The Gunning Fog Index: a useful tool for targeting an article to an audience

In the 1930s, psychologists began to research how our brains process written information. They found that the longer a sentence the more difficult it is for a reader’s short-term memory to hold its meaning, so clearly shorter sentences improve comprehension.

Readability formulas determine how difficult it is to read and understand a piece of writing.

Robert Gunning’s Fog Index (GFI) was one of the first efforts to quantify the readability. The number that results from the following calculation correlates to the grade level:

GFI = [(number of words / number of sentences) + number of ‘difficult words’] x 0.4

• Count the words and sentences in a representative passage of about 100 words.
• Divide the number of words by the number of sentences to give the average length of each sentence.
• Count the number of words of three or more syllables that are not (a) proper nouns, (b) combinations of easy words, or (c) made three syllables by suffixes such as ‘ed’, ‘es’ or ‘ing’.
• Add the average sentence length from step 2 and the number of ‘difficult’ words from step 3 and multiply by 0.4.

So the GFI in pulp novels is 8–10; that of tabloid newspapers is 10–12; while medical journals score 14–16. Not surprisingly, insurance policies score an daunting 18–20!

Professional writing should score between 10 and 15. Below 10, and you are in danger of over-simplifying your message. Over 15, and your reader may struggle to understand.