Fog Index

Devised by Robert Gunning

  1. Take a representative sample of at least 100 words (stopping at a full stop as soon after 100 as possible).
  2. Count the number of sentences within that sample.
  3. Divide the total number of words by the number of sentences (this figure is A).
  4. Count the number of words with three or more syllables in the sample. DO NOT COUNT proper nouns (words that must begin with a capital), hyphenated words, three-syllable words made so because they have had “ed” or “es” added on the end, such as allowed or explodes (this figure is B).
  5. (A + B) x 0.4 = Fog Index.

A Fog Index of 9 – 12 is acceptable. More than 12 means the reader has to put in more effort to understand the ideas in the piece.

Best-selling novels average 6-7

Government circulars average 23

Other writing measures (links open new windows)