The Gunning Fog index, created in 1944, is a commonly cited readability scoring formula. But, what do the scores mean? How did the formula come about? And when is the test most useful?

"There are limits relating to long sentences and long words that the craftsman does not go beyond. The writer’s restrictions may be conscious or unconscious, but they are there. If not, he does not win an audience." — Gunning, 1964

What is a Gunning Fog Index readability score?

The Gunning Fog formula generates a grade level, typically between 0 and 20. The formula estimates the years of formal education the reader requires to understand the text on first reading.

So, if a piece of text has a grade level readability score of 6 then this should be easily readable by those educated to 6th grade in the US schooling system, i.e. 11-12 year olds.

Text to be read by the general public should aim for a grade level of around 8. Text above a score of 17 should be taken to have graduate level readability.

Where did the test come from?

The Gunning Fog index was developed by Robert Gunning Associates, a consultancy set up by Robert Gunning in 1944. The aim of which was to help businesses to improve their writing.

Gunning was an American businessman experienced in the world of newspapers and publishing.

In a paper written by Gunning in 1964 he reflects on how, prior to setting up the consultancy, the readability of his own work was questioned.

At that time an editor of an educational publication, a readability assessment had been conducted by Columbia University on Gunning’s articles.

The results indicated that Gunning’s work was too hard to read. Observations included ‘too many different words per thousand… not enough simple sentences… and was too difficult for the intended audience’.

Outraged by the report and the idea of applying a yardstick to writing, Gunning composed a ‘scathing rebuttal’.

But, as he wrote he found that he was weighing up each word and sentence more carefully and favouring simpler prose. Like it or not, the readability test had influenced him to improve his writing for the benefit of the readers.

Gunning came to argue that writing was often too complex and this complexity was ‘fog that interfered with meaning rather than helped convey it’.

Robert Gunning Associates sought to create a test of readability that would help writers identify this fog so they could clear it.

A test of readability that was would be reliable yet simple. The result was the Gunning Fog index.

How does the Gunning Fog test work?

Gunning Fog ScoreThe formula for Gunning Fog is 0.4 [(words/sentences) + 100 (complex words/words)], where complex words are defined as those containing three or more syllables.

What is immediately apparent when looking at this calculation is its simplicity compared to some other readability tests.

For example, for the Flesch Reading Ease test, the numbers within the formula are rounded up to three decimal places 206.835-1.015 (words/sentences) - .836 (syllables/words).

At the time that the index was created, the ease of calculation of the Gunning Fog score would have been of particular importance as the calculation would have been done by hand.

Created as a human algorithm, the Gunning Fog formula could be followed and applied by anybody, no equipment necessary.

Even now when readability formulas are typically run on a computer rather than by hand, the simplicity of Gunning Fog’s formula is still highly praised.

The fact that the Gunning Fog index was designed as a human algorithm does, however, raise a few issues when translating the formula into computerised format. This results in some modifications to the rules of the original version.

For example, Gunning proposed that the test be conducted on excerpts of 10 sentences of text or around 100 words. Going much beyond this would make for a tiresome job if conducting the calculation by hand.

However, when a Gunning Fog score is calculated by software such as ReadablePro, the analysis is conducted on the text as a whole, regardless of how many sentences it contains.

In addition, the index states that proper nouns, familiar jargon, or compound words are not included in the analysis. Also, common suffixes such as -ed, or -ing are not counted as a syllable. So, in the statement Gloria followed her supernatural instinct, only supernatural would count as a complex word.

In the computer imitation of the formula, proper nouns are ignored if these fall at the beginning of a sentence but not within the lines as the algorithm struggles to detect these instances.

When it comes to common suffixes such as -ed, the complexity of the English language makes it difficult to machine detect when the -ed at the end of a word is a suffix and when it is not, e.g. mend-ed vs. moped.

So, while online versions of the Gunning Fog index mirror the basic calculation, necessary adaptations mean that the computerised version is not a perfect copy of the original formula as proposed by Gunning (1944).

When is the Gunning Fog Index most useful?

‘The Fog Index has served as an effective warning system against drifting into needless complexity in the mechanics of writing. Beginning writers need such warning. So do business men, engineers, scientists and all others who have to write as part of their jobs but are not primarily writers.’
- Gunning, 1964

Consistent with Gunning’s original aim, the Gunning Fog index is principally used as a tool to help writers keep their texts clear and simple.

One area where the Gunning Fog index is useful is in helping researchers write papers. For example, according to a paper written by a group of medical science journal editors, one thing to consider in trying to get a research abstract accepted is its clarity.

They suggest that when the writer gets to the stage of honing and perfecting their abstract, Gunning Fog can assist the writer in verifying that the text is not too difficult to read.

Given the competition and standards required to get work published in high impact journals, such advice may be appealing to researchers.

Even more so when the paper is written by the editors of a range of high ranking, international, peer-reviewed medical journals such as The World Journal of Surgical Oncology.

Consistent with this use of the Gunning Fog index for editing, research examining the editorial policies of leading science journals came up with a set of suggestions for improving the policies of these journals.

This included the suggestion that the Gunning Fog index was used by editors to measure readability and that authors were explicitly informed that the journal would use a readability index.

The authors describe the Gunning Fog Index as ‘simple to compute, widely used, and easily interpreted’.

They propose that a specific goal be set for the readability of any article, e.g. Journal of X will only publish papers with a Gunning Index of less than 16.

They also propose that the readability score of the paper be published on the first page of the paper.

Were such editorial advice taken on by journals, unnecessarily complex wording would become much harder to disguise.

The suggestion of simplifying academic writing may not fit comfortably with some. As Gunning himself says, "nearly anyone who loves words is annoyed at the outset by efforts to judge writing by counting such factors as sentence length and long words" - Gunning, 1964.

Indeed, complex, wordy, jargon heavy writing is worn by some as a badge of honour to illustrate just how clever and knowledgeable they are.

A study looking at the relationship between management research and academic prestige reported a positive correlation between Gunning Fog scores of management journal articles and faculty members’ perceptions of their prestige.

Passages that were more difficult as rated by the Gunning Fog index were rated by academics as higher in research competence. However, there is evidence that people do prefer reading simpler text.

In one study readability scores were compared between peer-reviewed journal articles and articles from throwaway journals.

So-called throwaway journals refer to those that contain no original investigation and are provided free of charge.

While you might think that throwaway articles would be less appealing than those that are peer-reviewed, they in fact have wide circulation and readership.

But why? In their comparison between the two types of journals, the researchers found that throwaway journals were easier to read.

Scores revealed peer-reviewed articles as significantly more likely than throwaway journal articles to generate a readability score too difficult even for medical writing.

So, although poorer methodologically, these throwaway journals still held appeal for physicians who no doubt enjoyed reading clearer, simpler texts.

In sum, whatever your field, the Gunning Fog index can help take the fog out of your writing leaving you with simpler, clearer text that your readers will be grateful for!

Ruth Colmer

Ruth is a freelance writer, researcher, and lecturer. She likes reading, cooking, writing stories, travel, and human beings.