Fog Index
In linguistics, the Fog Index is a test designed to measure the readability of a sample of English text. 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. So 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, professor of English at Oxford University.
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.
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Typical fog indices of selected magazines
- 12 — Atlantic Monthly
- 11 — TIME, Harper's
- 10 — Newsweek
- 9 — Reader's Digest
- 8 — Ladies' Home Journal
- 7 — True Confessions
- 6 — Comic books
Calculating the fog index
The fog index can be calculated with the following algorithm:
- Take a full passage that is around 100 words (do not omit any sentences).
- Find the average sentence length (divide the number of words by the number of sentences).
- Count words with more than three syllables, not including proper nouns (for example, Kathmandu), compound words, -ed or -ing as a syllable, or familiar jargon.
- Add the average sentence length and the number of words with more than three syllables.
- Multiply the result by 0.4
While the index is a good indication of reading difficulty, it still has flaws. Not all multisyllabic words are difficult. For example, the word spontaneous is generally not considered to be a difficult word, even though it has four syllables.
See also