Guide for the Use of the International System of Units (SI)

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 ºC is “degree Celsius” (the unit 
“degree” begins with a lowercase “d” and the modifier “Celsius” begins with an uppercase “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. [6], 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 [6] points out that there are three cases in which the final vowel of an SI prefix is 
commonly omitted: megohm (not megaohm), kilohm (not kiloohm), and hectare (not hectoare). In all other 
cases in which 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. [6] and this Guide, or a hyphen is used to separate the names of the individual 
units. 

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