Numbers as compound adjectives
 
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 Hyphenated

Expressions 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:

  • Two-and-a-half cakes
  • 3-and-a-quarter miles

Do Not Link the Adjective to the Noun with a Hyphen

Sometimes, 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.
Examples:

  • Two-and-a-half thousand (2,500)
  • (Here, the noun being modified is thousand. That is why there is no hyphen between Two-and-a-half and thousand.)
  • Two-and-a-half-thousand soldiers
  • (Here, the compound adjective is Two-and-a-half-thousand. In this example, thousand is part of the adjective modifying soldiers.)
  • 3-and-a-quarter million (3,250,000)
  • 3-and-a-quarter-million pounds
  • one-and-a-half dozen (18)
  • one-and-a-half-dozen eggs
  • Two and a half thousand (2,500)
  • (It is quite harsh to mark this wrong because there is quite a lot of leniency on whether you need to use hyphens to group the words in a compound adjective (especially if there is no ambiguity), but, technically, Two and a half thousand is 2 and 500, which equal 502.)
  • 3 and a quarter million (3,250,000)
  • (Technically, 3 and 250,000 equal 250,003.)
  • one and a half dozen (18)  
  • (Technically, 1 and 6 equal 7.)
 
ADJECTIVE?

An adjective is a describing word (e.g., red, big, beautiful, contagious). (See lesson Adjectives.)

COMPOUND ADJECTIVE?

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.

three-page document (< three-page is a compound adjective)
ironing-board cover (< ironing-board is a compound adjective)

See the lesson What are Compound Adjectives?
 

See also:

What are adjectives?
What are compound adjectives?
Hyphens in compound adjectives
How to write numbers in full
Starting sentences with numbers