Gunning Fog Index Definition

The Gunning Fox index is the number of years of formal education that a person requires to easily comprehend the text on an initial reading.

Gunning Fog Index = 0.4 x (ASL + ((SYW / words) x 100))


ASL = Average sentence length (the number of words divided by the number of sentences)

SYW = Number of words with three or more syllables

For reference, the New York Times has an average Fog Index of 11-12, Time magazine about 11. Typically, technical documentation has a Fog Index between 10 and 15, and professional prose almost never exceeds 18.

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