hyphens: numbers and units of measurement
Follow the guidelines below in deciding whether to use a hyphen with numbers and units of measurement.
Hyphenate compound cardinal and ordinal numerals from twenty-one (twenty-first) to ninety-nine (ninety-ninth) when they are written out:
- There are twenty-nine members on the committee.
Compound adjectives containing a number
Hyphenate a compound adjective in which one element is a cardinal or ordinal number and the other a noun:
- a two-car family
- a five-kilometre trek
- a $4-million project
- a first-class coach
- a third-rate play
- an 18th-century novel
However, do not hyphenate the compound adjective if the number, written in full, is itself a compound:
- a two hundred and fifty millilitre flask
In cases such as the above, use the abbreviated form (a 250 ml flask) if at all possible.
For further information, see numbers in compound adjectives.
Number before a symbol
Do not hyphenate before a symbol that is not a letter:
- a 100 °C thermometer
- a 30% increase
Do not hyphenate before a metric unit symbol:
- a 5 km trail
- 35 mm film
- a 100 W bulb
Number before a possessive noun
Do not hyphenate a possessive noun preceded by a number:
- one week’s pay
- 40 hours’ work
- three weeks’ vacation
- 10 months’ leave
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