The Fog Index is a rough measure that quantifies the readability of a written passage. The index was invented by an American called Robert Gunning. An index of around 7 is considered to be clear and precise. An index of 15 and above denotes a murky text that needs to be re-written. The Fog Index is calculated as follows:
- Count the number of words in a section of the written work. This section should be at least 100 words long.
- Count the number of complete thoughts in the sections. These thoughts are short sentences, or part of sentences that are separated by commas, colons or semicolons.
- Divide the number of words by the number of thoughts. This number is the average thought length.
- Count the number of words with three or more syllables in the section. Ignore proper names, compound names that are formed of simple words, and words which are three syllables because of the following suffixes: -ed, -s, and -es.
- Calculate the percentage of multi-syllable words in the section.
- Add this percentage to the average number of thoughts, and multiply the sum by 0.4 to obtain the Fog Index.