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Fog index
is it really worth the trouble?

Ubiquity, Volume 2022 Issue October, October 2022 | BY Philip Yaffe

Full citation in the ACM Digital Library  | PDF


Volume 2022, Number October (2022), Pages 1-4

Communication Corner: Fog index: is it really worth the trouble?
Philip Yaffe
DOI: 10.1145/3568307

Each "Communication Corner" essay is self-contained; however, they build on each other. For best results, before reading this essay and doing the exercise, go to the first essay "How an Ugly Duckling Became a Swan," then read each succeeding essay.

The Gunning Fog Index is supposed to provide a means of determining how easy or difficult a text will be to read. But does it? Can any formula really provide valid insights into the readability of a text? And if so, does it really matter?

I have never been a fan of prescriptions and checklists for good writing (and by extension good public speaking). I believe good writing and speaking both derive from a few basic principles that are easy to understand and, with some practice, easy to apply. Most of these principles were laid out in the earliest installments of "Communication Corner."

For people who like technical analyses of writing (and oral presentations), perhaps the most often cited is the so-called "Fog Index." This is widely described as "a proven method of analyzing written material to see how easy it is to read and understand."

Although I don't consciously use the Fog Index, I cite it here because it seems to have gained considerable currency among many amateur writers and speakers. I say I don't "consciously" use the index because I never sit down and actually do the required calculations. However I am certain that I regularly benefit from many of the principles and practices that supposedly underlie it.

The "ideal" Fog Index rating is 7–8. A rating above 12 indicates that the writing sample is too hard for most people to understand; a rating much lower than 7–8 indicates that many readers might find the text too simplistic and therefore off-putting.

Here is the recipe for calculating the Fog Index of a text:

  • Choose a sample of at least 100 words.
  • Count the number of words in the sample.
  • Count the number of sentences in the sample.
  • Count the number of "big (complex) words," defined as all words of three syllables or more.
  • Calculate the average sentence length, i.e. divide the number of words in the sample by the number of sentences.
  • Calculate the percentage of big (complex) words in the sample, i.e. divide the number of sentences into the number of big words.
  • Add the average sentence length to the percentage of big words.
  • Multiply the result by 4.

If you are mathematically inclined, the Fog Index might appeal to you as a quick, easy method for improving your writing. However, it has a number of flaws.

For instance, step 4 states all words of three syllables or more should be counted as big words, meaning words that are likely to be difficult for many people to instantly comprehend. But this is obviously false. For example, addition, automobile, education, hospital, intelligence, memory, reliable, restaurant, universal, university, etc. These would all qualify as "big words," but would anyone really have any difficulty understanding them?

The basic method also doesn't take into account the nature of the intended audience. Words that might be difficult for children and teenagers would be perfectly understandable to an audience of adults. Medical terms that might be difficult for a general audience would be perfectly understandable to an audience of doctors. And so on.

A more nuanced version of the Fog Index attempts to deal with such crucial differences by abandoning the simplistic notion that an index rating of 7–8 is in any way ideal. Instead, it tries to identify audiences for which any index result might be appropriate. It does so by categorizing the academic level most of the members of the audience will probably have achieved within the United States educational system.


So the question is: If the Fog Index is so flawed and its results so doubtful, is it really worth the trouble? The same criticism is true for other methods and means of trying to evaluate the readability of a text. So why bother?

In my 50 years of experience, I have found keeping one's eye firmly on the key principles of good writing (and by extension good speaking) achieves the best results. (You may benefit from taking a second look at the essay "First Write Like You Speak; Then Write Like You Write.")

Do you find it easier to say things than to write them? For instance, you have an idea for a new product for your company, or a new way for your organization to raise funds. You can easily describe it to a colleague in five minutes but putting it down on paper as a formal proposal takes hours and hours—and it still never seems to be right.

The essay "First Write Like You Speak; Then Write Like You Write" examines the source of this annoying phenomenon, and how you can turn it to your benefit.


Philip Yaffe was born in Boston, Massachusetts, in 1942 and grew up in Los Angeles, where he graduated from the University of California with a degree in mathematics and physics. In his senior year, he was also editor-in-chief of the Daily Bruin, UCLA's daily student newspaper. He has more than 40 years of experience in journalism and international marketing communication. At various points in his career, he has been a teacher of journalism, a reporter/feature writer with The Wall Street Journal, an account executive with a major international press relations agency, European marketing communication director with two major international companies, and a founding partner of a specialized marketing communication agency in Brussels, Belgium, where he has lived since 1974. He is the author of more than 30 books, which can be found easily in Amazon Kindle.

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