Practical tool
Check your writing with the Gunning Fog Index
The Gunning Fog Index estimates how many years of formal education a reader may need to understand a passage on first reading. It uses average sentence length and the percentage of words estimated to have three or more syllables. The score is a rough signal, not a verdict on accuracy, tone, accessibility or importance.
Key takeaways
- Use a representative sample, not a heading or single sentence.
- Inspect the causes of a score before rewriting.
- Keep necessary technical terms when they carry precise meaning.
- The calculator processes text only in your browser.
Local browser tool
Calculate a Fog score
Your text stays in this browser and is not uploaded or stored.
- Sentences
- 0
- Words
- 0
- Complex words
- 0
- Average sentence
- 0.0
- Complex-word share
- 0.0%
Enter a representative passage to see an estimate.
A worked edit: make the action clearer
Before: “Following completion of the initial assessment, participants are requested to undertake a subsequent revision of the documentation prior to submission.”
After: “Complete the initial assessment. Then revise the document before you submit it.”
The revision makes the order visible and replaces an indirect request with an action. It keeps the assessment, revision and submission as distinct steps. If the original instruction means revise only when feedback identifies a problem, add that condition explicitly; shortening the sentence must not remove it.
This example is too short to support a meaningful standalone readability verdict. Use a representative passage in the calculator, then review meaning, headings and necessary terminology. A lower Fog score cannot tell you whether the instructions are correct.
An assistant-run check with this site’s calculator on 23 September 2026 counted 20 words in one sentence before editing and 12 words in two sentences afterwards. Average sentence length therefore fell from 20 to 6 words. Those are counts for these exact examples, not measurements of comprehension or a reliable education-level estimate from such a short sample.
Move from a score to a better assignment
Use the student editing checklist to check the argument and evidence before polishing sentences.
For language assistance, our Grammarly assessment for students shows what one public-checker sample flagged and what still needed a human evidence check.
The formula
Fog score = 0.4 × (average sentence length + percentage of complex words). This browser calculator estimates syllables using letter patterns and excludes some endings and capitalised words. It is a heuristic approximation, not a complete linguistic analysis. Hyphenated forms are counted as one word; abbreviations and sentence boundaries can still be miscounted.
| Approximate score | Plain-English reading |
|---|---|
| 6 to 8 | Generally accessible everyday prose |
| 9 to 12 | Typical general or secondary-level material |
| 13 to 16 | Dense, academic or specialist prose |
| 17+ | Inspect long sentences and complex vocabulary closely |
A transparent calculation and a better comparison
For a hypothetical passage of 100 words in five sentences, with 15 words classified as complex, the average sentence length is 20 and the complex-word percentage is 15. The formula gives 0.4 × (20 + 15) = 14.0. This example demonstrates the arithmetic; it is not a test of a reader or a real passage.
Use Fog when you want to locate long sentences and potentially difficult words. Use a grammar checker for possible language errors. Ask an intended reader to explain or act on the passage when the question is whether it communicates successfully. My recommendation is to use the reader’s task as the final check; neither a low score nor a clean grammar report validates the meaning.
Capitalisation can change this calculator’s complex-word count because it uses capital letters as a rough proper-name clue. Necessary specialist words, unusual punctuation and non-English text can also distort results. Compare revisions with the same tool and inspect the counts rather than treating small score changes as precise improvement.
How to use the result
Start with the longest sentence. Split it only if the logical relationship remains clear. Then inspect complex words: replace an unnecessary abstraction, but keep an exact term when the audience needs it. Read the revision aloud and check whether the meaning, emphasis and rhythm improved.
A lower score is not automatically better. Legal, scientific and technical writing sometimes needs specialist vocabulary. A short sentence can still be vague, inaccurate or exclusionary. Pair the score with user testing, subject review and accessible structure.
What the score cannot measure
- Whether a claim is true or well sourced.
- Whether examples suit the reader’s background knowledge.
- Whether the tone is respectful and inclusive.
- Whether headings, layout and navigation support comprehension.
- Whether a difficult idea has been simplified beyond accuracy.