The Fog Index

This KnowledgeBase archive includes content and external links that were accurate and relevant as of September 30, 2019.

Robert Gunning developed a way to measure how hard something is to read. His Fog Index in The Technique of Clear Writing (McGraw-Hill) is considered the most reliable formula for testing your writing. It is not an index of how good your writing is, but of how easy it is to understand. Good writing is another subject, but all writing must be clear before it can be good. Here is the Fog Index formula:

1. Take a sample of your writing. Count the words in several complete sentences until you have about 100 words. Divide the number of words by the number of sentences to get the average sentence length.
2. Count the words with three or more syllables. Do not count those words that are capitalized or combinations of short, easy words (like bookkeeper or garbageman) or that are made three syllables by adding –ed or –es (like expanded or confesses). This gives you the percentage of "hard" words if your sample is about 100 words.
3. Add the average sentence length and the percentage of hard words (like 15.25 percent, not .1525). Multiply by .4.

Example: Apply the Fog Index to the three numbered paragraphs above, without the samples in parentheses. Figures like 100 count as we read them – one hundred (two words). Likewise, we read ".4" as point four or four tenths (two words either way). The three paragraphs above have 98 words and eight sentences – an average of 12.25 words per sentence. The "hard" words total 10 – slightly more than 10 percent. Adding average sentence length (12.25) and percentage of hard words (10.20) gives you 22.45. Multiply that by .4 and you have 8.98. See what this means below.

If you have a Fog Index of more than 12, you run a serious risk of not being understood – or even read. This chart is not a measure of intelligence level, or of subject matter level, but only of reading level. The introduction mentioned Rudolf Flesch who wrote The Art of Readable Writing. Flesch gave a formula to measure reading ease. He based it on scientific studies by Columbia University Teachers College. The Flesch method measures the ease of reading for elementary through college grades. The Gunning Fog Index measures the grade level for clear understanding of writing. We all learn to read more difficult words before we understand them. Thus, the Fog Index level is usually two grades or higher than the reading ease grade level. The Flesch formula has remained valid through later Columbia studies. So we have two measures for writing. One for a reading ease grade level (Flesch) and the other for a clear understanding grade level (Fog Index). Note: Most newspapers write at the 8th to 10th grade understanding level (fifth to eighth grade reading ease). Why? Most people will not read the news and features if they have to labor over articles as if doing school research.

Source:

"Write to reach your readers use real words and common sense" BPA corporate communications group February 1999. Captured from the worldwide web on March 7, 2003.

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