Gunning Fog Index: formula, complex words, and its gray areas
The Fog index adds average sentence length to the share of long words, then scales the result to a grade. Its rule for 'complex words' is simple to state and surprisingly hard to apply.
Robert Gunning was a consultant who worked with newspapers and businesses on clearer writing. He published the Fog index in his 1952 book The Technique of Clear Writing. His point was that most unreadable prose is not hard because of its ideas. It is hard because of “fog”: long sentences and long words that add effort without adding meaning.
The index puts a number on that fog. Like most grade formulas, it estimates the years of schooling a reader needs to follow the text on first reading.
The formula
Fog = 0.4 × [ (words ÷ sentences) + 100 × (complex words ÷ words) ]
Two numbers go in:
- Average sentence length, in words.
- The percentage of complex words, meaning words of three or more syllables (with exceptions, below).
Add them and multiply by 0.4. That’s all.
The design makes the index easy to compute by hand. Gunning’s procedure worked on samples of about 100 words, where the percentage of complex words is simply the number you counted.
A worked example
Suppose a 60-word passage has 3 sentences and 6 complex words.
- Average sentence length: 60 ÷ 3 = 20
- Percentage of complex words: 100 × (6 ÷ 60) = 10
- Fog: 0.4 × (20 + 10) = 0.4 × 30 = 12
Now split each sentence in two, keeping every word. Average length drops to 10 and the Fog falls to 0.4 × (10 + 10) = 8. Replace four of the six complex words with plain ones as well, and the percentage drops to 100 × (2 ÷ 60) ≈ 3.3, giving 0.4 × (10 + 3.3) ≈ 5.3.
The two inputs carry equal weight. One word per sentence is worth the same as one percentage point of complex words.
What counts as a complex word
The basic rule: a word with three or more syllables is complex. Gunning then excluded three groups, because they don’t make reading harder in the way long words usually do:
- Proper nouns. “Washington,” “Microsoft,” and “Elizabeth” don’t count.
- Familiar compound words built from short, easy words. “Bookkeeper,” “however,” and “butterfly” don’t count.
- Words that reach three syllables only through an -es, -ed, or -ing ending. “Created” (cre-at-ed) and “trespasses” (tres-pass-es) don’t count, because “create” and “trespass” have two.
Everything else with three or more syllables does: “important,” “information,” “company,” “deliver.”
The gray areas
Gunning’s exceptions depend on judgment. A person applying them will make calls a program cannot, and two people may make different calls.
What is a familiar compound? “Understand,” “however,” “everyone,” “afternoon,” and “underneath” are all made of short words. Most readers find them easy. But “understand” no longer means “stand under,” so is it still a compound? The rule doesn’t say.
Which endings strip cleanly? “Created” drops to “create” (two syllables), so it doesn’t count. “Interested” drops to “interest” (three syllables, or two if you say “in-trest”), so it might. “Employees” ends in -es on the page, but “employ-ees” isn’t the plural ending at all.
What about the syllable count itself? “Every” can be two syllables or three. “Different” can be two or three. “Business” is usually two but has three vowel groups. A rule-based counter and a careful human will split on these. See counting syllables.
What is a proper noun at the start of a sentence? A program sees a capital letter and can’t always tell “Organization” (the first word) from “Organization of American States” (a name).
What about numbers and acronyms? “2026” or “UNESCO” might count as zero, one, or several syllables depending on the tool.
Most software simplifies. Some programs count every word of three or more syllables. Others strip common endings first, or skip capitalized words. None reproduces human judgment on “familiar compounds.” That is why Fog scores for the same text often differ by a grade or more between calculators.
Tip: If you want stable Fog scores, use one tool and compare drafts within it. Treat the absolute number as approximate.
Lowering the fog
The two levers are exactly what the formula names.
Shorter sentences. Split at “and,” “but,” “which,” and semicolons when they join two full thoughts. The sentence length checker finds the long ones.
Fewer long words where short ones work. The complex-word count punishes abstract nouns most of all: “implementation,” “effectiveness,” “organizational,” “consideration.” Turning them back into verbs often fixes a sentence. See nominalizations.
Before: Management anticipates considerable improvement in organizational effectiveness following the implementation of the restructuring initiative.
After: Leaders expect the new structure to help teams work better.
The Before has 14 words, and 9 of them are complex. (“Following” escapes only because “follow” has two syllables.) The After has 10 words and none.
Keep the long word when it is the right word. “Insurance,” “medication,” and “deductible” are complex by the formula’s rule, and a policy document cannot do without them. Gunning’s own advice was to cut needless long words, not to ban long words.
Fog compared with the other formulas
Fog behaves much like SMOG, which also counts words of three or more syllables. It differs from Flesch–Kincaid, which uses the average syllables of every word. A text full of two-syllable words can look fine to Fog and harder to Flesch–Kincaid. A text that mixes plain one-syllable words with many three-syllable ones can look the other way round.
None is the “right” grade. Each is a different estimate built from the same rough signals. Readability scores explained covers what all of them leave out.
Try it on your own text
Scores
- Flesch Reading Ease–
Counts used
Hardest sentences
- Paste a few sentences to see which are hardest.