Style Guide > 1 Formatting > [1.144] Symbols
[1.144] Symbols
1.144 Symbols
Always define the units of measure that are used in the document, and their abbreviations and symbols, at the start of the text. Symbols that are widely known (distance, mass, volume, and time—see Standard abbreviations for symbols in common use that do usually not require definition). Repeat these definitions as often as you think is necessary to avoid any possibility of misunderstanding. Even the most widely used symbols, prefixes, and abbreviations are frequently used incorrectly. For example, what is the correct symbol for the kilotonne: kt, Kt, kT or KT?
The shortened form of an SI unit of measure is known as a symbol, and obeys different punctuation rules to abbreviations. Non-SI units can, however, use abbreviations. There are many exceptions and should be studied carefully.
1.145 Symbols, usage
The following is a summary of the more important rules for use of symbols.
If the symbol is well-known (°C, %, m, and kg, for example), it can be used without definition (see Units of measure, common symbols). Otherwise, define your symbols at first use and in the list of abbreviations in the front matter.
For a more complete description, refer to the following topics:
[1.146] Metric system
For guidelines on using SI symbols and when they should be spelled out, see Symbols, usage.
1.146 SI 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 following.
Note. The format and punctuation of SI symbols is an ISO standard, which is applied in Australia through AS ISO 1000–1998.
1.147 Punctuation, symbols
Symbols are always separated from the numerical quantity they follow by a non-breaking space, except for percent (%) and planar angle (° ' "),  which are the only symbols that follow the measurement without a space. Symbols can be preceded by a prefix, as in GB and kW, but must never be placed next to another symbol.
A symbol is never followed by a period, except the imperial symbol for inch, which always has a period. If the symbol is at the end of a sentence, normal punctuation is applied.
1.148 Plurals, symbols
The letter “s” is never added to a symbol to indicate a plural.
Note. The name of the unit is a noun, is not capitalized (except degree Celsius), and obeys the standard rules of punctuation when spelled out.
2 cms
5 Kg
1.2 cts
38 %
15 ° 45 ' 10 "
5 in.
2 cm
7 m
5 kg
1.2 ct
15° 45' 10"
38.6 °C
1.149 Case, prefixes
Symbols must be written as they are defined no matter where they occur in a sentence or title. Capitalizing a symbol changes its meaning. For example, t is the symbol for the tonne, and T is the symbol for the tesla and the tera prefix.
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. The symbol for litre is an exception, and is normally capitalized. Both L and l are acceptable but L is preferred, because the letter l is easily confused with the number 1. Note that the spelled-out unit is not capitalized (joule, for example) except for degree Celsius and degree Fahrenheit (but you must include the uncapitalized “degree”).
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. And the correct symbol for kilotonne is kt, and never Kt or kT.
The symbol for kilogram kg is the only SI symbol that includes a prefix. Only one prefix can be used on a symbol. Hence km is the symbol for (kilo-)metre, but mkg (milli-)(kilo-)gram is not permitted.
There are twenty SI prefixes. The most widely used prefixes are:
Note that the prefix k, although an anomaly (prefixes that signify a factor of greater than 1 are otherwise upper case), is never capitalized, as in kB, km, kg, kPa, and kL.
1.150 Derived units (exponent, factor, per)
The superscripts 2 and 3 are always used for “square” and “cubic”, respectively.
A middle dot · is used to separate multiplied symbols, as in kilowatt hour kW·h —symbols must not be placed next to each other.
Note. In cases where a non-standard format is in general use and does not cause confusion—kWh, for example—define it as an abbreviation. For more information, see Non-standard units.
The slash mark / is used for “per”. Furthermore, only one slash mark is allowed per symbol.
m/s2 (engineering)
m·s-2 (scientific)
sq km
Note. You will find instructions on how to type a middle dot, non-breaking space, thin space, and other special characters in Typing shortcuts.
1.151 Non-standard units
Although the correct SI format is always preferred, in cases where a non-standard format is widely accepted in an industry, it can be defined as an abbreviation and then used legitimately. This list also includes measurements for which there are no accepted SI symbols, such as machine-hour and bank cubic metre.
The terms bank cubic metre (in situ volume of rock in m3) and loose cubic metre (broken volume of rock in m3) are widely used in the mining and construction industries. They are also useful when expressing large volumes (1000 m3 = kBCM, not km3). The SI-compliant km3 (= GBCM) is rarely used.
[1.152] List of SI symbols
1.152 List of SI symbols
The base, derived, and acceptable SI units (International Organization for Standardization) and their symbols are identified in the following table. Other units shown are outside the SI system but are widely used. For simplicity, treat units that are outside the SI system as symbols (see also Imperial system, below).
Refer to Glossaries for symbols and abbreviations that are specific to an industry.
Angle unit equal to 160 of a degree.
Measurement of concentration (1100), usually volumetric or population.
Typography unit equal to 172 of a foot.
Angle unit equal to 160 of a minute.
[1.153] Imperial system
1.153 Imperial system
Imperial or traditional English units are less regular than SI units and are prone to error and misinterpretation. 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.
The recommended practice 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. For example, the abbreviation psi is commonly used for pounds per square inch, however the correct unit of measure is pounds force per square inch, which has the algebraic symbol lbf/in.2. In this case, using the more recognizable symbol psi, defined in the text as pounds force per square inch (lbf/in.2), avoids any confusion. A more general example is the symbol for pound (lb).
In general, do not add an SI prefix to an imperial unit (see Non-standard units for exceptions). The SI symbol formatting rules otherwise apply:
2 lb
2 pounds
1 pound
2 million tons
2 lb.
2 lbs
2 pound
2 Mton
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