Gunning-Fog Score Readability Calculator

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Gunning-Fog Score Readability Calculator

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Gunning’s FOG Index


A boom in readability tests occurred during the late 1900s, and one such test developed was the FOG Index. It was formulated by Robert Gunning in the 1960s. Gunning was a well-known businessman who has knowledge about newspapers and publishing.

Gunning has observed that many high school graduates had problems with reading, and a big problem with that was because of writing. Newspapers and business documents use up a lot of 'fog' and needless complexity. So he and his associates devised a formula to test the readability of texts and called it the FOG Index.


Gunning’s FOG Index highlights the number of words and syllables present in a text. It is best to get a sample passage of at least 100 words for this test. Using the Gunning FOG formula, you can directly estimate the number of years of formal education a person needs to understand the text.

That means, if your text scored a 6 in the Gunning FOG Index, it could be understood by people who are in 6th Grade and above.

If you want the general public to comprehend your text, it is a good idea to aim for a score of 8 since 8th grade English is the benchmark for conversational English.


What makes the Gunning FOG Test so beloved by many is because it has a simplified formula and an equally simple method. In a time where computers were not so commonplace, the Gunning FOG Readability Index was an easy and convenient method to use.

Before using the formula, you will need to choose a sample passage from your text, at least 100 words long or more. Count exactly how many words and sentences you have.

The formula for the FOG Index you need to use is:
FOG Score = 0.4 (ASL + PHW)

ASL = Average Sentence Length
ASL is calculated by dividing the total number of words by the total number of sentences in the sample passage.

PHW = Percent of Hard Words
To get the PHW, you must first count all the words in the sample passage with three or more syllables, provided that they are NOT:

  • Proper nouns
  • Hyphenated words
  • Two-syllable verbs turned into three syllables because of –es or –ed
    Once counted, you need to divide this number by the number of words in the sample passage to get your PHW.

Grade Conversion

Understanding the Gunning FOG Index score meaning is quite simple. That’s because you get a typical result between 0 and 20, and the direct result matches up the grade level someone needs to understand your text.

As a guide, here is a simple conversion table you can use:

FOG Score School Level Comprehension
0-5 5th Grade and below Very easy to read
6 6th Grade Easy to read
7 7th Grade Fairly easy to read
8 8th Grade Conversational English
9-12 9th & 12th Grade Fairly difficult to read
13-16 College Difficult to read
17 College Graduate Very difficult to read
18-20 Professional Extremely difficult to read


The Gunning FOG Index was first developed to help writers keep their texts clear and concise. As a businessman in publishing, Gunning's formula is very apt for newspapers, novels, etc. You can even use this readability test for today's written products, such as blogs and online articles.

What's more, the Gunning FOG Index is also widely used by researchers of different industries. You can use the FOG Index to check whether or not your paper's abstract is clear and understandable by the audience intended. Science journals and medical journals make use of the Gunning FOG Index quite a lot.

How to Improve Score

It’s quite easy to improve your score in the Gunning FOG Index. All you need to do is keep your words simple. Use the jargon that comes up in daily conversations. It’s also best to stick to simple sentence structures for better clarity.

If you are pressed for time and can’t calculate manually, a modern FOG Index calculator can help you determine your text’s score. Then, edit accordingly.

Further Readings