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Gunning Fog Index

Measure how much fog long words and sentences add to your writing.

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Robert Gunning built this in 1952 out of frustration with corporate writing. Sentence length and three-syllable words carry equal weight, which is why jargon shows up so clearly.

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years of formal education needed

Your longest sentences

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Quick Tool Overview

The Gunning Fog Index measures the years of formal education a reader needs, weighting sentence length and words of three or more syllables equally. Because the formula names complex words explicitly, the tool lists yours — the most directly actionable output of any readability measure.

What it does
Calculates the Gunning Fog readability index
You give it
A report, article or piece of business writing
You get back
A years-of-education figure and every complex word
Best for
Cutting jargon out of business and technical writing
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About the Gunning Fog Index

Robert Gunning was a businessman rather than an academic, and he built the Fog Index in 1952 out of frustration with corporate writing that nobody could understand. His insight was that difficulty comes from two things: long sentences, and words of three or more syllables.

The formula weights those equally, producing a number that corresponds to the years of formal education a reader needs. A score of 12 means a text needs a high school graduate; above 17 means a postgraduate. Gunning's own advice was that anything above 12 is too hard for general readership. Because the formula names complex words explicitly, this tool lists the ones in your text — which is the most directly actionable output of any readability measure.

Key features of the Gunning Fog Index

Complex words listed

Every three-syllable word in your text, which is half the formula and the half you can fix.

Gunning's exclusions applied

Proper nouns, familiar compounds and words made long only by a suffix are excluded, as specified.

The under-12 threshold

Gunning's own limit for general readership, marked clearly against your score.

Explains a high score with short sentences

Because the other half is jargon density, which is exactly the writing the index was built to catch.

Real benchmarks

Popular newspapers, business writing and academic papers, so the number has context.

How to use the Gunning Fog Index

  1. 1

    Paste your text

    A report, an article or any business writing.

  2. 2

    Read the fog index

    The years of education a reader needs — 12 means a high school graduate.

  3. 3

    Review the complex words

    Every word of three or more syllables is listed, since those are half the formula.

  4. 4

    Cut and re-check

    Replace the complex words that have simple equivalents and watch the score fall.

Frequently asked questions

Three or more syllables, with three exceptions Gunning specified: proper nouns, familiar jargon, and compound words made of simple parts. Words made complex only by a suffix such as -ed or -es are also excluded.

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