Using Abbreviations or Symbols
In the International System of Units (SI),
the units do not have "abbreviations". They have
symbols. The unit symbols do not follow the
grammatical rules for abbreviations, because they follow the
mathematical rules for symbols instead. These rules include the
- A symbol is never followed by a period (unless, of course, it happens to
fall at the end of a sentence).
- The letter "s" is never added to a symbol to indicate a plural.
- In other words, 2 minutes is written 2 min, not 2 min. or 2 mins.
- Symbols are case-sensitive and must be written as they are defined.
- There is a tradition in the metric system that the first (or only)
letter of an unprefixed unit symbol is capitalized if (and only if) the
unit's name comes from a proper name. Thus W is the symbol for the watt
and A is the symbol for the ampere, because
these units are named for scientists.
- It makes a big difference whether a symbol is capitalized or not, because
often the same letter represents different units: t stands for the tonne
and T for the tesla, for example.
- There is one loophole in the rule on capitalization: it's acceptable
to use the symbol L instead of l for the liter, since the letter l is
so easily confused with the number 1.
- The case of symbol prefixes is specified, upper and lower, and must
not be changed. For example, the symbol for kilo- is k-, so kW and not
KW is the symbol for the kilowatt.
- The superscripts 2 and 3 are always used for "square"
and "cubic", respectively.
- Thus the square kilometer, for example, is written km2, not
- A raised dot (also called a middle dot or half-high dot) is recommended
when symbols are multiplied. It is permissible to use a space instead, but
symbols should not be placed next to one another with nothing between them.
- For example, A·h is the recommended symbol for the ampere hour.
A h is also permitted, but not Ah or amp hr.
- The slash (solidus) / is used for "per". Furthermore, only one slash is
allowed per symbol.
- This means the SI unit of acceleration is written m/s2 rather
than m/s/s, even though it is often spoken "meters per second per second".
(Negative exponents can also be used: m/s2 can be written m·s-2.)
- Symbols are separated from the numerical quantity they follow by a space.
- Thus 5 kilograms is written 5 kg, not 5kg.
In the English customary systems there are no "official"
symbols or abbreviations. For many English units a variety of abbreviations
are used. Sometimes these abbreviations duplicate metric symbols; for example,
"A" is sometimes used in English for the acre
instead of the ampere. It would be better to
use the symbol "ac" for the acre. In this dictionary, the first symbol listed
is either the official one or the one that should be preferred in order to minimize
confusion of units.
The policy of this dictionary is to define symbols for the traditional English
units and to apply the SI rules to their use. This is done for both consistency
and clarity. It avoids a number of outstanding problems caused by the traditional
abbreviations for the English units, especially the following.
- In the English systems, there is no general agreement as to whether abbreviations
for units are capitalized or not. When English-speaking writers extend this
informal practice to metric symbols, they sometimes create wild errors, such
as "10 ML" used for 10 milliliters when it actually means
10 megaliters. Even standard dictionaries in English sometimes
give the wrong case for metric symbols! In this dictionary, the SI convention
for capitalization is applied (with a very few exceptions) to the symbols
used for the English units: a letter is capitalized only if it comes from
a proper name. Thus we write Btu (not BTU) for the British thermal unit.
- In English traditional unit abbreviations, the letter "p" is often used
as an abbreviation for "per", "sq" or "s" as an abbreviation for "square",
and "cu" or "c" as an abbreviation for "cubic". These are all bad ideas,
despite their long usage, and they lead to confusing abbreviations. Although
it is traditional to write "psi" for pounds per square inch, the symbol
would be much clearer. (In fact, the correct symbol should really be lbf/in2,
since the pounds in question are pounds of force, not pounds of mass!) A
traditional abbreviation such as CFM (cubic feet per minute) is often not
understood by general readers; ft3/min
has a much better chance of being understood.
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July 14, 2004