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The Fog Index and Readability Formulas

"Readability formulas" determine if documents are written at the correct reading level for their targeted audience. 

Gunnings’s Fog Index is one of the best known and measures the level of reading difficulty of any document.

The formula for the index is as follows:

( (average number of words per sentence) + (number of words of 3 syllables or more) ) * 0.4 = Fog index

The Fog Index level 'translates' the number of years education a reader needs to understand the material. The "ideal" score is 7 or 8; anything above 12 is too hard for most people to read.

The Fog Index does not determine if the writing is too basic or too advanced for a particular audience; instead, it helps you decide whether a document could benefit from editing or using "plain language" techniques.

How to Use the Fog Index

1. Select a short passage and count the number of words. For a lengthy document, select several different passages and average the Fog Index.

For example:

SINCE John Snow, a rail-company boss, was picked this week to replace Paul O’Neill as America’s treasury secretary, the talk in economic and financial circles has been all about the effect his appointment might have on economic policymaking. Among staffers at the Federal Reserve, though, much of the gossip continues to be about when—and whether—they themselves will get a new boss. The central bank’s current chairman, Alan Greenspan, has been in the job since 1987; his current term expires in 2004. So, along with many Fed-watchers, insiders want to know whether Mr Greenspan will, if he seeks to, be reappointed.

Number of words = 102

2. Count the number of sentences within the passage.

Number of sentences = 4

3. Count the number of big words (3 or more syllables). Exclude words in which "es" or "ed" form the third and final syllable, hyphenated words like "state-of-the-art", and compound words like "newspaper."

Number of "big words" = 9

4. Calculate the average sentence length by dividing the number of words by the number of sentences.

Average sentence length = 102/4 = 25 words

5. Calculate the percentage of big words by dividing the number of big words by the number of words, and multiplying by 100.

Percentage of big words = (9/102) * 100 = 11.30%

6. Add the average sentence length to the percentage of big words and multiply that result by 0.4; that’s the Fog Index score.

Fog Index = (25 + 9) * 0.4 = * 0.4 = 13.6

The example passage requires an 14th grade level of reading ability. Using technical terms or jargon tends to increase Fog Index scores. If these are words that the audience would be familiar with, you don’t need to count them as big words.

Mark Twain and the Fog Index

The Bible, Shakespeare and Mark Twain all have Fog Indexes of about 6.
Time, Newsweek, and the Wall St. Journal average about 11.

If you find your Index soaring into the teens beware you've lost most of your audience in the dense fog.

Other Popular Indexes

Flesch Reading Ease Score

  • Rates text on a 100-point scale; the higher the score, the easier it is to understand the document.
    Most standard documents aim for a score of 60 to 70.

Flesch-Kincaid Grade Level Score

  • Rates text on a U.S. grade-school level (same yardstick that the Gunning Fog index uses).

Appraisal of Indexes

Popular because they:

  • Reduce to simple formulas the complex work of writing

  • Provide a convenient check and measure of the level of one’s writing

  • Possess the glitter of mathematical exactness

  • Can be calculated by word processing software


  • Low writing styles can result from a slavish use of readability indexes—a monotonous succession of short sentences and simple words can make your writing dull and uninteresting to read.

  • Indexes frequently give conflicting results

Best Use of Indexes

Use the formulas as a general guide. Formulas will not replace the clear and logical thinking that is the foundation of all clear writing.

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