unit.

    This includes all metric units, including degrees of temperature. Adding a space before degrees Celsius (°C) is a convention that has been slow to catch up, but the space is the international metric standard. For example, use 82 °C, not 82°C, and 910 m, not 910m.

  2. Do not put hyphens before metric units, even when they are used in an adjectival sense.

    In the Imperial system, hyphens between numbers and units were (and are) common. This is not recommended metric practice, though it has persisted in Canadian use.

    So, while you might write "six-inch nails," or even "6-inch nails," they would be 15 cm nails in metric, not 15-cm nails. It might be a 6-mile run, but it is a 10 km run. Think of it as less romantic if you must, but do not put hyphens before metric units.

  3. The case of the letters for the metric symbols matters very much. The symbol Mm means "megametres," or one million (1 000 000) m, while mm means "millimetres," or one-thousandth (0.001) m.

  4. Do not put a period after the symbol for a metric unit.

    The only time there would be a period following a metric unit would be if it was at the end of a sentence.

  5. Do not add an "s" to make metric units plural. They are plural already.

    E.g. 1 m, 2 m (not 2 ms, as ms would mean "milliseconds")

  6. For modern metric measurement, signal units of area with an exponent of 2 for square units, and units of volume with an exponent of 3 for cubed units.

    E.g. m2, m3

    A common error that authors make in manuscripts is not to check that their superscripts are formatted properly: m2 does not mean "metres squared"; it must be m2.

    For typesetting, you can use the superscript 2 and 3 characters: on a PC, they're ALT 0178 and ALT 0179 (typed on the numeric keypad).

    The units litre (L) for volume and hectare (ha) for area may be used in place of the standard SI units of m3 and m2.

  7. Choose metric prefixes that will be easily known to your reader, while making the number as easy as possible.

    The most commonly known prefixes are kilo- and milli-. Since the advent of computers, people have become more familiar with tera-, giga-, and mega-. Since the advent of microbiology and nanotechnology, people have become more familiar with micro- and nano-. But have a thought for your readers before getting too fancy.

    A full list of metric prefixes is available from The Canadian Style online.

    Example:  It is better to write "370 m" than "37 dam" or "3.7 hm".

    It is better to use whole numbers than decimal numbers less than one, so "370 m" is better than "0.37 km," unless for some reason this number is being compared with a number of other distances expressed in km.

    Use common unit prefixes. Everyone knows m and km and cm and mm, but fewer people would recognize hm, dam, and dm, much less remember what factor of ten the prefix represents.

  8. Metric unit symbols must always be set in plain text and not italics. Letters in italics are reserved for mathematical and statistical variables. (Variables are letters that represent numbers.)