The Gunning Fog Index estimates the years of formal education needed to understand a piece of text on first reading.
Use Gunning Fog when your goal is plain business writing — Robert Gunning created it for newsroom editors who needed a quick way to catch jargon. It is especially useful for evaluating financial reports, legal summaries, and corporate communications where multi-syllable words are easy to slip in unconsciously.
Gunning Fog and SMOG both penalise polysyllabic words, but SMOG uses square-root scaling and is more conservative. Compared to Flesch–Kincaid, Fog is harsher on vocabulary and gentler on sentence length. Use Fog for business; use SMOG for health.
The Gunning Fog index was created by Robert Gunning to identify text that is unnecessarily "foggy" or hard to understand.
It combines average sentence length with the percentage of "complex" words — words of 3+ syllables, with allowances for proper nouns, suffixes, and compound words.
A score of 12 corresponds to high-school senior level. Time and Newsweek typically score around 11.
A word of 3 or more syllables, excluding proper nouns, familiar compound words (like "everywhere"), and words made polysyllabic by common suffixes (-ed, -es, -ing).
Aim for under 12 for general audiences. Anything above 16 is considered very difficult to read.
Yes. The Fog index was popularised by Robert Gunning specifically for business and journalistic writing.
Input
The implementation requires consideration of consistency, observability, and serialization.
Output
Gunning Fog Index: 18.4 — Difficult (post-graduate).
Eleven words, but four are 3+ syllables. The Fog index multiplies the complex-word ratio by 100 and adds average sentence length, so dense vocabulary dominates the score.
Classic 0–100 readability score.
readabilityU.S. grade level for your text.
readabilityHealth & medical text grade level.
readabilityNo syllables needed — just characters.