Intro to units of measure
This chapter provides usage guidelines for units of measure and lists abbreviations and symbols for frequently used units.
Follow these guidelines when you use units of measure in documentation:
When to spell out: In user documentation, spell out units of measure and give the abbreviation in parentheses on first occurrence. Repeat the spelled-out version in new sections and chapters if the unit symbol or abbreviation is obscure and if the audience requires it.
20 gigabytes (GB) of memory
Subsequent occurrences: 20 GB of memory
Always spell out nonmetric units of measure in text (for example, 17-inch display). It’s OK to abbreviate such units in tables and technical specifications (Display size: 17 in.).
Noun: When you use a unit symbol or abbreviation as a noun, insert a space between the number and the abbreviation, and use the preposition of before the unit the value quantifies.
20 GB of memory
Note that Apple Marketing Communications style for many abbreviations is to close up the number and the abbreviation (20GB of memory).
Adjective: When you use a spelled-out unit of measure in a compound adjective, hyphenate the compound.
17-inch display
3-meter cable
When you use a unit symbol or abbreviation in a compound adjective, don’t hyphenate; add a space between the number and the abbreviation.
20 nA battery
30 GB capacity
Capitalization: With the exception of degrees Celsius, Fahrenheit, and Rankine, units of measure derived from a proper name aren’t capitalized when spelled out, but their unit symbols are capitalized. (For example, the unit symbol for joule is J.)
Plurals: Unit symbols and abbreviations are unaltered in the plural forms (for example, lb., not lbs.).
International System of Units: Use the unit symbol for units of the International System of Units (SI) after first occurrence. If the unit symbol for meter could be confused with another term in your document, spell out meter.
The maximum size for a memory card is 35 mm.
The Ethernet cable is 40 meters long.
Don’t use a period after an SI unit (unless the unit appears at the end of a sentence).
Mixing symbols and names: Don’t mix unit symbols and names (m/second) or unit symbols and abbreviations (J/sec.). Don’t mix a prefix name with a unit symbol (kiloHz), or a prefix symbol with a unit name (khertz).