Readability science
The Gunning fog index: a 73-year-old formula that still works
The Gunning fog index turns 73 this year. It is older than the internet, older than personal computers, older than anyone who currently works in SEO. And it is still the score professional writers reach for when Flesch reading ease is being too generous.
This post is what fog actually measures, how to read the number and the one thing it is brilliant at that Flesch will not catch.
What is the Gunning fog index?
The Gunning fog index is a readability score that estimates the years of formal education a reader needs to understand a piece of writing on the first read. Higher numbers mean harder writing. A fog score of 12 reads like the writing of a final-year sixth-former. A fog score of 17 reads like a research paper.
Robert Gunning published it in his 1952 book The Technique of Clear Writing. He was a publishing consultant who spent the 1940s trying to fix dense business writing for US newspapers and corporations. He thought most adult writing was “foggy” with long sentences and pointless polysyllabic words, and he built the formula to prove it.
What is the Gunning fog formula?
The formula is two counts and a multiply.
0.4 times ((average words per sentence) + (percentage of complex words))
A “complex word” in Gunning’s definition is any word with three or more syllables. There are exceptions. Proper nouns are excluded. Words ending in common suffixes like “-ed” or “-es” that only add a syllable are excluded. Compound words built from two short words are also excluded.
So a paragraph with an average sentence length of 18 words and 10% complex words scores 0.4 * (18 + 10) = 11.2. That is the fog score. Read it as “needs roughly 11 years of formal education”, which in the UK means a Year 12 student should manage it without help.
What is a good Gunning fog score?
Lower is easier. The table below maps fog scores to UK audiences.
| Fog score | Reading level | Audience |
|---|---|---|
| 6 | Year 7 | Comfortable plain English. Tabloid front pages sit here. |
| 8 | Year 9 | NHS plain English target. Comfortable for most adults. |
| 10 | Year 11 | UK broadsheet news. Standard blog target. |
| 12 | Year 13 (sixth form) | Upper limit for general public writing. |
| 14 | Undergraduate | Quality magazine and journal writing. |
| 17+ | Postgraduate | Academic papers. Too dense for the web. |
For a marketing site or blog, aim for fog 10 or lower. For anything healthcare, government or legal-services facing a non-specialist audience, push for fog 8.
Why is Gunning fog harsher than Flesch?
Both Flesch and Gunning fog look at sentence length. Only Gunning counts how many of your words are long.
Flesch averages syllables across every word. A paragraph with one 8-syllable word and 99 one-syllable words averages 1.07 syllables per word, which barely moves the Flesch score. Gunning fog treats that same paragraph as having 1% complex words, which is also barely a hit. So far so similar.
Where they diverge is volume. A paragraph with ten 3-syllable words and 90 one-syllable words averages 1.20 syllables per word for Flesch, which moves the score a few points. For Gunning fog, that is 10% complex words, which adds a full grade level to the result. Fog punishes the *concentration* of hard words, not the average. That is closer to how a real reader experiences the prose.
This is why technical writing tends to score acceptably on Flesch and badly on Gunning fog. The hard words are not spread out, they are clustered. Fog notices.
How to lower a Gunning fog score
Three moves cover most of the work.
Shorten the longest 10% of your sentences. Find every sentence over 25 words. Most of them are doing two jobs and should be split at the first “and” or “but” that joins complete thoughts. This alone will drop your fog score by 1 to 2 points on a typical draft.
Replace one long word per paragraph. You do not have to strip every polysyllabic word. Cutting one per paragraph is usually enough to drop the complex-word percentage below 10. “Utilise” becomes “use”. “Subsequently” becomes “then”. “Necessitate” becomes “need”. “Demonstrate” becomes “show”. Pick the worst offender. Move on.
Leave the words that genuinely have to stay. “Fiduciary”, “anaphylaxis” and “indemnity” cannot be replaced with a shorter word without losing the meaning. Keeping them costs you fog points. The alternative is rewriting the sentence into something less precise, which is worse. WP AI Explainer lets readers tap any hard word on a WordPress page and see an instant plain-English explanation, so you can keep the precise word and not lose the reader. The fog score does not improve. Comprehension does.
What Gunning fog does not catch
Fog is good at structural difficulty and useless at semantic difficulty. A sentence can score fog 6 and still be incomprehensible.
“The yield curve inverted” scores fog 5. It is also gibberish to anyone who has not spent ten minutes reading about bond markets. Fog cannot tell the difference between a hard word and a foreign concept. To fog, “horse” and “anaphylaxis” both register as one word. Only the syllable count separates them, which has nothing to do with whether your reader knows what they mean.
This is the same blind spot every formula has. Fog does not measure jargon. It measures the *shape* of jargon, which is correlated but not the same thing.
How to check Gunning fog on a WordPress post
WordPress does not show Gunning fog out of the box. Yoast surfaces Flesch only. Three good ways to get fog on a draft.
Readable.com runs Flesch, Flesch-Kincaid, Gunning fog, SMOG and Dale-Chall in parallel and shows you all of them. Paste your draft in for a 30-second answer.
Hemingway editor does not show fog explicitly but its “grade level” output is calibrated against Gunning. A Hemingway grade of 9 means your prose sits roughly at fog 9.
SEO Review Tools readability checker is the free option that shows Gunning fog alongside everything else. Useful if you do not want to sign up for anything.
None of these will help the reader once the post is live. For that, install WP AI Explainer. It does for hard words on a live page what Gunning fog did for hard words in a 1952 draft. The 73-year-old formula spots the problem. The plugin fixes it.
Frequently asked questions
What is the Gunning fog formula?
0.4 times the sum of (average words per sentence) and (percentage of words with three or more syllables). Output is a US grade level. Lower means easier.
What is a good Gunning fog score?
For a public blog or marketing site, aim for fog 10 or lower. For healthcare, government or legal-services writing aimed at the general public, push for fog 8 or lower. Anything above 12 will lose most adult readers on the first read.
Why is my Gunning fog score worse than my Flesch score?
Fog punishes clusters of long words. Flesch averages syllables across the whole document. A paragraph with ten 3-syllable words in 100 will barely move Flesch but will add a full grade level to fog. Technical writing scores acceptably on Flesch and badly on fog for this reason.
Does Gunning fog work for UK English?
Yes. The formula counts syllables and words. English syllable structure is the same in UK and US writing. The grade-level output maps to US schooling but you can translate it: fog 8 equals UK Year 9, fog 12 equals Year 13, fog 16 equals undergraduate.
Want this on your own site?
WP AI Explainer turns confusing words into instant tooltips on every WordPress post. Free on wordpress.org, with a pro option for sites that need bulk scanning.