The Gunning Fog Index, developed by American businessman Robert Gunning in 1952, is a grade-level readability formula built for business and professional writing. It targets "fog" — the needless complexity that keeps readers from understanding text on the first pass. Its distinctive feature is that it counts complex words, defined as words of three or more syllables, and weights them heavily. The formula is:
0.4 × [ (words / sentences) + 100 × (complex words / words) ]
The result is a US grade level. Worked example: a 100-word passage in 5 sentences containing 10 complex words yields 0.4 × (20 + 100 × 0.10) = 0.4 × (20 + 10) = 0.4 × 30 = 12 — a twelfth-grade reading level. A widely cited rule of thumb is that anything above a Fog index of 12 is too hard for a general audience, and business writing should sit around 8–10.
Gunning Fog is popular for auditing corporate reports, insurance policies, and technical documentation because polysyllabic jargon is exactly what it penalises. There are conventions about which words are exempt from the "complex" count: proper nouns, common compound words, and verbs made three syllables only by adding "-es" or "-ed" (like "created") are traditionally excluded, though automated tools vary in how strictly they apply this — which is why Fog scores differ slightly between tools. Because it keys on long words rather than raw sentence length, it can diverge from Flesch–Kincaid; comparing the two, plus SMOG, gives a more robust read on overall readability.