How to Compute Gunning's Fog Index

Robert Gunning's Fog Index was one of the first efforts to quantify the readability of textual material. The number that results from the following calculations is meant to correlate to grade level.


Count the words and sentences in a representative passage of about 100 words


Divide the number of words by the number of sentences to give the average length of each sentence


Count the number of words of three or more syllables that are not (a) proper names, (b) combinations of easy words, or (c) made three syllables by suffixes such as &endash;ed, &endash;es, or &endash;ing


Add the average sentence length from step 2 and the number of "difficult" words from step 3.


Multiply the resulting sum by 0.4.

Gunning Fog Index = [(number of words / number of sentences) + "difficult words"] x 0.4

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