Unit symbols are printed in roman (upright) type regardless of the type used in the surrounding text. They are printed in lowercase letters unless they are derived from a proper name, in which case the first letter is a capital letter.
An exception, adopted by the 16th CGPM (1979, Resolution 6), is that either capital L or lowercase l is allowed for the litre, in order to avoid possible confusion between the numeral 1 (one) and the lowercase letter l (el).
A multiple or submultiple prefix, if used, is part of the unit and precedes the unit symbol without a separator. A prefix is never used in isolation, and compound prefixes are never used.
Unit symbols are mathematical entities and not abbreviations. Therefore, they are not followed by a period except at the end of a sentence, and one must neither use the plural nor mix unit symbols and unit names within one expression, since names are not mathematical entities.
In forming products and quotients of unit symbols the normal rules of algebraic multiplication or division apply. Multiplication must be indicated by a space or a halfhigh (centred) dot (·), since otherwise some prefixes could be misinterpreted as a unit symbol. Division is indicated by a horizontal line, by a solidus (oblique stroke, /) or by negative exponents. When several unit symbols are combined, care should be taken to avoid ambiguities, for example by using brackets or negative exponents. A solidus must not be used more than once in a given expression without brackets to remove ambiguities.
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 mm^{2} or square millimetre), cc (for either cm^{3} or cubic centimetre), or mps (for either m/s or metre per second). The use of the correct symbols for SI units, and for units in general, as listed in earlier chapters of this Brochure, is mandatory. In this way ambiguities and misunderstandings in the values of quantities are avoided.


m, metre
s, second
Pa, pascal
, ohm
L or l, litre
nm, not mµm
It is 75 cm long,
not 75 cm. long
l = 75 cm,
not 75 cms
coulomb per kilogram,
not coulomb per kg
N m or N · m
for a newton metre
m/s or _{} or m s^{–1},
for metre per second
ms, millisecond
m s, metre times second
m kg/(s^{3} A),
or m kg s^{–3} A^{–1},
but not m kg/s^{3}/A,
nor m kg/s^{3} A
