Heavy Fog road signOne of the first lessons in copywriting is learning how to write for the reader. You wouldn’t expect to find the same type of words used for an aircraft manual, a recipe book, an online shop, and a sales email, because the audiences for each are different.  Being an aircraft engineer doesn’t stop you cooking or shopping of course, but the style of writing will change according to the readers.

One quick and useful tool that writers use is called the Gunning Fog Index.  It was introduced in 1952 in the book The Technique of Clear Writing by Robert Gunning. It remains one of the quickest and most accurate ways to work out how easy it is to read your writing.

Why should writing be easy to read?

Writing that is easy to read is usually aimed at, or lower than, the reading level of a standard young teenager. The Fog Index shows the number of years of education a reader needs to be able to understand what you have written.

Writing that is dense, full of complicated words and technical terms, is going to be a chore to plough through. Unless the reader has to read it – it’s a text book or an instruction manual – they are very likely to struggle through a page or so, and then give up.  You do not want this to happen to anything of yours!

Writing that is easy to read is more popular, more effective, and more memorable than very formal, difficult prose.

How to find your Fog Index

To work out the Fog Index of your writing, there are three simple steps.

1. Count the average number of words per sentence.

To do this, take a couple of paragraphs and count the number of words (A) and the number of sentences (B). Divide (A) by (B) and this will give you the average number of words per sentence.

Average number of words per sentence = number of words / number of sentences.

2. Count the number of long words.

Using the same couple of paragraphs, count the number of words that have three syllables or more. Don’t include proper nouns or compound words, and don’t count suffixes (-ed -ing etc) as syllables.

3. Add these two numbers together and multiply by 0.4

Fog Index = (Average number of words per sentence + number of long words) * 0.4

There are plenty of programmes that can do this for you!

Once you have the score, what does it mean?

The resulting score tells you how many years of schooling a reader needs to be able to understand your writing without many problems. A Fog Index of 10 relates to 10 years of formal education, and the reading ability of a fifteen year old.

This is the same level used by quality newspapers and serious publications. It is too formal for a lot of writing. A low score doesn’t mean dumbing down, however, Shakespeare scores around 6!

A general guide is:

  • 6 – 8 comfortable for most people to read
  • 9 – 10 easy enough for anyone with a general level of education
  • 11 – 13 understandable for someone with further education
  • 14 – 16 best kept for people with a university education
  • 17 + too difficult for non-technical writing.

Problems with the Fog Index

The Fog Index is not perfect – it’s not fair to class all long words as difficult, for example. Using the 3+ syllables rule means that elephant, potato, education, and engineer would count towards the score.  None of them is likely to confuse the average woman in the street.  That said, the index remains one of the most accurate methods and a favourite with writers everywhere.  It’s worth keeping in your tool box.

There are other methods to measure the readability of your writing and this blog will cover those, as well as other useful writing tools as we go along.


This blog post has a Fog Index of about 10, by the way.