The Fog index estimates the years of formal education someone needs to read a passage on first pass. Unlike Flesch it ignores total syllables and looks only at the proportion of complex words — those of three syllables or more. The passage loaded below scores 24.7.
The formula is 0.4 × ((words ÷ sentences) + 100 × (complex words ÷ words)). Robert Gunning devised it in 1952 while consulting for newspapers, and the design intent shows: it punishes exactly the two habits that make business writing turgid, long sentences and abstract polysyllabic nouns.
The complex-word term is scaled by 100, so it reads as a percentage. If ten per cent of your words are polysyllabic, that term contributes 10 before the 0.4 multiplier — the same as adding ten words to every sentence. That equal weighting is what makes Fog different from the Flesch family.
Below the calculator sits the sentence length, complex-word share and the resulting Fog table: 4 rows — Plain narrative at one end, Formal policy prose at the other — which is usually faster than entering a value you have seen before. The page opens with your text already set to Sustainable procurement policies require suppliers to document emissions across their entire distribution network before tendering., so there is a finished result on screen before anything is typed. Alongside the number, 4 bands label it — very readable through heavy — because a bare figure rarely tells you whether it is unusual. The pages people most often open next are SMOG Readability Calculator and Flesch-Kincaid Grade Calculator.
The formula
Fog- Index value, read as years of education
words ÷ sentences- Average sentence length
complex- Words of three or more syllables
0.4- Scaling factor that maps the sum onto school years
How to use this calculator
- Paste a paragraph or more of continuous prose.
- The gauge shows the Fog index.
- Check the complex-word percentage below it.
- Aim for 12 or lower for a general business audience.
Worked examples
The loaded policy sentence
It has 15 words in 1 sentence and 7 complex words, so 0.4 × (15.0 + 46.7) = 24.7. Both terms are high at once, which is the classic profile of institutional prose.
The same measurement on plain narrative
The plain sample scores 3.6 with only 0 complex words in 27. The entire 21.1 point gap comes from vocabulary and sentence length moving together.
Checking the Research summary row
The reference table lists Research summary, giving Words per sentence: 12.5, Complex words: 7 (28.0%), Fog index: 16.2. Entering that above should reproduce the same figures — if it does not, the input is being read differently to the way the table assumes.
How to read your score
Frequently asked questions
What counts as a complex word?
A word of three or more syllables. Gunning originally excluded proper nouns, familiar jargon and compounds made of short words, and also words made long only by a suffix such as -ed or -es. This calculator counts all of them, which nudges the score slightly higher.
What Fog score should I target?
Gunning argued that anything above 12 was too hard for most people. Business writing guidance still uses 12 as the ceiling, and plain-language public communication usually aims for 8 to 10.
How does Fog differ from Flesch-Kincaid?
Fog counts only words past a three-syllable threshold, so it is insensitive to whether your short words are one or two syllables. Flesch-Kincaid uses the full syllable average. Fog therefore reacts sharply to a handful of long abstractions.
Is the complex-word count exact?
No. A word counts as complex when the syllable estimate reaches three, and that estimate is based on vowel groups rather than pronunciation. Fog is particularly sensitive to it, since one word moving in or out of the complex set shifts the index directly.
What do the bands under the fog index mean?
They label the figure so it can be read without a reference point of your own: Very readable from 0 to 8; Plain English from 8 to 12; Demanding from 12 to 16; Heavy from 16 upwards. The band is a reading aid only — the number itself is exact, and a value sitting just over a boundary is not meaningfully different from one just under it.
What formula does the Gunning Fog Index Calculator use?
It computes Fog = 0.4 × ((words ÷ sentences) + 100 × (complex ÷ words)), where Fog is index value, read as years of education; words ÷ sentences is average sentence length; complex is words of three or more syllables; 0.4 is scaling factor that maps the sum onto school years. The same expression is printed on the page above the inputs, so the result can be reproduced by hand or in a spreadsheet.
Fog index across the sample passages
| Passage | Words per sentence | Complex words | Fog index |
|---|---|---|---|
| Plain narrative | 9.0 | 0 (0.0%) | 3.6 |
| Business copy | 13.5 | 2 (7.4%) | 8.4 |
| Research summary | 12.5 | 7 (28.0%) | 16.2 |
| Formal policy prose | 15.0 | 7 (46.7%) | 24.7 |
Complex words are those the syllable estimator puts at three or more syllables.