Gunning Fog Index Calculator

Gunning Fog index: years of schooling needed to read the text

Robert Gunning (1952) came up with this index to estimate how many years of schooling a reader needs to get a passage on the first pass. The formula is fog = 0.4 · (words/sentences + 100·complex_words/words). Here complex_words means words with 3 or more syllables, leaving out proper nouns, compound words and verb inflections. Score it like this: 12 ≈ US high-school senior, 17+ ≈ post-graduate, ~8 ≈ broad mass audience. For reference, the Wall Street Journal hovers around 11 and Reader's Digest around 9. In practice you want 7–8 for marketing copy, 9–12 for news, and 12+ is fine in technical or academic writing.

Applications

Journalism style guides lean on it (WSJ and Reuters both aim for roughly 11). It shows up in marketing and landing-page copy, where a lower fog tends to lift conversion. Teachers use it to grade ESL writing and to level EFL textbooks. It also helps with plain-language compliance in healthcare and legal documents, and in teaching academic writing.

FAQ

What counts as a "complex word"? Any word with 3 or more syllables, but with three exceptions: proper nouns (e.g., "Mississippi"), compound words made from simple parts ("bookkeeper"), and words that only cross the threshold because of a verb suffix ("-ed", "-es", "-ing").

Does it work for Portuguese? Only sort of. Portuguese leans on more polysyllables by nature, so the raw fog comes out inflated. For Portuguese text, reach for the PT-adapted Flesch instead, since Fog was calibrated for English.

How is it different from Flesch-Kincaid? Both spit out a grade level. The difference is what they weigh: Fog keys on word complexity (3+ syllables), while Flesch-Kincaid works off average syllables per word. The two usually land within 1–2 grades of each other.

What's a good target? For a mass audience, 7–9. For an educated general reader, 10–12. For a specialist or academic audience, 13–17. Push past 17 and even PhDs will struggle on a first read.