The gunning fog index

One 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 readability test 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 in high school. The Fog Index shows the number of years of education a reader needs to be able to understand what you’ve 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 don’t 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. The reader can understand the text on the first reading and doesn’t need to keep re-reading it.

Key factors include the number of sentences and average sentence length.

How to find your Fog Index

blackboardTo 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 = divide the number of words / number of sentences.

Short sentences are easier for everyone to read, regardless of their grade level. You see?

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.

Complex words are difficult. They take longer to understand.

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

Fortunately, there are plenty of websites where we can have our text critiqued and programs, such as Grammarly, with inbuilt readability formulas to do all the counting for us! Plus all sorts of writing tips, such as proper nouns vs common nouns. Compound words vs complex words.

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 the sentences written are dumb, for example 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 Fog Scores

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 person in the street.

That said, the index remains one of the most accurate methods and a favourite with writers everywhere. So 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.

And just so you know, this blog post has a Gunning Fog Score of about 10.

joanna brown

About the author: Joanna Brown
Joanna is a business copywriter who specialises in creating web content that gets companies communicating better. You can find her at The Word Hen or on Twitter as @TheWordHen.