Gunning FOG Index

I have been reading novels and books for as long as I can remember, but until Monday (2nd), I had never heard of the Gunning FOG Index. So, I set out to discover more about it.

This Index was created by Robert Gunning, an American publisher of textbooks, who observed that many of his high school graduates were unable to read. However, he soon discovered that it wasn’t because of illiteracy but because there was too much unnecessary complexity, or fog, in the text, which hindered their progress. Forming the first consulting firm specialising in readability, in 1944, Robert Gunning worked with more than sixty daily newspapers and popular magazines, aiding their editors to write to their target audience by using his FOG Index Readability Formula. Robert Gunning published a book, The Technique of Clear Writing and created an easy-to-use FOG Index, in 1952.

The FOG Index works on a scale of 1 – 12, with 1 being the simplest and 12 being the hardest to read.

To give you an idea of the FOG Index for different publications:

Time Magazine, Newsweek and the Wall Street Journal – about 11

Reader’s Digest – between 8-9

The Bible, Mark Twain and William Shakespeare – about 6

The equation for the FOG Index or Readability Formula is below:

  1. Take a sample passage of at least 100-words and count the number of exact words and sentences.
  2. Divide the total number of words in the sample by the number of sentences to arrive at the Average Sentence Length (ASL).
  3. Count the number of words of three or more syllables that are NOT (1) proper nouns, (2) combinations of easy words or hyphenated words, or (3) two-syllable verbs made into three with -es and -ed endings.
  4. Divide this number by the number or words in the sample passage. For example, 25 long words divided by 100 words gives you 25 Percent Hard Words (PHW).
  5. Add the ASL from Step 2 and the PHW from Step 4.
  6. Multiply the result by 0.4.

Readability Level = 0.4 (ASL + PHW) 

Here is a link to an on-line Gunning FOG Index calculator.

It is quite a fascinating area and, if you like writing as much as I do, it can help you to reach your target audience. It is not, however, an exact science or perfect formula, but it does give you a very good idea as to your writing level and your reading level. I tried it out on a passage from Lou Morgan’s The Patron Saint of Wishful Thinking, and found it had a readability level of 8-9.


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