hyphens: numbers and units of measurement

Follow the guidelines below in deciding whether to use a hyphen with numbers and units of measurement.

Compound numerals

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 18^th-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