What is the Fog Index?

The Fog Index was developed by Robert Gunning, an American businessman, in 1952. It 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. If a passage has a fog index of 12, it has the reading level of a US high school senior. Texts that require a close-to-universal understanding generally require an index of less than 8. The Bible and Shakespeare reportedly have a fog index of 6.