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2.11 Fractions

Hyphenate fractions used as modifiers and written in full, unless the numerator or denominator already contains a hyphen:

  • a one-third share
  • twenty-fiftieths calcium

but

  • one thirty-second
  • twenty-nine fiftieths calcium (In this case numerals are preferable.)

Exception

one and a half months

Usage is divided on whether fractions used as nouns should be hyphenated. We recommend that the hyphen not be used in such cases:

  • Four fifths of the load was wheat, and one fifth barley.