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SI Units (Metric system) (2)
This tip is the second of several about expressing the units of the Système international d'unités—also known as SI, or (incorrectly) the
metric system. Last month's tip concerned the purpose and structure (prefix+base) of SI units. This month's tip lists the correct abbreviations of the
more commonly used units.
Formation of the Abbreviations
Part of the "rationality" of the SI measurement system is the simpler formation of SI unit abbreviations. Essentially, every derived unit
abbreviation takes the form:
Looking again at the base units of length, area, volume, and mass, here are the names of the units and their abbreviations:
- Length: metre, abbreviated "m"
- Area: square metre, abbreviated "m2" OR are, abbreviated "a"
- Volume: cubic metre, abbreviated "m3" OR litre, abbreviated "L"
- Mass: gram, abbreviated "g"
The abbreviations for the common prefixes are:
- µ (micro: 0.000 001)
- m (milli: 0.001)
- c (centi: 0.01)
- d (deci: 0.1)
- da (deca: 10)
- h (hecto: 100)
- k (kilo: 1 000)
- M (mega: 1 000 000)
- G (giga: 1 000 000 000)
Capital or Lower Case?
In general, SI prefers lowercase for both spelled-out units and their abbreviations. But notice the use of capital letters above. As prefixes,
capitals are normally used only for multipliers higher than 1, and only when the prefix must be differentiated from a lowercase use of the same
letter. For base units, capital letters are normally used when the unit was proposed and defined by an individual after whom the unit is named. Take,
for example, the bel (a unit that measures the intensity of sound). Abbreviated, the bel is "B", because it was named after Alexander
Graham Bell. Others include the hertz ("Hz", Heinrich Hertz), the newton ("N", Sir Isaac Newton), and the watt
("W", James Watt).
The "K", "b", and "B" of Kb (kilobits) and KB (kilobytes) are not SI. Neither is the "K" as a generic
stand-in for 1000 ("He makes 50K per year"). In SI, "K" is a kelvin (one "degree" on the scale that measures
temperature from absolute zero). The "K" is capitalized because the kelvin scale is named for William Thomson, Lord Kelvin, who proposed it.
Ensure then, that in context, the meaning of "K" in written materials is always clear.
Also notice the abbreviation for litre. An uppercase "L" was adopted to distinguish it from the numeral 1.
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