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Formatting Unit Names, Unit Symbols, Prefixes and Values

Formatting Unit Names, Unit Symbols, Prefixes and Values

Owned by Shira Nagel
Last updated: Feb 20, 2016

Spacing

The value of a quantity is written as a number followed by a unit symbol with a space between them. This includes the percent sign (%) and the symbol for degrees of temperature (°C).
For example:

  • 12 V, 10 Hz
  • 30.2 °C, but not 30.2°C, nor 30.2° C

An exception is for the symbols for plane angular degrees, minutes, and seconds (°, ′, and ″), which are placed immediately after the number with no intervening space.
For example:

  • 30° 22′ 8″

Prefixes are attached to unit symbols without a space between the prefix name or symbol and the unit name or symbol.
For example:

  • k in km, M in MPa, G in GHz.

This last rule also applies to prefixes attached to unit names.
For example:

  • kilometre not kilo metre.

A prefix is never used in isolation, and compound prefixes are never used.
In English however, when the name of a derived unit is formed from the names of individual units by multiplication, then either a space or a hyphen is used to separate the names of the individual units.
For example:

  • pascal second, or pascal-second

There should be no spaces between sub- and superscripted parameters and their associated variables.
For example:

  • Vc not V c

Where a symbol or value with a subscript is followed by a superscript, for example VC2, we have found that latex would place the 2 directly above the c, so it looked too much like a fraction. In this case you can either:

  • Add a space to separate the subscript from the superscript to make them separately readable.
  • Insert brackets around the value that the superscript applies to.

For example:

  • (VC)2


Any line-break inside a number, inside a compound unit, or between number and unit should be avoided.

Formatting numbers and the decimal marker

If the number is between +1 and −1, then the decimal marker is always preceded by a zero.

Unacceptability of abbreviations for units

It is not permissible to use abbreviations for unit symbols or unit names, such as sec (for either s or second), sq. mm (for either mm2 or square millimeter), cc (for either cm3 or cubic centimeter), or mps (for either m/s or meter per second).

Typeface

Unit Symbols and are written in upright (Roman) type, so as to differentiate from the italic type used for quantities. By consensus of international standards bodies, this rule is applied independent of the font used for surrounding text.
For example:

  • Unit symbols: m for metres, s for seconds
  • Quantities: m for mass, s for displacement


Prefix names and symbols are also printed in roman (upright) type regardless of the type used in the surrounding text.

Case

Names of units start with a lowercase letter, even when the symbol for the unit begins with a capital letter. This also applies to "degrees Celsius", since "degree" is the unit.
Symbols for units begin with a capital letter when they are named after an individual.
For example:

  • ampere, A; kelvin, K; hertz, Hz; coulomb, C

Otherwise they always begin with a lower case letter.
For example:

  • metre, m; second, s; mole, mol

The symbol for the litre is an exception: either a lower case letter or a capital L may be used, the capital being allowed in this case to avoid confusion between the lower case letter l and the number one, 1.
Uppercase and lowercase prefixes are not interchangeable. For example the quantities 1 mW and 1 MW represent two different quantities; 1 milliwatt = 0.001 watts, and 1 megawatt = 1000000 watts. Prefix names are printed in lowercase letters.

Pluralisation

Symbols of units do not have a plural form.
For example:

  • 25 kg, not 25 kgs

Punctuation

Unit symbols are not followed by a full stop except at the end of a sentence. 
 

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