What counts as a complex word in the Fog index?

A word of three or more syllables, excluding words that only reach three through an -es, -ed or -ing ending and excluding capitalised tokens assumed to be proper nouns. Both the Fog count and the raw three-syllable count are shown so you can see the difference.

Why does the formula exclude proper nouns?

Because Gunning assumed a capitalised word is a name or a place that the reader already knows or cannot avoid, so penalising it for length would distort the grade. The exclusion follows the original definition rather than inventing a local rule.

What Fog score should I aim for?

Gunning treated scores around 17 as hard for most readers and around 12 or below as suitable for a general audience. For specialist audiences a higher score is expected. The tool reports the grade rather than declaring a target.

Why is the score multiplied by 0.4?

The factor calibrates the raw sum of sentence length and complex-word percentage onto a school-grade scale. It is a scaling constant from the original index, not a tunable weight, and the unmultiplied inputs are shown so the arithmetic can be checked.

How does the Fog index differ from Flesch-Kincaid?

Both use sentence length, but Fog counts three-syllable complex words as a percentage while Flesch-Kincaid uses average syllables per word. Fog also excludes proper nouns and some suffixed words, which is why the two grades often differ by a level or more.

Gunning Fog Index Calculator

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Exclude capitalised words (proper nouns)

Fog index

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The Gunning Fog Index Calculator applies the Fog formula to your text and shows exactly which words it counted as complex, so the grade can be traced back to the vocabulary that produced it.

What is Gunning Fog Index Calculator?

The Gunning Fog index answers a narrow question: how many years of schooling does a reader need to understand this passage on one reading? The Gunning Fog Index Calculator computes it and, just as importantly, shows which words the formula treated as complex, because that list is where the edits come from.

The arithmetic is short. Multiply 0.4 by the sum of the average sentence length in words and the percentage of complex words. Complex words are the crux. For the Fog index a complex word is one with three or more syllables, with two documented exclusions: words that reach three syllables only through an -es, -ed or -ing ending, and words that are capitalised in the source, which are presumed to be proper nouns rather than difficult vocabulary. Those exclusions are why the complex-word count here is lower than a simple three-syllable tally, and both figures are shown so the difference is visible rather than hidden.

The 0.4 multiplier is a calibration, not a magic number. It maps the raw sum onto a school-grade scale so that the result lands near the years of education a reader needs. That is the whole intent of the index, and it means the number should be read as a grade estimate rather than as a quantity of difficulty.

Gunning's own guidance was that a Fog score of about 17 marks writing that is hard for most readers, and around 12 or below is comfortable for a general business or news audience. The tool reports the grade label alongside the score so the number is not left to interpretation, but it attaches no pass or fail. A technical paper for specialists is expected to score high, and rewriting it down would remove precision the audience needs.

Because the formula rewards short sentences and short words, it responds quickly to editing. Splitting one long sentence and replacing a few three-syllable words with shorter synonyms can move the index by more than a grade, and because the complex-word list is shown, you can see which substitutions the formula would notice. That list is also the index's weakness: it counts syllables rather than difficulty, so a familiar three-syllable word is penalised as heavily as an obscure one, and a short technical term is not penalised at all.

Two sample caveats apply. The index is quoted for texts of roughly a hundred words or more and is unstable below that, and it is an English measure whose syllable heuristic, documented on the page, is right most of the time and occasionally wrong. The counts panel lets you check the arithmetic by hand.

The index is most sensitive to vocabulary, so it rewards a plain word list more than short sentences. That is worth knowing before rewriting: replacing three long words with short ones usually moves the Fog score further than splitting a sentence, even though splitting is the more common advice. The complex-word list shows exactly which substitutions the formula would notice, which turns the grade into a specific, finite editing task rather than a vague instruction to write more simply.

How to use Gunning Fog Index Calculator

Paste your text; the Fog index and the counts update as you type.
Read the index and the grade label, then the complex-word count and the average sentence length.
Look at the complex-word list to see which specific words the formula penalised.
Split a long sentence or shorten a flagged word and watch the index move.
Compare against the reference bands before deciding whether the score is a problem for your audience.

When to use Gunning Fog Index Calculator vs related tools

Use the Fog index when you want a grade estimate driven mainly by vocabulary and want to see the complex-word list. When a syllable-only formula is preferred — for health material or a word list that is hard to control — the SMOG Index Calculator counts polysyllables and ignores sentence length entirely.

For a full comparison across Fog, Flesch, SMOG and the letter-based formulas, the Readability Grade Scorer reports them side by side, and the Flesch-Kincaid Score Calculator breaks out the two Flesch formulas from the same engine.

Privacy & Security

This tool runs entirely in your browser — no data ever leaves your device. There is no server round-trip, no upload, no logging, and no account required. Your input is processed locally using client-side JavaScript and is never stored, transmitted, or accessible to anyone else. When you close the tab, everything disappears.