Writing Tips

SI Units (Metric system) (3)

This tip is the third in a series about expressing the units of the Système international d'unités—also known as SI, or (incorrectly) the metric system. October's tip concerned the purpose and structure (prefix+base) of SI units. November's tip listed the correct abbreviations of the more commonly used units. This month's tip discusses how the unit names and abbreviations are used in running text.

Name or Abbreviation?
Because SI unit names and abbreviations are linked, in use, to numbers, the choice between using a unit name and a unit abbreviation depends on how the number is expressed. If the number is spelled out, use the unit name; if the number is expressed in digits, use the unit abbreviation. For example:

• We walked three kilometres.
• This Formula One track is 11.2 km long.

Spacing, Punctuation, and Pluralization
The SI unit name or abbreviation is always separated from the preceding number by one space. When the unit abbreviation is used, the space should usually be a hard space, as discussed in the May 1997 tip.

SI abbreviations are never terminated by periods (except when they fall at the end of a sentence). Similarly, SI abbreviations never take an added "s" to mark the plural.

SI and "Metric"
Finally, why is it incorrect to call SI "the metric system"? "The metric system" properly refers only to the system of weights and measures created in France around the time of the French Revolution. "Metric" encompasses only the measures of length, area, volume, and mass based on the metre and kilogram. Here's what Napoleon Bonaparte¹ had to say about the subject:

The scientists had another idea which was totally at odds with the benefits to be derived from the standardization of weights and measures; they adapted to them the decimal system, on the basis of the metre as a unit; they suppressed all complicated numbers. Nothing is more contrary to the organization of the mind, of the memory, and of the imagination.... The new system of weights and measures will be a stumbling block and the source of difficulties for several generations.... It's just tormenting the people with trivia!!!

Some people today would say the same about SI. But as nations and cultures continue to exchange ideas, world standards such as SI will become more common. And keeping to standard means keeping clarity in your writing.

¹ The Oxford Dictionary of Quotations. 3rd Ed. 1980.