Sunday, April 19, 2009

Readability Formulas: The Fog Index

math formulaReadability formulas provide an easy way to tailor your writing for a particular audience. There are a handful of different formulas. The most common is probably the Fog Index. It is quite easy to calculate the Fog Index for an article.

To calculate the Fog Index:

1. Select at least 100 words that appear continuously (10–12 sentences is ideal) in an article.

2. Calculate the average sentence length in the group of sentences (now referred to as the “passage”). To calculate the average sentence length:

  • Count the total number of words in the passage.
  • Divide the total number of words in the passage by the number of sentences.
3. Count the number of words with three or more syllables in the passage.

Note: do not include proper nouns or words whose common suffixes (-es, -ed, -ing) bring the total number of syllables to three. If the root word contains three or more syllables, discount this rule.

4. Calculate the percentage of words in the passage that have three or more syllables. To calculate the percentage:
  • Divide the number of words with three or more syllables by the total number of words in the passage and multiply by one hundred.
5. Add the average sentence length (from step 2) and the percentage of three or more syllable words.

6. Multiply that number by 0.4

The result is the Fog Index rating for the passage.


Regarding the exceptions for words with three syllables or more, there are some sources that claim that compound words (closed and hyphenated) should also be excluded. Of course, this would apply only to compound words that are three or more syllables after being joined. If one of the words alone already exceeds three syllables, presumably the rule wouldn't apply.

I didn't include the rule here for the simple reason that the only reliable online Fog Index calculator that I am aware of does not exclude compound words.

Online Fog Index Calculator

That omission could be due to limitations on the computer script that calculates the Fog Index. Regardless, the Fog Index rating won't be affected too much either way.

Here's another online tool that calculates the Fog Index for entire websites or blogs:

Calculate Readability of Website

What Does the Fog Index Rating Mean?

So what does that magic number indicate?

The Fog Index rating supposedly indicates the number of years of formal education required to read a piece of writing. In theory, the higher the Fog Index rating, the more difficult a passage is to read. However, it is important to remember that a passage with a lower Fog Index will not only appeal to less educated readers. A Fog Index of between 7–8 is probably the most accessible to the widest audience.

Remember also that a passage may be more or less readable depending on how well written it is, regardless of the Fog Index. In other words, sometimes a passage with a higher Fog Index rating is more readable than a passage with a lower Fog Index rating.

So don't attach too much significance to the Fog Index. Simply get a feel for the kind of writing that is representative of different Fog Index ratings. One way to do that is to calculate the Fog Index for number of different articles that you have written. Another way is to calculate the Fog Index for books or magazines that you like to read.

Amazon's Text Stats

Amazon recently added a new feature to the books they sell online. It's called Text Stats, and one of its statistics is the Fog Index for each particular book. However, the feature is not available for all books.

To locate Text Stats for a book on

1. Open in your computer's browser.

2. Select Books in the search menu.

Amazon search books

Enter Of Mice and Men in the search box.

4. Click the first title that appears.

5. Scroll down to Inside This Book.

Amazon Text Stats

6. Click Text Stats.

There you will see that the Fog Index for Of Mice and Men is listed at 4.9. A pretty accurate indication of years of schooling required in this case. I recall reading the book in grade five or six.

As mentioned, this feature is not available for every book on Amazon. When you do a search for a book, only the ones that have an Inside This Book graphic over the book may have the feature.

Benefits for Writers

lost in the fogThe Fog Index provides numerous benefits for writers. Most importantly, it allows you to shape and edit your writing for a particular audience.

Imagine that you are trying to pick up extra work by submitting articles to various publications. Simply calculate the Fog Index of some articles from recent editions of the magazines you are targeting. Tailor your writing so it is within the Fog Index of the kind of articles they publish.

I hope you can use the Fog Index to assist you as a writer. Remember, without clear, organized writing, readability formulas are of little use. However, if a document has a Fog Index rating over 12, it is safe to say that some readers will get lost in the fog.


William DuBay said...

For the history and research of the readability formulas, see my two books "Smart Language: Readers, Readability, and the Grading of Text" and "Unlocking Language: The Classic Readability Studies." For a short introduction to readability research see my free online paper, "The Principles of Readability" at

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