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In linguistics, the Gunning fog index is a test designed to measure the readability of a sample of English writing. The resulting number is an indication of the number of years of formal education that a person requires in order to easily understand the text on the first reading. That is, if a passage has a fog index of 12, it has the reading level of a U.S. high school senior. The test was developed by Robert Gunning, an American businessman, in 1952.[1]

The fog index is generally used by people who want their writing to be read easily by a large segment of the population. Texts that are designed for a wide audience generally require a fog index of less than 12. Texts that require a close-to-universal understanding generally require an index of less than 8.

Calculating the Gunning fog indexEdit

The Gunning fog index can be calculated with the following algorithm:

  1. Take a full passage that is around 100 words (do not omit any sentences).
  2. Find the average sentence length (divide the number of words by the number of sentences).
  3. Count words with three or more syllables (complex words), not including proper nouns (for example, Djibouti), familiar jargon or compound words, or common suffixes such as -es, -ed, or -ing as a syllable.
  4. Add the average sentence length and the percentage of complex words (ex., +13.37%, not simply + 0.1337)
  5. Multiply the result by 0.4

The complete formula is as follows:

0.4*\left( \left(\frac{\mbox{words}}{\mbox{sentence}}\right) + 100\left(\frac{\mbox{complex words}}{\mbox{words}}\right) \right )

While the index is a good indication of reading difficulty, it still has limitations. Not all multisyllabic words are difficult. For example, the word "asparagus" is generally not considered to be a difficult word, even though it has four syllables.

In the late 1960s this index was calculated differently. (The technique of clear writing by Gunning, R. McGraw-Hill International Book Co; New York, NY 1952.) The original formula demanded independent clauses to be counted as separate sentences. The logic was that readers perceive independent clauses as distinct ideas, as they are cognitively synonymous with sentences, and thus measured clarity of ideation within sentence elements as part of the score. Obviously, this would change the sentence count for some writers more than others. In the 1980s, though, it was decided by others that this grammatical distinction was beyond the ability of computers and was of little value.

There is much writing, "The Rise and Fall of the Roman Empire" by Gibbon for example, where the difference between a score using the modern algorithm shown above and a hand-done score using the original formula would vary greatly.

The Gunning Fog Index was changed to make it computer-friendly but counting independent clauses to follow the original Gunning-Fog formula remains an option.


The following paragraph, from the Wikipedia article on "logorrhea", has a Gunning-Fog Index of 16.6.

The word logorrhoea is often used pejoratively to describe prose that is highly abstract and contains little concrete language. Since abstract writing is hard to visualize, it often seems as though it makes no sense and all the words are excessive. Writers in academic fields that concern themselves mostly with the abstract, such as philosophy and especially postmodernism, often fail to include extensive concrete examples of their ideas, and so a superficial examination of their work might lead one to believe that it is all nonsense.

See alsoEdit


  1. Plain Language At Work Newsletter, 23 March 2004,

External linksEdit

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