The Gunning Fog index estimates the years of schooling needed to understand a text on first reading. It uses two things: how long your sentences are and how many of your words are complex. Both are countable by hand.
How it works
The formula is: Fog = 0.4 x (average sentence length + percentage of complex words). Average sentence length = words / sentences. Percentage of complex words = complex words / words x 100. A complex word has three or more syllables, excluding proper nouns, compound words made of simple words, and verbs that reach three syllables only by adding -ed or -es. Scores under 8 suit a wide audience, 12 is about high-school senior level, and 17 and above is college graduate level.
Step by step
- Pick a sample of about 100 words, ending at a sentence boundary.
- Count words and sentences for average sentence length.
- Count complex words of three or more syllables, applying the exclusions above.
- Divide complex words by total words and multiply by 100 for the percentage.
- Add average sentence length and the percentage, then multiply by 0.4.
- Read the result as years of schooling needed.
Worked example
A sample has 100 words, 5 sentences, and 12 complex words. Average sentence length = 100 / 5 = 20. Complex percentage = 12 / 100 x 100 = 12. Fog = 0.4 x (20 + 12) = 0.4 x 32 = 12.8, about high-school senior to college freshman level. Shorten sentences to 15 words on average and cut complex words to 8, and Fog becomes 0.4 x (15 + 8) = 9.2, readable by a much wider audience.
Common mistakes
- Counting proper nouns as complex words, which inflates the score.
- Counting words like “created” as complex when -ed only pushed them to three syllables.
- Using a sample far below 100 words, where one rare word swings the percentage.
- Treating the score as a quality judgement. Clear writing for experts can legitimately score high.
Try it with the free tool
You can follow every step above by hand, but the free tool below does the same calculation instantly. Try your own numbers in it, then check one result by hand so you trust the method as well as the answer. Open the full tool here: open the tool.
Checklist before you finish
- Write down every input number first, with its unit, before you calculate anything.
- Work through the steps in order, and do not skip the unit conversions.
- Check the answer against the worked example above: is it the right size, and does the direction make sense?
- Round only at the end, and keep the unrounded figure for any follow-up calculation.
Practice with your own numbers
Pick three cases of your own and work each one by hand. First, try a simple round-number case you can check mentally. Second, try a realistic case with awkward numbers, the kind you will actually meet. Third, try an edge case, such as a very small or very large input, and confirm the answer still behaves sensibly. If any case gives a surprising result, re-check the units and the formula step by step before you trust it.
If your answer looks wrong
Most errors come from a small set of causes. Check that you used the same units throughout, that you divided where the formula divides and multiplied where it multiplies, and that you applied any percentage as a decimal. Check the order of operations: powers and brackets come before multiplication and division, which come before addition and subtraction. Finally, test the method on the worked example in this guide. If your method reproduces the example answer, the method is right and the inputs deserve a second look.
Quick tips
To lower a Fog score, split long sentences first, then replace complex words. Sentence length usually moves the score faster than vocabulary for most drafts.