Gunning Fog Index Calculator
Grading Text by Sentence Length and Complex Words Together
The Gunning Fog Index, developed by Robert Gunning in 1952 for evaluating business and technical writing, combines two of the most common causes of hard-to-read prose — long sentences and complex, multi-syllable vocabulary — into a single grade-level number. It remains a standard reference alongside Flesch Reading Ease and SMOG for checking whether writing matches its intended audience.
The Formula
A "complex word" here means one with three or more syllables, with common suffixed forms (-es, -ed, -ing) evaluated on their base word so that ordinary inflections like "created" or "walking" don't get penalized just for their ending. The result reads as a U.S. grade level — a Fog Index of 12 suggests the text is suited to a 12th-grade (high school senior) reading level.
Where This Matters
- Business and technical writing — the formula was built specifically for this kind of content, where unnecessary jargon and long sentences commonly creep in.
- Editing for a general audience — a Fog Index consistently above 12 often points toward writing that's harder than necessary for a broad readership.
- Comparing drafts after an edit pass — tracking Fog Index before and after simplifying sentence structure and vocabulary shows whether the edit actually improved accessibility.
Worked Examples
| Complex words | % complex | Fog Index |
|---|---|---|
| 20 of 200 | 10% | 12.0 |
| 40 of 200 | 20% | 16.0 |
General guidance treats a Fog Index around 7-8 as ideal for a broad general audience, with scores above 12 considered difficult for most readers.
How to Use This Calculator
- Paste or type your text into the text box.
- Select Calculate to see the Fog Index grade level along with word, sentence, and complex-word counts.
Related Calculations
Compare against the Flesch Reading Ease Calculator and SMOG Index Calculator to triangulate readability from multiple formulas.