When used as compound adjectives, expressions like four-and-a-quarter and two-and-a-half should be hyphenated to aid your reader and to eliminate ambiguity.
Terms Like One-and-a-Half Should Be HyphenatedExpressions like three-and-a-half (as in three-and-a-half ounces) are classified as compound adjectives and should be hyphenated. This is done to group the words together to show they are all part of the same adjective. For example:
Do Not Link the Adjective to the Noun with a HyphenSometimes, the tricky bit is spotting where the compound adjective ends and the noun starts. This is important because the noun should not be joined to the adjective with a hyphen.
An adjective is a describing word (e.g., red, big, beautiful, contagious). (See lesson Adjectives.)
A single adjective made up of two or more words is called a compound adjective. The words in a compound adjective are often linked with a hyphen (or hyphens) to show they are part of the same adjective.
What are adjectives?
What are compound adjectives?
Hyphens in compound adjectives
How to write numbers in full
Starting sentences with numbers