Gunning Fog Index Calculator

Aim for 8 to 10 for a wide audience and under 12 for anything meant to be read quickly. Above 17 you are writing for people who are being paid to read it.

Add average words per sentence to the percentage of complex words, then multiply by 0.4. A complex word is three or more syllables, excluding proper nouns, familiar compound words and verb forms made three syllables only by an -ed or -es ending.

Three syllables or more. Gunning's original rules excluded proper nouns, easy compounds like bookkeeper, and words that only reach three syllables through an inflected ending. Automated tools including this one apply the syllable rule directly, so expect a slightly stricter score than a hand count.

Flesch-Kincaid averages syllables across every word, so many short words dilute a few long ones. Fog counts what share of words are long, so jargon shows up more sharply. For business and technical writing Fog is usually the more revealing of the two.

Because your text has a concentration of long words rather than a uniform spread. That gap between the two scores is itself the signal: it means jargon, not sentence structure, is what is making the page hard.

It penalises them accurately - a three-syllable term is harder for a newcomer regardless of how routine it feels to you. Whether that matters depends on your reader. For internal engineering docs, ignore it; for a landing page, do not.

Robert Gunning, a US businessman who founded a consultancy in 1944 to fix what he called 'foggy writing' in newspapers and business documents. He published the index in 1952.

At least 100 words, and ideally several hundred. The complex-word percentage is unstable on short samples - one technical term in a 20-word snippet swings the score by several grades.