The Gunning fog index is an algorithm that aims to indicate how readable your writing is by calculating the (roughly) the number of years of formal education you need to have had in order to understand the text after a single reading.

It’s quite simple, you can calculate it by hand. Based on Wikipedia’s description it works like this:

  1. Pick a continuous passage of the text of around 100 words. Don’t include partial sentences.
  2. Calculate the average number of words per sentence.
  3. Count the number of ‘complex’ words. Complex words are those that:
    • Have at least 3 syllables. Do not count common word endings like -es, -ed or -ing as syllables. For example “created” counts as 2 syllables.
    • Are not proper nouns, familiar jargon or compounds of simple words such as “sunflower”.
  4. Add together the average sentence length and the percentage of complex words
  5. Multiply the result by 0.4.

Of course you don’t have to calculate it by hand. You can paste some text into this website amongst others and it’ll do the calculation for you.

It must be said that the various website options don’t all give you the same answer, which I’d guess reflects the slightly subjectivity needed when determining common word endings, compound words and the other exceptions to the general rule. Or of course faulty programming.

In summary, the formula looks like:

0.4 * ((count of words / count of sentences) + (100 × count of complex words / count of words))

As an example, the first few sentences of this post would thus be calculated as:

0.4 * ((113 / 9) + (100 * 11 / 113)) = ~ 8.9

The estimate is then that you’d likely was 9 years of formal education to easily understand it, making you, in US terms, a high school freshman.

It’s one of these things that can only ever be a rough approximation, and is presumably targeted only at the English language. But I can see it being of some use in checking whether what you write is vaguely suitable for the audience you intend it for, as well as providing some insight into what makes text readable.

It is only one of very many proposed formulae for calculating readability - it just happens to be the first one I came across. The Readability Formulas website gives a lot more info on a lot more formulae of that type, including an opinion on which to use when and an online calculator that can calculate several at once.