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.
SINCE John Snow, a rail-company boss, was picked this week to replace Paul
ONeill as Americas 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
whenand whetherthey themselves will get a new boss. The central banks
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; thats 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 dont 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
Flesch-Kincaid Grade Level Score
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 ones
Possess the glitter of mathematical exactness
Can be calculated by word processing software
Low writing styles can result from a slavish use of readability
indexesa 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