9 Rules and Style Conventions for Spelling Unit Names
The following eight sections give rules and style conventions related to
spelling the names of units.
9.1 Capitalization
When spelled out in full, unit names are treated like ordinary English nouns.
Thus the names of all units start with a lower-case letter, except at the
beginning of a sentence or in capitalized material such as a title.
In keeping with this rule, the correct spelling of the name of the unit °C is
"degree Celsius" (the unit "degree" begins with a lower
case "d" and the modifier "Celsius" begins with an
upper-case "C" because it is the name of a person).
9.2 Plurals
Plural unit names are used when they are required by the rules of English
grammar. They are normally formed regularly, for example, "henries"
is the plural of henry. According to Ref. [8],
the following plurals are irregular: Singular - lux, hertz, siemens;
Plural - lux, hertz, siemens. (See also
Sec. 9.7.)
9.3 Spelling unit names with prefixes
When the name of a unit containing a prefix is spelled out, no space or hyphen
is used between the prefix and unit name (see
Sec. 6.2.3).
| Examples: |
milligram |
but not: |
milli-gram |
| kilopascal |
but not: |
kilo-pascal |
Reference [8] points out that there are three
cases where the final vowel of an SI prefix is commonly omitted: megohm
(not megaohm), kilohm (not kiloohm), and hectare (not
hectoare). In all other cases where the unit name begins with a vowel, both the
final vowel of the prefix and the vowel of the unit name are retained and both
are pronounced.
9.4 Spelling unit names obtained by multiplication
When the name of a derived unit formed from other units by multiplication is
spelled out, a space, which is preferred by
Ref. [8] and this Guide, or a hyphen
is used to separate the names of the individual units.
Example: pascal second or pascal-second
9.5 Spelling unit names obtained by division
When the name of a derived unit formed from other units by division is spelled
out, the word "per" is used and not a solidus. (See also
Sec. 6.1.7 and
Sec. 9.8.)
Example: ampere per meter (A/m) but not:
ampere/meter
9.6 Spelling unit names raised to powers
When the names of units raised to powers are spelled out, modifiers such as
"squared" or "cubed" are used and are placed after the unit
name.
Example: meter per second squared (m/s2)
The modifiers "square" or "cubic" may, however, be placed
before the unit name in the case of area or volume.
| Examples: |
square centimeter (cm2) |
cubic millimeter (mm3) |
| ampere per square meter (A/m2) |
kilogram per cubic meter (kg/m3) |
9.7 Other spelling conventions
A derived unit is usually singular in English, for example, the value
3 m2 · K/W is usually spelled out as "three
square meter kelvin per watt," and the value
3 C · m2/V is usually spelled out as "three
coulomb meter squared per volt." However, a "single" unit may be
plural; for example, the value 5 kPa is spelled out as "five
kilopascals," although "five kilopascal" is acceptable. If in
such a single-unit case the number is less than one, the unit is always
singular when spelled out; for example, 0.5 kPa is spelled out as
"five-tenths kilopascal."
| Note: |
These other spelling conventions are given for completeness; as indicated
in Sec. 7.6, it is the position of this
Guide that symbols for numbers and units should be used to express the
values of quantities, not the spelled-out names of numbers and units.
Reference [4] also requires that a symbol for
a number be used whenever the value of a quantity is expressed in terms of a
unit of measurement. |
9.8 Unacceptability of applying mathematical operations to unit names
Because it could possibly lead to confusion, mathematical operations are not
applied to unit names but only to unit symbols. (See also
Sec. 6.1.7 and
Sec. 9.5.)
| Example: |
joule per kilogram or J/kg |
or J · kg-1 |
| but not: |
joule/kilogram |
or joule · kilogram-1 |
