Readability formulas were developed in the 1920s in the United States to test the readability of school text books.

Common readability formulas are the Gunning Fog Index, Flesch Reading Ease and Flesch-Kincaid. The Gunning Fog Index is one of the most popular. Here's how it works:

1. Count the number of words in a sample text (at least 100). Then divide the total number of words by the total number of sentences. You now have the average number of words per sentence.

2. Count the number of polysyllabic words (three or more syllables) and divide that figure by the total number of words in your sample. Calculate the percentage of polysyllabic words.
   no. of polysyllabic words     x    100
   no. of words                            1

3. Add the two totals and multiply the total by 0.4. This figure is the Fog Index in years of education.

Typical Fog Index Scores Fog Index Resources
Reading ages of well-known material according to Juicy Studio http://juicystudio.com/services/readability.php are:

6	TV guides
8	Reader's Digest
8-10	Most popular novels
10	Time, Newsweek
11	Wall Street Journal
14	The Times, The Guardian
15-20	Academic papers

This website will also calculate the reading age of your website.

Readability formulas are useful to:

• give an early warning that your writing is too dense and you need to revise it
• show measurable improvements in written documents.

Readability formulas have their limitations as they can't tell you:

• how complex the ideas are
• whether the content is interesting and appropriate for the reader
• whether the layout and design is attractive.

Further reading
If you enjoyed reading this article, you may also like to read:
Writing executive summaries
Editing and proofreading tips
Dealing with abbreviations
Writing endings
First, second and third person
Trouble getting started
Make numbers manageable
What is irony?
Writing recommendations
Writing quotations

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