Guilford College Writing Manual

This is the official Guilford College Writing Manual. A collaboration between the English Department and the Hege Library.

Gunning's Fog Index

Want to see at a glance how your diction and syntax choices affect your prose's readability?

Professional editors often use readability formulas to gauge how well writers are meeting an audience's reading levels, and one of the most famous of readability formulas is Gunning's Fog Index.

It's premise is that the bigger the words you use and the more complex your sentences, the more difficult your prose will be to read. You may or may not agree with this premise, but Gunning's Fog Index is fun to use anyway. And it enables writers to do some good old-fashioned counting. Along the way, you will become more aware of the role of these two important stylistic factors.

Here are the steps:

  1. Choose a sample of your writing that is at least 100 words long.
  2. Count the exact total number of words. Then find the average length of your sentences by dividing the total number of words by the number of sentences.
  3. Count the total number of words that have three syllables or more. Exclude the following from your count:
    • proper nouns (e.g., names or other words that are capitalized. If a noun is capitalized because it is the first word in a sentence but wouldn't ordinarily be capitalized, count it)
    • compound nouns which combine short easy words like housekeeper (house + keeper)
    • verbs whose third syllable is "es" or "ed" (e.g., donated, trespasses)
  4. Add the numbers representing (a) the average sentence length and (b) three-syllable-plus words.
  5. Multiply the total from step four by 0.4. The number you get is the Fog Index. It tells you approximately how many years of school a person would need to have had in order to process your writing easily. If the total is, say 10.4, we're talking about a tenth-grade reading level. If we're talking about a total over 20, well . . . .

One implication of the Fog Index becomes clear if we look at the two versions if the following memo. The first version has a fog index of between 16 and 17, which suggests that it's pitched to an audience of college graduates. The second version has a fog index of between 7 and 8. What if the audience to which the memo is being addressed consists of high-school graduates? Is there any reason for the higher level of complexity?

Version 1:


Since general safety conditions are one of the responsibilities you have within your department, we would appreciate your disseminating the following information.

In a recent inspection of all areas the most obvious hazard detected was the manner in which office equipment is placed or used in relation to personnel movement or traffic.

All personnel in your area should be made aware of possible Safety Hazards and take precautionary measures at all times so that a high "Safety Level" may be maintained.

  1. Electrical cords on equipment such as computers, printers, etc., should not be permitted to lie loose on the floor where the possibility of someone tripping over them exists.
  2. When not in immediate use, desk and file drawers should be kept closed at all times.

Version 2:


Safety in your department is your responsibility. Please see that your people are made aware of hazards and that they take measures to prevent them.

A recent inspection revealed a chief hazard to be office equipment placed in the way of people moving about.

  1. Electrical cords of computers, printers, etc., should not lie loose on the floor. Someone may trip over them.
  2. Keep desk and file drawers closed when not in use.