When numbers are used as the first part of a compound adjective, use a hyphen to connect them to the noun that follows them. This way, the reader knows that both words function like a unit to modify another noun. This applies whether the number is written in words or in digits.
In some compounds with numbers—namely, with the number at the end of the compound—a hyphen is also not required. Take a look at the examples below, where the compound clearly functions as one standalone unit:
Hyphen in Compound Adjective With Fractions
When using a fraction (e.g. half or quarter) as part of a compound adjective, it should be hyphenated so the reader understands which fraction is modifying which noun.
Hyphen With Number of Years
Use hyphens when you are using someone’s age as a label.
Don’t use hyphens when you’re just talking about a span of time.