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Gunning Fog Index Calculator

Measure readability by sentence length and complex word percentage

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Measure How Much Education Your Text Demands

Robert Gunning developed the Fog Index to help business writers cut through unnecessarily dense prose. The idea is simple: long sentences and big words make text foggy. The formula quantifies that fog as a grade level, giving you a single number to track as you edit. It remains one of the most widely cited readability measures in journalism, government, and corporate communication.

Two Factors: Sentence Length and Word Complexity

The Fog Index adds two numbers before scaling: the average number of words per sentence and the percentage of complex words. Each factor can independently raise the score. A text with short sentences but many three-syllable words scores high because of vocabulary. A text with simple words but sprawling sentences scores high because of syntax. The most readable texts keep both factors low.

What Makes a Word Complex

In the original formulation, a complex word has three or more syllables but is not a proper noun, a familiar compound, or a word inflated to three syllables only by -es, -ed, or -ing. This calculator counts syllables using a vowel-group algorithm and strips common suffixes before checking the threshold, which prevents words like"created" (two base syllables) from being falsely flagged. The list of complex words is shown so you can decide which ones to simplify.

Real-World Benchmarks

Reader's Digest averages a Fog Index around 8 to 9. Most best-selling novels score 7 to 10. Academic journals often exceed 15. US government plain-language guidelines recommend keeping public-facing documents below a Fog of 12. If your text is for a general audience and scores above 12, the complex-word list this tool provides is your editing starting point — look for words you can replace with shorter, more common alternatives.

Limitations to Keep in Mind

The Fog Index does not account for word familiarity —"basketball" has three syllables and counts as complex, even though every English speaker knows it. It also does not consider text structure, visual formatting, or reader motivation. Use it alongside other readability formulas for a more balanced view, and always complement automated scores with a human judgment call about whether your specific audience will find the text accessible.

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What is the Gunning Fog Index? ▸
The Gunning Fog Index is a readability formula created by Robert Gunning in 1952. It estimates the number of years of formal education a reader needs to understand a text on first reading. The formula focuses on two factors: average sentence length and the percentage of complex words, defined as words with three or more syllables. A Fog Index of 12 means the text requires about 12 years of education, roughly a high school senior level. Lower scores indicate easier text.
What is the Gunning Fog formula? ▸
The formula is: Fog Index = 0.4 × (average sentence length + percentage of complex words). Average sentence length is total words divided by total sentences. Percentage of complex words is the count of words with three or more syllables divided by total words, multiplied by 100. Complex words exclude proper nouns, familiar compound words like"everything," and words made three syllables only by common suffixes like -es, -ed, or -ing. The 0.4 multiplier scales the sum to approximate a US grade level.
What is a good Gunning Fog score? ▸
Most popular magazines and newspapers score between 8 and 12. Time magazine averages around 11, and the Wall Street Journal around 11 to 12. General-audience web content should target 8 to 10 for comfortable reading. Technical documentation for professionals can score 12 to 17 without issue. Fog scores above 17 indicate extremely dense text that even well-educated readers will find taxing. If your audience is broad and your score exceeds 12, consider breaking long sentences and replacing multi-syllable words with simpler alternatives.
What counts as a complex word? ▸
A complex word in the Gunning Fog formula is any word with three or more syllables, with three exceptions. Proper nouns are excluded because their length does not reflect vocabulary difficulty. Familiar compound words like"however" and"everything" are sometimes excluded in stricter implementations. Words that reach three syllables only because of common inflectional suffixes like -es, -ed, or -ing are also excluded in the original formulation. This calculator uses the standard syllable-count method and excludes common suffix inflation to stay close to Gunning's intent.
How does Gunning Fog compare to Flesch-Kincaid? ▸
Both formulas use sentence length and word difficulty, but they measure word difficulty differently. Flesch-Kincaid uses average syllables per word — a continuous measure that counts every syllable. Gunning Fog uses a binary threshold — a word is either complex (three-plus syllables) or not. This makes Fog more sensitive to the proportion of hard words but less sensitive to differences among easy words. In practice, the two formulas usually agree within two grade levels. Fog tends to score higher on texts with many three-syllable words even if those words are common.
Can I use the Fog Index for non-English text? ▸
The formula was calibrated for English. You can run it on other languages, but the syllable threshold of three may not be appropriate. Languages like German and Finnish have naturally longer words due to compounding and agglutination, which would inflate the complex-word percentage and overestimate difficulty. For non-English readability, use a formula designed for that language or adjust the complexity threshold accordingly.
Is any data sent to a server? ▸
No. All calculations run locally in JavaScript inside your browser. Your text is never uploaded, stored, or shared with anyone.