Gunning Fog Index Calculator
The Gunning Fog Index measures the years of formal education a reader needs to understand a piece of writing on the first read. A score of 12 corresponds to a US high school graduate. Above 17 reads as college-graduate territory.
What the Gunning Fog Index measures
The Fog Index was developed in 1952 by Robert Gunning, a business-writing consultant who had spent two decades trying to figure out why so much business and political writing was so badly misunderstood. He coined the word fog to describe writing that obscures rather than communicates — and built a formula that quantified it.
The Fog score combines two indicators of difficulty: average sentence length (long sentences are harder to track) and the percentage of complex words (defined as words with three or more syllables, excluding proper nouns and common suffixes like -ing, -ed, -es). The result is a single number you can read as years of education.
How to interpret your Gunning Fog score
| Fog | Reading level | Typical content |
|---|---|---|
| 6 | 6th grade | Children's content, marketing taglines |
| 7–8 | Middle school | Most blog content, simple business writing |
| 9–11 | High school | Newspapers (NYT typically lands at 11–12) |
| 12 | High school graduate | Wall Street Journal, Time, Newsweek |
| 13–16 | College | Academic prose, white papers |
| 17+ | Graduate | Legal documents, technical manuals |
For most professional writing aimed at a broad audience, target a Fog of 8 to 10. Government and healthcare communications often target 7 or below per plain-language guidelines. Anything above 12 starts to feel like work — and is the threshold where reader drop-off accelerates.
When to use the Gunning Fog Index
- Business writing — proposals, internal memos, executive briefs.
- Policy and compliance documents being rewritten in plain language.
- Press releases targeting non-specialist journalists.
- Healthcare patient information — Fog is a US-favoured formula in the medical-writing world.
- Investor communications — the SEC's Plain English Handbook references Fog as a benchmark.
- Translated content being audited for accessibility.
How the Gunning Fog Index is calculated
Average sentence length is added to the percentage of complex words, and the sum is multiplied by 0.4 to scale it onto a years-of-education axis. Complex words are defined as words of three or more syllables, excluding proper nouns, compound words, and common suffixes.
Because the percentage of complex words is multiplied by 100 inside the formula, vocabulary changes have an outsized effect on the score. Replacing five three-syllable words with shorter alternatives in a 200-word passage will drop the Fog by roughly 1 grade level — the same effect as cutting your average sentence length by 5 words.
Gunning Fog vs other readability formulas
Flesch-Kincaid Grade Level uses total syllables per word; Gunning Fog uses the percentage of words with three or more syllables. The practical difference: Fog penalises a single very long word more harshly than Flesch-Kincaid does. A passage with one "incomprehensible" plus many short words will Fog higher than it Flesch-Kincaids.
SMOG also uses complex words but is calibrated for healthcare writing and demands at least 30 sentences for a stable reading. For shorter passages, Fog gives a usable signal where SMOG produces noise.
Flesch Reading Ease returns a 0 to 100 score on the opposite scale (higher is easier). Use Reading Ease if you prefer a single percentage-style number; use Fog if you want to talk about reading levels in years of education.
Frequently asked questions
What does a Gunning Fog score of 12 mean?
A Fog of 12 means a typical US high school graduate (12 years of formal education) can read your text on the first pass. Most major newspapers — Wall Street Journal, Time, Newsweek — write at this level.
What is a good Gunning Fog Index score?
Target 7–8 for a general audience, 8–12 for educated professionals, below 7 for marketing copy. Above 12 starts to feel academic.
How is Gunning Fog different from Flesch-Kincaid?
Both weight sentence length, but Fog adds the percentage of complex (3+ syllable) words instead of total syllables. Fog penalises jargon more aggressively, which is why policy and technical documents often score worse on Fog than on Flesch-Kincaid.
Does Gunning Fog count proper nouns as complex?
The original 1952 specification excludes proper nouns, compound words, and common suffixes (-ing, -ed, -es). Most modern implementations approximate this rule but can't perfectly identify every proper noun without a named-entity recognition pass — so passages heavy with long names may Fog slightly high.
How do I lower my Gunning Fog score?
Two levers, in order of impact: replace 3+ syllable words with shorter alternatives ("utilise" → "use", "facilitate" → "help"), then split sentences longer than 20 words. Vocabulary changes typically move the score faster than sentence-length edits.
Where did the Gunning Fog Index come from?
Robert Gunning, a Pittsburgh-based writing consultant, published the formula in 1952 after years of helping business and political writers be understood. The word fog describes writing that obscures rather than communicates.