Eight Style Points for Metric Measurements

Certain style points for metric are different from the Imperial measurement system.

Though some folks seem to intend to cling to historical styles (e.g. writing metric based on Imperial practices) for decades to come, here I 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. Use 910 m, not 910m. (This is often done incorrectly in news media, but that does notmake it acceptable in scientific papers. Use the correct international convention for formal writing.)

This convention extends to degrees of temperature. Add a space before degrees Celsius (°C). For example, use 82 °C, not 82°C.

Note: For Fahrenheit, most people would omit the space. That is still acceptable if you are writing degrees Fahrenheit. When you write in Celsius, use a space. If you have both Celsius and Fahrenheit in a paper, I suggest that you use the space for both for consistency.

2. Do not put hyphens before metric units, not even in an adjectival sense. 

In the Imperial system, hyphens between numbers and units were (and are) common. This is not recommended metric practice, though you may see some people continuing to use the Imperial habit when they write metric. Correct them. JMetric measurements use spaces, not hyphens.

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 capitalization of the letters for the metric symbols matters.

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. m^2, m^3

A common error that authors leave in manuscripts is not to check that their superscripts are formatted properly; “m2”’ does not mean metres squared; it must be m^2.

The units litre (L) for volume and hectare (ha) for area have been approved for use with the standard SI units of m^3 and m^2.

7. Never put metric units in italic type. 

Letters in italics are variables. Metric prefixes and units must be set in plain type.

8. 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 on Termium, at http://www.btb.termiumplus.gc.ca/tcdnstyl-srch?lang=eng&srchtxt=metric+prefixes&i=1&cur=1&nmbr=14&comencsrch.x=0&comencsrch.y=0

You can also see more you want to know about metric on the Bureau Internationale des Poids et Mesures, the international organization of standards for metric, at https://www.bipm.org.

While choosing a familiar prefix, also make the number as easy as possible (usually between 0.1 and 1000).

Example: Rather than describing a distance as 12 dam or 1.2 hm, write it as 120 m. It is better to use whole numbers than decimal numbers less than one, so 120 m is better than 0.12 km, unless for some reason this number is being compared with a number of other distances expressed in kilometres. It is also better to use common prefixes instead of the rarer ones, which may not be known. 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 goes with.

Only one symbol can be used with each metric unit.

 

The 7 Basic Units and the Prefixes

The SI system (aka the metric system) is easy and accessible and based on tens, and most people know it. The Système International is built on seven prescribed units: m, kg, s, A, mol, K, and cd, which are modified by prefixes for various powers of ten. Some other units such as the litre (L), hour (h), degree (°), and hectare (ha) are acceptable additions for metric practice. Further explanation of the slight difference between metric and SI is outside the scope of this article but is readily available elsewhere on the Internet.