Fog Index

A readability test aimed to determine the level of text difficulty and how easy it is to read

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What is the Fog Index?

The fog index refers to a readability test that aims to determine the level of text difficulty, or how easy a text is to read. The index provides a reader with the number of years of education that he or she hypothetically needs to understand and digest a particular text on the first reading.

How to Calculate the Fog Index

To calculate a fog index score, we need to know the three following components:

  1. Average sentence length
  2. Percentage of long words present in the text
  3. Sum of the average sentence length and the percentage of long words

To calculate the average sentence length, we need to divide the number of words by the number of sentences in a text sample.

To find the percentage of long words, we divide the number of long words by the total number of words and multiply the result by 100. A long word is defined as a word with at least three syllables.

Finally, the sum between the average sentence length and the percentage of long words is computed to arrive at the fog index.

There are a few exceptions to what is defined as a long word. Here is a list of the exceptions for long words:

  • Words starting with a capital letter such as company names
  • Combined “short-words” such as “share-holders” or “over-draft”
  • Words with three syllables just because we added “ed” or “es” (e.g., created or practices)
  • Short three-syllable words, such as “media”

Ideal Fog Index Level

For business writing, we aim for a fog index of between 20 to 25. For longer and more technical text scripts, such as official reports, we aim for 30 to 35. Anything more than these suggested figures is getting too complex to be understood by the average reader. The easier it is for the reader to understand what you are saying, the more likely they will want to read your message.

Ideal Fog Index Level

Example of the Fog Index Calculation

Let’s look at an example of the fog index calculation below. Here is a passage taken from our Business Writing Fundamentals course.

Sample Text - Original

The passage above only consists of a single sentence with 87 words. Thus, we need to divide 87 by 1 to arrive at 87 average sentence length.

Next, count the number of hard words. In the passage above, there are 11 hard words. Eleven hard words divided by 87 total words and multiplied by 100 gets us the percentage of hard words, which is equal to 13.

Finally, summing up 13 and 87, we obtain a fog index of 100. That is much higher than the numbers mentioned above.

Here is a revised version of the text passage, which is much shorter and easier to read.

Sample Text - Revised

The fog index in the revised version is 18, given there are 51 words, five sentences, and four hard words. You’ll notice that with an easier-to-read passage comes a much lower index.


In essence, readability is the ease of understanding a written text by a reader. The readability of the text depends on its content (e.g., vocabulary complexity) and its presentation (font size, line height, etc.).

Higher readability makes it easier for a reader to perceive the given information in the text and read faster. The fog index is aimed at increasing reading speed and comprehension, while also reducing reading fatigue.

Limitations of the Fog Index

The fog index comes with a few disadvantages. Even though the index serves as a good indicator of a hard text, not all difficult words are complex. It assumes a word to be hard-to-read based on the number of its syllables.

For example, the word “interesting” is hardly considered a difficult word by most individuals. Nonetheless, it comprises four syllables and, therefore, is considered as hard by the fog index.

Related Readings

CFI offers the Commercial Banking & Credit Analyst (CBCA)™ certification program for those looking to take their careers to the next level. To keep learning and developing your knowledge base, please explore the additional relevant CFI resources below:

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