Gunning Fog Index Calculator
The Gunning Fog Index estimates the US school grade a reader needs to understand a piece of writing. A score of 12 is the last year of high school. Higher means harder.
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What the Gunning Fog Index measures
The Fog Index was published in 1952, in The Technique of Clear Writing, by Robert Gunning, a writing consultant who founded his firm in 1944.
The Fog score combines two measures: average sentence length and the percentage of complex words. This calculator counts a word as complex when it has three or more syllables. It does not skip proper nouns or numbers, and it skips a word whose third syllable comes only from -ing, -ed or -es. The result is a single grade-level number.
How to interpret your Gunning Fog score
| Fog | Label on this calculator |
|---|---|
| Up to 5 | Elementary |
| 6–8 | Middle school |
| 9–12 | High school |
| 13–16 | College |
| 17+ | Graduate |
No US federal rule sets a Fog score. Pick the grade your readers need.
When to use the Gunning Fog Index
- Business writing — proposals, internal memos, executive briefs.
- Policy and compliance documents being rewritten in plain language.
- Press releases for non-specialist journalists.
How the Gunning Fog Index is calculated
Average sentence length is added to the percentage of complex words, and the sum is multiplied by 0.4. Complex words are counted as described above.
The percentage of complex words counts as much as sentence length. Replacing five complex words with shorter ones in a 200-word passage lowers the Fog by 1.0, the same as cutting average sentence length by 2.5 words.
Gunning Fog vs other readability formulas
Flesch-Kincaid Grade Level uses total syllables per word; Gunning Fog uses the percentage of words with three or more syllables. The practical difference: Fog counts a three-syllable word and a six-syllable word the same, while Flesch-Kincaid counts every syllable.
SMOG also counts complex words. McLaughlin's SMOG method uses 30 sentences, so it is less suited to short passages.
Flesch Reading Ease returns a 0 to 100 score on the opposite scale (higher is easier). Use Reading Ease if you prefer a 0 to 100 number; use Fog if you want to talk about school grades.
Frequently asked questions
What does a Gunning Fog score of 12 mean?
A Fog score of 12 puts the text at about a 12th-grade level, the last year of US high school.
What is a good Gunning Fog Index score?
It depends on your readers. No standard sets a Fog target. Lower scores suit a wider audience.
How is Gunning Fog different from Flesch-Kincaid?
Both use average sentence length. Fog adds the percentage of words with three or more syllables, while Flesch-Kincaid counts every syllable. Fog treats a three-syllable word and a six-syllable word the same.
Does Gunning Fog count proper nouns as complex?
On this calculator, yes. It counts any word of three or more syllables, including names and numbers. It skips a word only when the third syllable comes from -ing, -ed or -es. Other tools may count differently.
How do I lower my Gunning Fog score?
Replace words of three or more syllables with shorter ones, and split long sentences. One percentage point fewer complex words lowers the score by 0.4, the same as one word less per average sentence.
Where did the Gunning Fog Index come from?
Robert Gunning published the formula in 1952 in his book The Technique of Clear Writing. He had founded a writing consultancy in 1944.