Gunning Fog Index calculator

Paste your writing to get its Gunning Fog Index — the years of formal education a reader needs to understand it on first reading — with the complex-word count and working shown. Free, instant, and private to your browser.

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The Gunning Fog Index formula

Fog Index = 0.4 × [(words ÷ sentences) + 100 × (complex words ÷ words)]

Words ÷ sentences — average sentence length.
Complex words ÷ words — the share of words with three or more syllables, as a percentage.
Complex words — three or more syllables, not counting proper nouns, hyphenated compounds, or words that only reach three syllables through -es, -ed, or -ing (so “whispering” is not complex, but “beautiful” is).

Worked example

Here is the Gunning Fog Index worked through for our sample bedtime story — press Try a sample story above to load it. It’s a short tale about an owl who can’t sleep, and it counts up like this:

Words: 118
Sentences: 18
Complex words: 1

0.4 × [(118 ÷ 18) + 100 × (1 ÷ 118)] = 3.0

So the story scores 3.0 — Grade 3 · ages 8–9.

What your score means

Score	Level	Who reads it easily
1 or below	Kindergarten–1st grade	Ages 5–7 · picture books, first readers
2–3	2nd–3rd grade	Ages 7–9 · early chapter books
4–5	4th–5th grade	Ages 9–11 · confident young readers
6–8	Middle school	Ages 11–14 · most adults read this comfortably
9–12	High school	Ages 14–18 · news features, business writing
13–16	College	Academic and specialist writing
17+	Graduate	Dense professional or scientific prose

What is a good Gunning Fog score?

Gunning’s rule of thumb: text meant for a wide audience should score below 12, and text almost everyone can understand usually scores 8 or lower.

Scores of 17 and up correspond to postgraduate reading — common in academic papers and legal writing, and a warning sign almost anywhere else.

Where it comes from

Robert Gunning, an American businessman who had worked in newspaper and textbook publishing, introduced the index in his 1952 book The Technique of Clear Writing. He argued that much business and newspaper writing was full of “fog” — needless complexity that hid the point.

The index gives equal weight to long sentences and long words, and it remains a favourite of editors and communications teams because it’s simple enough to work out by hand on a 100-word sample.

Limitations

Not every long word is hard. “Family”, “banana”, and “everybody” count as complex, while short but unfamiliar words like “eke” or “qua” don’t. The exclusions for names and suffixes also vary between tools, which is the main reason Fog scores differ from checker to checker.

Like other formulas, it measures surface features only — it can’t tell whether a text is well organised, whether terms are explained, or whether the reader already knows the subject.

Gunning’s by-hand method

Gunning designed the index to be worked out with a pencil. Take a passage of about 100 words that ends at the end of a sentence, and count its sentences to get the average sentence length.

Then go through it word by word and mark every complex word, skipping names, hyphenated words and -es/-ed/-ing forms. Turn that into a percentage of all words, add it to the average sentence length, and multiply by 0.4.

Because the two parts are simply added, one extra word per sentence and one extra complex word per 100 words raise the index by exactly the same amount: 0.4 of a grade.

Gunning Fog vs SMOG

Both formulas are built on long words of three or more syllables. The Fog Index adds sentence length directly and excludes names and suffix-only words; SMOG counts every long word, scales the count to 30 sentences and takes a square root.

On our sample bedtime story, with a single long word in 18 sentences, Fog gives 3.0 and SMOG 4.5. SMOG’s built-in floor of about 3 means it rarely scores simple text as low as Fog does.

Frequently asked questions

What does a Gunning Fog Index of 10 mean?

That a reader needs about ten years of formal education — a US high-school sophomore level — to understand the text on first reading.

What counts as a complex word in the Fog Index?

Any word of three or more syllables, except proper nouns, hyphenated compound words, and words that only become three syllables by adding -es, -ed or -ing.

How do I calculate the Gunning Fog Index by hand?

Take a passage of around 100 words. Divide the words by the sentences, work out what percentage of words are complex, add the two numbers, and multiply by 0.4.

Why is my Fog score higher than my Flesch-Kincaid grade?

The Fog Index treats every complex word as a full percentage point, so text with a handful of long words scores noticeably higher than it does on Flesch-Kincaid, which averages syllables across all words.

Is the Gunning Fog Index good for children’s writing?

It works, but it’s blunt at the low end: with almost no three-syllable words, the score is mostly sentence length times 0.4. The Flesch-Kincaid Grade Level or the reading level checker give more detail for early readers.