HesapHanem

Gunning Fog Index Calculator

Measure how complex your writing is with the Gunning Fog index.

How it is calculated

Paste English text. The tool counts W words (anything separated by whitespace), S sentences (every full stop, exclamation mark or question mark ends one, and a run such as ?! or ... counts once; commas do not end a sentence; at least 1) and C complex words, then computes:

Fog = 0.4 × (W / S + 100 × C / W)

A word counts as complex when it has three or more syllables. The tool estimates syllables by counting groups of consecutive vowels (a, e, i, o, u, y) in the letters of the word and dropping one for a silent final e. Only the letters a to z are looked at, so digits, punctuation and accented letters are ignored. It does not exclude suffixes or proper nouns, so it can overcount. The index is rounded to two decimals and roughly corresponds to a US school grade.

Worked example

Take the sample text “The committee approved the proposal after a comprehensive evaluation. Employees must complete the mandatory registration before Friday. Questions are welcome.” It has 20 words and 3 sentences, giving 20 / 3 = 6.67 words per sentence. The tool finds 6 complex words (approved, proposal, comprehensive, evaluation, mandatory, registration), which is 100 × 6 / 20 = 30%. Fog = 0.4 × (6.67 + 30) = 14.67. Note that "approved" is counted as complex by the vowel-group rule even though it is usually said in two syllables.

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Common mistakes

  • Running non-English text through it. Only plain a to z letters are examined, so Turkish and other accented text is mis-scored and short words may be counted wrongly.
  • Pasting text with abbreviations or decimals such as "e.g." or "3.5", which create extra sentence endings and lower the words-per-sentence part.
  • Pasting a few words with no full stop. It counts as one sentence, so a title or a heading scores very differently from the full text.
  • Treating the score as exact. The syllable count is an estimate, so "approved" and similar words are flagged complex and the figure is a guide, not a verdict.
  • Trying to lower the score only by chopping sentences. Complex-word density counts for as much as sentence length.

Sources

About this calculator

This calculator computes the Gunning Fog Index, a readability formula developed by Robert Gunning in 1952: Grade level = 0.4 × [(words / sentences) + 100 × (complex words / words)], where a complex word has three or more syllables (here estimated by counting vowel groups in each English word, with no exclusions for suffixes or compound words). The result approximates the years of formal education needed to understand the text on first reading.

Editors, technical writers, UX writers, and students use it to check whether a document, article, or set of instructions is written at an appropriately accessible level — a Fog Index above 12 is generally considered too complex for a general audience.

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Comments (1)

  • Editorial team

    Every full stop ends a sentence for this tool, so abbreviations such as e.g. or decimals like 3.5 split sentences and pull the score down. Clean those out first if you want a fair reading. Syllables are estimated from vowel groups, so names and acronyms can be miscounted, and very short snippets swing widely. Use it to compare drafts of one text rather than to judge a single number. What kind of writing are you checking: web copy, manuals or reports?

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