5.08 Fractions

In non-technical writing, spell out simple fractions, especially when used in isolation:

  • half of one percent
  • half an inch or one-half inch
  • a quarter of an inch or one-quarter inch
  • three quarters of an inch or three-quarters inch (not inches)
  • three-quarter length

When a fraction is used adjectivally, place a hyphen between the numerator and the denominator unless either of these elements is itself hyphenated:

  • four-fifths inch
  • a three-quarters majority

Fractions such as the last two, which lend themselves to confusion, are better expressed in numerals.

For the use of hyphens with fractions, see 2.11 Fractions.

It is incorrect to use th or ths after fractions expressed in numerals:

  • 1/25 not 1/25th
  • 3/100 not 3/100ths

A fraction expressed in numerals should not be followed by of a or of an:

  • 3/8 inch not 3/8 of an inch

If the sentence seems to require of a, the fraction should be spelled out.

Mixed numbers (combinations of a whole number and a fraction) should be given in numerals:

  • but time and a half for overtime