Fog Index
Gunning Readability Formula


Updated Spring 2019 as a supplement to Professor Smith's textbooks
Strategic Planning for Public Relations and Becoming a Public Relations Writer, (Routledge/Taylor and Francis).



Studies show that shorter sentences are easier to read, which contributes to the reader's overall understanding of a piece of writing. These readability studies suggest that general audiences have the best comprehension with sentences of about 16 words. That's an average. Some will be longer. Some shorter. As the writer, you will want to give variety and rhythm to your text.


The Gunning Readability Formula, also known as the Fog Index, is a good tool for the public relations writer or marketing communicator. This is a simple way to measure the level of reading ease or difficulty for any piece of writing. 


The Fog Index is built on the premise that short words and sentences are easy to understand, long words and sentences more difficult. The index does not determine directly if the writing is too basic or too advanced for the audience. It does not even measure reading comprehension. Rather it is a mathematical calculation of the ease or difficulty of reading. To indicate this ease or difficulty, the index reflects the equivalent of a grade level. 


As the writer, you will decide what level is appropriate for your intended readers. You also will decide how much readers are interested in what you are writing, for this too will affect how you write. These decisions are part of the strategy of public relations. In analyzing your publics, you assess their reading abilities and their interest in the subject matter.


Reading 'Below' Your Educational Level: Remember that people always can understand writing that is less than their highest educational achievement without necessarily feeling that the writing is beneath them. General interest newspapers, for example, usually are written on about an eighth-grade level, and Associated Press wire copy is written at the level of an 11th-grade reader. The news media deal with some very sophisticated and complicated subjects in a style that makes their writing accessible to virtually all adults.


Reading 'Above' Your Educational Level: At the same time, people can understand writing that is "beyond" their educational level, but they will have to work a bit harder. Comprehension is impacted not only by reading ease but also by the reader's motivation or desire to know the information. Robert Gunning himself wrote: "It is not enough, for instance, to ask, 'What can people read?' If a person's motive is strong enough, he will plow through any complexity of words, signs, or hieroglyphs. The other day I watched a twelve-year-old boy reading a radio repairman's manual. The volume was not only clogged with technical terms; it was atrociously written as well. I doubt I would have read it with a whip over me, but he went through it from cover to cover" (1952. The Technique of Clear Writing. New York, McGraw-Hill, p13-14). Also, a person who is already familiar with the subject matter could read above grade level with less difficulty than someone not familiar with the issue and with concepts and vocabulary associated with it. 


A note of caution: The Fog Index and other readability measures are merely formulas for measuring textual difficulty. While this certainly is related to comprehension, don't assume that merely assigning a grade level to a particular piece of writing necessarily means that a reader either will be interested in the piece or will (or will not) be able to understand it. Remember also that readability formulas are based on generalities, such as short words being easier to read than long words. Yet short, uncommon words such as "turgid" or "pique" are probably more difficult to read (certainly more difficult to understand) than longer but familiar words such as "television" or "elephant."



The Fog Index is simple to calculate


There are many online calculators that indicate the Fog Index for a passage of writing. One that I frequently use is Another site,, provides not only the Fog Index but several other readability grade indicators.


If you wish to calculate the Fog Index manually, here are a few easy steps. 

1. Select a 100-word passage. For a lengthy piece of writing, select several different 100-word passages and average the Fog Index.

2. Count the number of sentences. If the passage does not end on a sentence break, calculate a percentage of the final sentence in the passage and add this to the count.

3. Divide the number of sentences into 100 to determine the average sentence length.

4. Count the number of long words in the passage. These are words of three or more syllables. But do not count words in which es or ed form the third and final syllable, hyphenated words like state-of-the-art, or compound words like newspaper.

5. Add the average sentence length and the number of long words (totals from steps 3 and 4).

6. Multiply this total by 0.4 *. This number indicates the approximate grade level of the passage.


* Ron Smith's Personal Note: Several people have emailed me from around the world asking about the Fog Index, particularly asking about the derivation of the 0.4 factor. I have searched the literature and have found no explanation of this. I presume that the factor is simply a necessary ingredient for the formula to yield a grade equivalent, something that may make sense to more mathematically inclined persons but which I am willing to take on faith. If anyone can share information on the 0.4 factor, I would appreciate hearing from you. You can email me at