The Gunning Fog Index was devised to bring simplicity to writing. Unnecessary complexities that a document may contain render it unreadable for the people it is intended for. With a background in textbook publishing, Robert Gunning came up with this readability formula in 1952 which came to be called the Fog Index, as it aimed to remove the ‘fog’ and clutter of any document.
The Fog Index uses two variables – average length of the sentence, and the number of words with more than two syllables for every 100 words. The formula is as following where complex words are words with more than two syllables:
The score is an indication of the number of years a person requires to be able to understand the given text in the first reading. Therefore, a fog index of 10 would mean the reader should have a reading level of a 10th grader.
The Fog Index is used by writers and publishers who would like their reading to reach out to a wider audience. An index of 12 is considered suitable for a fairly large readership, while an index of 8 would definitely expand the audience.
However, the formula is based on the myth that all words with more than two syllables are difficult. Most multi-syllabic words are simple to read and pronounce, and don’t always make a document difficult to comprehend.
When the formula was first introduced, clauses within a sentence were considered as separate sentences. This was based on the logic that readers view these clauses as distinct from the rest of the sentence. However, once the formula was applied by computers, this distinction was removed, and all clauses are now considered as words within a sentence.
Computing the index is easy as Joe’s Web Tools online readability calculator include the Gunning Fog Index. The index will indicate the level of complexity of your document.