Gunning Fog Index

The Gunning Fox index was developed by Robert Gunning and is one of the simplest and most effective manual tools for analyzing readability. Gunning defines hard words as those with more than two syllables. To get to a fourth-grade readability level, you need to write with an average sentence length of eight words and no more than one out of 50 words being three or more syllables. It is relatively easy to calculate and accurate within one grade level.
The ideal score for readability with the Fog index is 7 or 8; anything above 12 is too hard for most people to read. The Bible, Shakespeare and Mark Twain all have Fog Indexes of about 6. Time, Newsweek, and the Wall St. Journal average about 11.

To analyze a writing using the Fog Index:

  1. Select a sample of at least 100 words. Count the number of sentences. Divide the total number of words in the sample by the number of sentences to get the average sentence length (ASL).
  2. Count the number of words with three or more syllables in the sample. Don’t count: 1) proper nouns; 2) hyphenated words; or 3) two-syllable verbs made into three with -es and -ed endings.
  3. Divide this number by the number or words in your sample. For example, 15 long words divided by 100 words gives you 15 percent hard words (PHW).
  4. To get the fog index, add the average sentence length and the percent hard words and multiply this by .4. The formula looks like this: (ASL + PHW) .4 = Grade Level. This is the number of years of schooling the reader would have to have to understand the writing sample.