The index uses the following formula:
Reading level (grade) = (Average no. of words in sentences + Percentage of words of three or more syllables) x 0.4
The resulting number is your Gunning Fog Index.
The Gunning Fog Index gives the number of years of education that your reader hypothetically needs to understand the paragraph or text. The Gunning Fog Index formula implies that short sentences written in plain English achieve a better score than long sentences written in complicated language.
For example, 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.