Gunning Fog Index Calculator — Check Your Text’s Fog Score

Q: What is a good Fog score?

For a general audience, aim for a Fog score under 12. Scores of 8 to 10 suit wide public communication, 10 to 12 business writing; anything above 14 is specialist or academic territory.

Q: What counts as a complex word?

A word of three or more syllables, excluding proper nouns, familiar compound words, and words that only reach three syllables by adding -ed or -es.

Q: Fog vs Flesch-Kincaid — which should I use?

Both are grade-level estimates that usually land within a grade of each other. Fog is the traditional choice for business and plain-language audits; Flesch-Kincaid is the default in word processors and schools.

Q: Who created the Fog Index?

American readability consultant Robert Gunning, published in 1952 in his book The Technique of Clear Writing.

The Gunning Fog Index is one of the oldest and most trusted readability formulas: it converts the average length of your sentences and the share of complex words in your text into a single number — an estimate of how many years of formal education a reader needs to understand it on first reading. A score of 8 is easy reading; a score of 18 is academic prose. This free Gunning Fog index calculator computes your score instantly in your browser: paste your text and it counts sentences, words and complex words, then applies the formula.

The Fog Index is especially popular for plain-language audits of business writing, news articles and government documents — anywhere the goal is writing that ordinary readers can grasp the first time. Nothing is uploaded; the calculation happens entirely on your device.

The Fog formula, worked out by hand

The formula is: Fog = 0.4 × (average sentence length + percentage of complex words). “Complex words” means words of three or more syllables — with three important exclusions that this calculator applies:

Proper nouns are not counted (e.g. “Microsoft”, “Pakistan”).
Easy compounds are not counted — familiar compound words like “bookkeeper” or “afternoon”.
Words that only reach three syllables by adding -ed or -es are not counted (e.g. “created” from “create”, “watches” from “watch”).

Here is a fully worked example. Take a 200-word passage split into 10 sentences:

Average sentence length: 200 words ÷ 10 sentences = 20.
Complex words: after applying the exclusions above, 25 words qualify as complex. 25 ÷ 200 = 12.5%.
Fog score: 0.4 × (20 + 12.5) = 0.4 × 32.5 = 13.

A Fog score of 13 means the passage reads at roughly a first-year college level — fine for a specialist report, but too demanding for a general audience, where you would aim below 12.

What you can use the Fog Index for

Business writing audits: many companies audit proposals, reports and internal communications against a Fog target to keep writing crisp.
Annual reports and investor documents: long, dense reports are the classic Fog use case — the index pinpoints whether sentences and vocabulary are drifting out of reach.
Journalism: editors use Fog-style checks to keep news copy accessible to a broad readership.
Plain-language compliance: government agencies and financial firms in several countries are expected to write public documents in plain language — Fog is one of the accepted measures. Pair it with our full readability checker to cross-check with Flesch-Kincaid and SMOG.

Honest limitations

The formula penalises necessary technical vocabulary. A medical leaflet full of correctly used terms like “cardiovascular” will score high even when its real readers understand it perfectly — Fog cannot tell a needed term from a pompous one.
It is not designed for fiction or creative writing, where long sentences and rich vocabulary are deliberate choices, not problems.
It needs a decent sample of text — very short texts give unstable scores, since one long sentence can swing the result several points.
The score estimates years of education; it does not certify that anyone will actually understand your text. Clear structure and accurate content still do most of the work.

Frequently asked questions

What is a good Fog score?

For a general audience, aim for a Fog score under 12 — roughly final-year high school or below. Scores of 8–10 suit wide public communication; business writing typically targets 10–12. Anything above 14 is specialist or academic territory.

What counts as a complex word?

A word of three or more syllables — excluding proper nouns, familiar compound words, and words that only reach three syllables by adding -ed or -es. So “communication” counts, but “Microsoft”, “bookkeeper” and “created” do not.

Fog vs Flesch-Kincaid — which should I use?

Both are grade-level estimates and usually land within a grade of each other. Fog is stricter about long sentences and is the traditional choice for business and plain-language audits; Flesch-Kincaid is the default in word processors and schools. Running both — as our readability checker does — gives you the most reliable picture.

Gunning Fog Index Calculator: how it works

Who created the Fog Index?

The index was developed by American businessman and readability consultant Robert Gunning and published in 1952 in his book The Technique of Clear Writing. He created it to help businesses and newspapers measure whether their writing was too hard for their readers.

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