Flesch-Kincaid vs Gunning Fog — Which Readability Formula Should You Use?
Compare Flesch-Kincaid Grade Level and Gunning Fog Index readability formulas. Learn how they work, when to use each one, and which actually gives more accurate results for your writing.
| Feature | Flesch-Kincaid Grade Level | Gunning Fog Index |
|---|---|---|
| Formula basis | Sentence length + average syllables per word | Sentence length + percentage of complex words (3+ syllables) |
| Score range | 0–18 (grade levels K through college) | 6–20 (years of formal education needed) |
| Best for web content | Yes — aim for grade 6–8 | Yes — aim for Fog Index 8–10 |
| Academic use | Widely used in education and textbook research | Less common in academia; more popular in journalism schools |
| Ease of interpretation | Very intuitive — 8 means an eighth grader can read it | Intuitive — 12 means 12 years of education needed |
| Industry adoption | Government, military, healthcare, education, Microsoft Word | Journalism, corporate communications, marketing |
| Complex word handling | Uses average syllables across all words | Flags words with 3+ syllables as complex |
| Limitations | Misses word familiarity — doesn't notice unfamiliar short words | Penalizes common long words like 'everything' or 'understand' |
Verdict
Both formulas are useful — use them together for the full picture. Flesch-Kincaid is your choice when you need to meet government or institutional plain-language standards, since it's the most recognized and is in Microsoft Word. Gunning Fog is great for journalists, marketers, and business writers who want to identify overly complex vocabulary. For web content, aim for a Flesch-Kincaid Grade Level of 6–8 and a Gunning Fog Index of 8–10. UnicornToolbox's Readability Checker shows both at once so you can compare them.
How the Flesch-Kincaid Formula Works Under the Hood
The formula is: (0.39 x average sentence length) + (11.8 x average syllables per word) - 15.59. Two things drive the score: sentence length (measured in words per sentence) and how many syllables your words have on average. Longer sentences and more syllables push the grade level up. Example: text with 15-word sentences and 1.5 syllables per word scores around grade 7. Rudolf Flesch and J. Peter Kincaid developed this for the U.S. Navy in 1975 to assess how readable their technical manuals were. It became the most widely used readability metric in English since then.
How the Gunning Fog Index Identifies Complex Writing
The formula is: 0.4 x (average sentence length + percentage of complex words). Complex words are anything with 3+ syllables, excluding proper nouns, familiar jargon, and compound words. The key difference from Flesch-Kincaid is that Gunning Fog specifically counts and penalizes complex words instead of averaging syllables. This makes it excellent at spotting unnecessarily fancy vocabulary. Robert Gunning created this formula in 1952 specifically for newspaper and business writing, where clear, direct language matters most. It's still popular in those fields.
Practical Tips for Improving Both Scores Simultaneously
The single best improvement is breaking up long sentences — aim for 14–18 words average by splitting compound sentences and using periods instead of semicolons. To lower Gunning Fog specifically, replace complex words: 'help' instead of 'facilitate,' 'use' instead of 'utilize,' 'start' instead of 'commence.' For Flesch-Kincaid, reduce average syllable count by preferring one- and two-syllable words. Read your text aloud — if you stumble or run out of breath, it's too long. UnicornToolbox's Readability Checker shows both scores in real time, so you see the impact of every edit immediately.
Frequently Asked Questions
Aim for grade 6–8. This means most adults, even those with lower literacy, can understand you. Major publications like The New York Times write at about grade 8. For specialized professional content, grade 10–12 might be okay if your readers have expertise in that area.
Target 8–10. The Wall Street Journal averages about 11, Time Magazine about 10. For maximum accessibility, keep it under 10. Anything above 14 means the text is hard to read and should be simplified for general audiences.
Absolutely, and you should. They measure different things — Flesch-Kincaid focuses on overall syllable density, Gunning Fog targets complex vocabulary. Using both gives you a better sense of what to fix. If one is high and one is acceptable, you know exactly what the problem is.
Gunning Fog treats every word with 3+ syllables as 'complex,' including everyday words like 'important,' 'computer,' and 'everything.' That's why text with lots of normal long words scores higher on Gunning Fog than Flesch-Kincaid, which spreads syllables across the whole calculation.
Readability formulas have limits. They only measure surface features like sentence length and syllable count. They can't judge concept difficulty, logical structure, or whether readers already know the topic. A physics textbook with short sentences and simple words might score as 'easy' even though the concepts are advanced. Use readability scores as one input, not the final word.