Gunning Fog Index

The Gunning Fog Index measures readability. And what is good readability? It’s when you write something and the person reading clearly understands what you are communicating. It can be challenging to ensure that what you write makes sense to average readers. It's not always easy to hold your intended audience's attention so that they want to go on reading, which is where the Gunning Fog Index comes in.

The index was developed by Robert Gunning, an American businessman

The index estimates whether the formal education you've received matches your reading comprehension. Gunning figured out that school students couldn’t read because they couldn’t write properly. He believed that newspapers and business writing contained complicated text that students couldn’t comprehend.

So he took readability to the workplace

He developed a “team” that was dedicated to working on readability. He even went to those same newspapers, magazines, and business document companies and worked with them. The aim was that their target audiences would enjoy and understand what they were reading and reduce unnecessary complexity.

How he worked it out to use the Gunning Fog Index

In order to calculate a score for readability using the Gunning Fog Index, Gunning took 100 words of text and syllables from a writing sample. He then counted how many sentences were in that group of 100 words. That gave him the Average Sentence Length (ASL).

Then he counted the words in the sentences that had three or four syllables in them, words that weren’t proper nouns, hyphenated words, or two-syllable verbs. He divided the number he arrived at by the total number of words that were in the 100-word passage. This gave the Percentage of Hard Words (PHW).

Then he added the ASL and the PHW together. The result was multiplied by 0.4 which was the grade level. His formula looks like this, for instance: Grade level= 0.4 (ASL + PHW).

What the score means

The whole idea behind the Gunning Fog Index is that sentences that are short and written in plain English receive better scores than sentences that are lengthy with complicated and long English words in them in terms of readability. 

Using this qualitative analysis, Gunning discovered that a Gunning Fog Index score of 7 or 8 was perfect. He also discovered that if the score was over 12, the text would be too complicated and difficult for most people to read. If text, for instance, was written at a graduate level, then it would have a score above 17.

The Gunning Fog formula generates a grade level for readers at scores between 0 and 20

The grade level you are at in your education will determine whether you can understand the text. For instance, if you are a 6th grader, the ideal score for your readability expectations is around 6. People who are school graduates will find reading easy at around an 8th-grade level.

The Gunning Fog Index method isn’t perfect

It might be thought of as quite an accurate measure to score readability and assess readability expectations. However, the Gunning Fog Index does come with some limitations. For instance, in the case of multi-syllable words, it doesn’t take into account that not all of them are difficult to read or understand.

Why is readability so important?

If you own a website and you are putting out “difficult” material for your visitors to read, they are simply going to navigate away to “greener pastures.” As a website owner, you want to increase your click rates, right? If you want people to engage with your content, you need to understand what their reading expectations are.

Who are you putting out content for? Is it for the medical sector? Is it for young kids? Naturally, this is going to impact what kind of writing you deliver. Understanding readability formulas can help you alter your writing and meet the expectations of your different audiences.

How to improve your score

Want to know how to improve your Gunning Fog Index score? Here’s a free readability test tool you can use. All you do is enter your website URL, and the free automatic readability checker will calculate your score using a readability metric. This will help you ensure that your writing is suitable for your intended audience.

The simplicity of the Gunning Fog formula is highly praised today, and there are modern versions that people claim work even better. Check out Readable, for instance. Readable evaluates text just like Gunning's formula does, but it is more modernized and improves accuracy in terms of readability.

Gunning Fog simply helps to reduce the difficulty of writing papers

For researchers, reducing the difficulties of complicated, difficult reading is very appealing. Think of those difficult international medical journals. Writers of these have become aware of the use of the Gunning Index because it is so easily interpreted.

For instance, the Journal of X-Ray Science and Technology will only publish papers with a Gunning Index score of less than 16. There's evidence that people prefer reading simpler text. Physicians who read throwaway journals because they are more appealing than the highfalutin, prestigious journals are an example [1].


The bottom line is, the Gunning Fog Index is still helpful today. It's able to estimate what level of text you can read according to your formal education level. If you want to determine how difficult or easy an article is so you can understand it fully, use the Gunning Fog Index as a readability metric. Your readers will thank you.

All Readability Tests:

Here is a list of all readability tests.

Jonathan Gillham

Founder / CEO of I have been involved in the SEO and Content Marketing world for over a decade. My career started with a portfolio of content sites, recently I sold 2 content marketing agencies and I am the Co-Founder of, the leading place to buy and sell content websites. Through these experiences I understand what web publishers need when it comes to verifying content is original. I am not For or Against AI content, I think it has a place in everyones content strategy. However, I believe you as the publisher should be the one making the decision on when to use AI content. Our Originality checking tool has been built with serious web publishers in mind!

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