Devised by Robert Gunning
- Take a representative sample of at least 100 words (stopping at a full stop as soon after 100 as possible).
- Count the number of sentences within that sample.
- Divide the total number of words by the number of sentences (this figure is A).
- 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).
- (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
The key is short words and short sentences. If you you use long words, it is even more important to keep the sentence short.