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Readability Statistics

Readability tests are a measure of the ease of comprehension of a written text. The results are obtained solely through the analysis of word and sentence characteristics of the subject text and do not have the benefit of an actual statistical survey of human readers.

Typical characteristics used in the analysis include number of words, number of syllables per word, sentence length, and number of sentences. The test may also compare words found within the subject text to a list of “common” words.

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New Dale Chall

The Dale Chall formula was created by Professor Edgar Dale and Jeanne S. Chall (who actually founded the Harvard Reading Lab), and focuses mainly on sentence length and uncommon, or difficult words. This leads to a final “grade” score that tells us which level of education and individual might be expected to have in order to fully comprehend the text.

Most readability formulas were actually developed with education in mind – for scholastic textbooks, where simple communication of key points is crucial.

But that doesn’t mean they’re only good for textbooks – readability itself is a universal concept, because people simply aren’t going to pay attention to, or feel involved in, writing that they find impenetrable.

Powers, Sumner, Kearl Write

This one was created by R. D. Powers, W. A. Sumner and B. E. Kearl in 1958, part of a journal named “A Recalculation of Four Adult Readability Formulas.”

It focuses on the number of words, average sentence length, and number of syllables.
The formula is as follows:

Reading age = 0.0778 (average sentence length) + 0.0455 (number of syllables) + 2.7971

The Powers, Sumner, Kearl Write formula is best for children’s books, and isn’t considered particularly useful for those above a reading age of 10.

Spache (Revised)

Similar to the Powers, Sumner, Kearl Write formula, the Spache test is also aimed at the younger set. It was published by George Spache in 1952, in The Elementary School Journal, and uses both a word list and sentence length to determine readability for children.

The actual formula is:

(0.121 x average sentence length) + (0.082 x percentage of unique unfamiliar words) + 0.659

The revised edition incorporates a different word list, and probably other tweaks, but like other readability tests it gives a final education-level grade that the writing would be suitable for.


The Coleman-Liau index test was designed by Meri Coleman and T.L. Liau, and, again, gives you an approximate US grade level for the text. Different to some other tests, this one actually looks at the number of characters in the text, rather than the syllables in words.

The thinking behind this one seems to be that it’s easier for machines to perform calculation based on raw numbers (characters) rather than needing to be able to figure out syllables in a block of text.

The formula is:

CLI = 0.0588L – 0.296S – 15.8

Where L is the average number of letters per 100 words, and S is the average number of sentences per 100 words.

How useful this one could be in general is up for debate.


Created by Harry McLaughlin, this 1969 formula means Simple Measure Of Gobbledygook.

To find the SMOG grade, the formula is:


Polysyllables are words of more than two syllables.

This formula is most widely recommended for use in healthcare materials.


A great “catch all” and one of the most widely-known readability tests, the Flesch-Kincaid (or F-K) readability test is another that offers a final reading grade level based on US education.

The original version, Flesch Reading Ease, was devised by Rudolph Flesch while he was working for the Associated Press, but the US Navy later adapted it into the Flesch-Kincaid score we use today.


AutoCrit provides results from both the original Flesch Reading Ease test (score out of 100) and the updated Flesch-Kincaid readability test (approximate grade level).

With the original Flesch Reading Ease test, the higher the score, the more accessible it is. Typically, it tends to go up to 100, but you can actually get phrases that go higher than that – such as “the cat sat on the mat” which I believe is around 116.

You can also convert the Flesch Reading Ease score a grade level using the following table:

 Score  School Level
 100 – 90  5th grade — Very easy to read.
 90 – 80  6th grade — Easy to read. Conversational English for consumers.
 80 – 70  7th grade — Fairly easy to read.
 70 – 60  8th & 9th grade — Plain English.
 60 – 50  10th to 12th grade — Fairly difficult to read.
 50 – 30  College level, difficult to read.
 30 – 10  College graduate, very difficult to read.
 10 – 0  Professional, extremely difficult to read.

The Flesch-Kincaid test is pretty much the most-used one today. If you have any experience in marketing, you’ve most likely met a BUNCH of people who are absolutely obsessed with it, trying to get their grade score as low as possible – like a badge of honor that their copy is easily read by just about anyone.


Automated Readability Index

Interestingly, this one was developed for real-time tracking of readability on electric typewriters, and like the Coleman-Liau test, it looks at the number of characters in text rather than the syllables per word.


You can convert the Automated Readability Index using the following table:


 Score  Age and Grade Level
 1  5-6 — Kindergarten
 2  6-7 — First/Second Grade
 3  7-9 — Third Grade
 4  9-10 — Fourth Grade
 5  10-11 — Fifth Grade
 6  11-12 — Sixth Grade
 7  12-13 — Seventh Grade
 8  13-14 — Eighth Grade
 9  14-15 — Ninth Grade
 10  15-16 — Tenth Grade
 11  16-17 — Eleventh Grade
 12  17-18 — Twelfth grade
 13  18-24 — College student
 14  24+ — Professor


Gunning Fog Index

Again, we’re getting grade scores here from the Gunning Fog index, which was developed by a consultant named Robert Gunning.

In his work, he helped organizations to clean up their writing and make it more accessible to wider markets, so he developed the test to more easily tell which pieces of writing needed work.

The formula for Gunning Fog is:

0.4 [(words/sentences) + 100 (complex words/words)]

Here complex words are defined as those containing three or more syllables.

You can see it’s a much more simple test than the likes of the Flesch-Kincaid, because you only have 0.4 and 100 in terms of the calculation numbers (ignoring what you put in for the word counts). The simplicity of this one is often praised, but it’s generally ignored in favor of the Flesch Kincaid.

Linsear Write

Linsear Write was actually developed by the US Air Force. Like the SMOG formula, this one is highly specialized – designed to gauge the complexity of air force technical manuals. The source of the idea traces back to a 1966 style manual called Gobbledygook Has Gotta Go, which pointed out the importance of shorter words.

Even when something is highly technical and susceptible to an overload of jargon, it still needs to be accessible!

Here, “easy” words (with two or fewer syllables) are given 1 point. “Hard words” are given 3 points. Divide the points by the number of sentences in a 100-word sample, and then adjust the provisional result r:

If r > 20, Lw = r / 2.
If r ≤ 20, Lw = r / 2 – 1.

Your result is, as with other tests, a required US grade level for reading.

Why should you care about Readability Statistics?

Well, according to the folks at, your book is accessible to 85% of the public if it has a readability grade of 8 or lower.

And if you improve your readability from a grade 12 level to a grade 5 level, you get an 83% increase in the number of people who will finish reading.

That’s a massive difference!

The bottom line

Readability statistics can be a useful but blunt tool. The resulting scores provided are only as relevant as the data and statistics used as input. Many of the tests are not applicable to the genre and audience and the resulting scores/grade levels can be misleading.

That being said, when used in the right context, these analytic tools are a quick and efficient way to determine how well your audience will comprehend your work.