NIST Guide to the SI, Chapter 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 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.

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 Secs. 6.1.7 and 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 [3] 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 Secs. 6.1.7 and 9.5.

Example:  joule per kilogram or J/kg or J·kg^-1but not:  joule/kilogram or  joule·kilogram^-1