Gunning Fog Index Calculator

The Gunning Fog Index, created by Robert Gunning in 1952, estimates how hard a piece of English writing is to read. It returns a U.S. school grade level: a score of 12 means a high-school senior can read the text on the first pass. Lower scores mean easier text. Gunning suggested aiming for a Fog index near 7 to 8 for a general audience.

The formula is 0.4 times the sum of the average sentence length and the percentage of complex words. In symbols: Fog = 0.4 * ((words / sentences) + 100 * (complex words / words)). Average sentence length is your total words divided by your total sentences, and the complex-word percentage counts how many of your words have three or more syllables.

A complex word has three or more syllables, with three exceptions Gunning defined. Proper nouns (capitalized names) do not count, hyphenated compound words like book-keeper do not count, and common verb endings do not count when removing -es, -ed, or -ing leaves a word of two syllables or fewer (so created and remained are not complex). This calculator applies all three rules automatically.

Shorten your sentences and swap long words for shorter ones. Because the formula weights average sentence length and the share of three-syllable words equally, splitting one long sentence into two and replacing a few polysyllabic words can drop your score by several grade levels. Most newspapers and consumer writing aim for a Fog index of about 8 to 10.

The calculator estimates syllables with a standard vowel-group method rather than a pronunciation dictionary, so the complex-word count can differ slightly from tools that look words up. Your score may vary by roughly a point from another calculator, but the reading level and the comparison between two drafts stay reliable. Use it as a guide, not an exact measurement.

Use a representative sample of at least 100 words, ideally a few full paragraphs. Very short text can swing the score because a single long sentence or one complex word changes the averages a lot. For a long document, sample several sections and compare their scores to find the passages that need editing.