Coleman-Liau Readability Index

The Coleman-Liau Index is unlike most other reading formulas. It doesn’t subscribe to formulas that contain syllables. What it does do is take into account how long words are. Even though it does differ from the other reading indexes, the Coleman-Liau Readability Index is still one that is commonly used today.

Jonathan Gillham

Who created the Coleman-Liau Readability Index?

Two linguists by the name of T.L. Liau and Meri Coleman created this readability index in 1975. They wanted to assess the readability of the textbooks that were being used in public schools in the USA. The idea behind this formula was that syllable counting lacked accuracy and took too much time. 

Because they didn’t fancy the idea of manually counting syllables, Coleman and Liau thought about using a machine to do the job. This machine would be able to count the number of letters in a word that was in the text. It would be more accurate than a machine that counted syllables in a word.

With the Coleman-Liau Reading Index, it is not necessary to count syllables

Finally, people could get a grade level of text without counting syllables. People could also understand the text. The readability score was easy to interpret. It showed people directly what grade level was needed to read the text.

Say, for instance, your index score was 6. That would mean that kids who were 6th graders in the U.S. schooling system would be able to understand the text that was being read.

If you are a person who is writing for the general public, your best bet is to aim for an index score of between 8 and 10. That is because 8th through 10th-grade kids mostly use conversational English.

Let’s check out the formula

If you want to achieve the best results for accuracy using the Coleman-Liau Index, use a sample piece that has at least 300 words. The formula is pretty straightforward.

CLI (Coleman-Liau Index) = 0.0588L (L= average number of letters for every 100 words) – 0.296S (S= average number of sentences for every 100 words) – 15.8

Check out this easy conversion table

It shows the possible scores you might get using the Coleman-Liau Index. See what they mean:


Why use the Coleman-Liau Index?

Firstly, it is used for educational purposes. It is an excellent predictor of readability for teaching purposes and for textbooks. Today, its use has broadened quite a bit. It is now used in other industries too, apart from educational institutions. Medical documents have also been subjected to the Coleman-Liau Readability Test.

This readability index can be compared with texts that are in Spanish or German too. Just don’t forget, though, that this index was formulated for use in the English language. That means that the grade level result you are trying to achieve in a non-English text will end up as a meaningless result.

What’s the best way to improve your score?

It is not that difficult to improve your score on the Coleman-Liau Index. All you need to do is make sure that you write easily and simply. Make use of common words, as if you are speaking to someone conversationally. Don’t use highfalutin words that few people understand.

If you are writing the text for an audience, you will understand that the simpler the words used the better. If you are writing for elementary school kids, but your Coleman-Liau score indicates that you are using a high school reading level, you might need to check your content again. Is it simple enough?

Another example is that you probably shouldn’t achieve writing for the 3rd-grade level if your intention is to reach college students. That’s the bottom line –  in order to communicate effectively with your audience, your writing should reflect the grade level of those you want to read your text.

Here’s a free readability test tool. It will help you to calculate your score with just a few clicks.

Why is readability so important?

The average American adult reads between a 7th and 9th-grade level. It is important to understand the readability expectations of the people you want to reach. That’s how you will keep people on your website. If you’ve got text that is difficult to understand, your audience will gradually navigate away from your site. If it’s too simplistic, on the other hand, that can also cause visitors to move away.

Your target audience is also important. If you are writing for a medical publication, your text might be more advanced than if you were writing text for elementary school students.


The Coleman–Liau index is a readability formula. It has been designed to test how easily text can be understood. It is similar to the Automated Reading Index but is unlike a lot of the other indexes because it relies on characters instead of syllables in a word.

Different opinions have arisen as to its accuracy compared to the other indexes. With the Coleman-Liau index, the characters are more accurately counted by computer programs than syllables.

The Coleman-Liau Index is applicable to all types of writing. This is because other reading level indexes use more complicated methods by counting syllables, which makes it more difficult to calculate with a computer and difficult to do without a computer. Try the Coleman-Liau and see how easy and reliable it is.

Jonathan Gillham

Founder / CEO of Originality.AI 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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