Readability Indices

It’s widely known that easy and enjoyable reading helps learning and comprehension. Most people prefer reading “plain English,” and tend to turn off when a passage is too difficult to read. So when we speak of a text’s readability score, or of a readability index, what we mean is: How easy is it for readers to understand?

Measuring readability is important for a number of reasons. For one, teachers need to know if their students are capable of writing at their grade level, or whether they need more schooling in a particular area. Readability scores also help teachers and school systems grade whether a textbook is right for their students.  

Writers also need readability scores - especially those who write for children and pre-teens. They’ve definitely helped me make sure I’m writing for the correct audience; plus, they’ve taught me how to increase or decrease my usage of complex sentences depending on my readership. Never write outside your audience!

PaperRater Premium-Only Module: Readability Scores

PaperRater is proud to offer a NEW series of twelve readability scores as part of our premium service. Just sign in, visit our proofreader, enter your text, and receive a full, side-by-side comparison of the most common readability indices.

Why do you offer twelve indices? Shouldn’t I use just one?

Each readability index uses different criteria to create a score. As you’ll see, some use input based on syllables, and others based on word length. Plus, the equations used are slightly different.

So we give our users a set of twelve to eliminate bias and to provide you a wider range of input. That way you can choose for yourself which ones you want to incorporate into your work.

Here’s a quick breakdown of each index we provide:

Automated Readability Index (ARI)

The ARI grades text based on a combination of word and sentence structure. Computers find it difficult to analyze syllables, so the ARI uses a formula based on the number of characters per word, although it’s debatable whether counting characters or syllables is more helpful.

Coleman-Liau Index

Creators Meri Coleman and T. L. Liau constructed this readability score for the Office of Education to standardize textbooks in the United States. Like the ARI, it operates on the assumption that characters per word is a better indicator of readability than syllables. From Wikipedia: “L is the average number of letters per 100 words and S is the average number of sentences per 100 words.”

Dale-Chall Readability Formula

The Dale-Chall Readability Formula uses a different kind of input. Instead of using the number of characters in a word, it calculates the approximate grade level of a text by measuring “hard words.”

What exactly are hard words? The Dale-Chall list contains a list of about 3,000 words known by at least eighty percent of the children in the fifth grade. Words considered difficult are those not listed. The higher the score, the higher the text’s grade level.

Flesch Reading Ease
Unlike the first two indices, the Flesch Reading Ease calculates readability by the average sentence length and the average number of syllables per word. Text is rated on a scale from one to a hundred; the lower the score, the harder the text is to read. Plain English is set at 65, with the average word containing two syllables. The average sentence contains 15 to 20 words.

By the way, the above passage received a score of 73.7 on the Flesch Reading Ease, which means it’s slightly easier to read than plain English.

Flesch-Kincaid Grade Level

The Flesch-Kincaid Grade Level is a companion index to the Flesch Reading Ease, and uses the same inputs (average sentence length + average number of syllables per word). However, the measurements are weighted differently, and so it produces a score as an approximate grade level (e.g. 1-12, or higher).

Fry Readability Formula

Instead of taking a complete word count, the Fry Readability Formula randomly chooses three 100-word samples throughout the text. It then counts the average number of syllables and the average number of sentences per hundred words, and plots them onto a graph. The intersection of the two averages represents the appropriate reading level. It is used widely in the healthcare industries.

Gunning Fog Index

The Gunning Fog Index is calculated by the average length of a sentence and the percentage of complex words. Its inventor, Robert Gunning, complained that then-current writing was too complex (had too much fog) and needed to be simplified. Complex words in this case are described as having three or more syllables. Yet while complex words can be a good indicator of readability, it fails to account for the fact that words with three or more syllables are not necessarily difficult to comprehend.


LIX, or the Lasbarhetsindex Swedish Readability Formula, was specially designed for the readability of texts in foreign languages. Its formula uses (a) the number of words; (b) the number of periods; and (c) the number of long words (more than six letters). Multicultural teachers prefer its emphasis on long words and average sentence length to predict readability.

\text{LIX} = \frac{A}{B} + \frac{C \cdot 100}{A}

Linsear Write

Similar to the Flesch readability indices, Linsear Write helps calculate a text’s readability by sentence length and the number of “hard words” - words with three or more syllables. It was adopted by the U.S. Air Force to grade the readability of their flight manuals.

Raygor Estimate Graph

A simple readability estimate, the Raygor Estimate allows you to calculate an approximate grade level by taking 100 words from your text and counting the number of sentences. Then you count the number of words within your sample that contain six or more letters. Plot your points on the graph below, and you will receive an approximate U.S. grade level.


SMOG is a playful acronym for “Simple Measure of Gobbledygook,” and was developed as a replacement for the Gunning fog index. It is estimated by taking three 10-sentence samples from a piece of text, counting the words with three or more syllables, estimating the square root of the number of words, then adding three.

Spache Readability Formula

This formula was designed for third-grade texts or below, comparing the amount of “unfamiliar words” to the number of words per sentence. “Unfamiliar words,” in this case, are determined to be words that those in third grade or below do not understand. It is recommended to use the Space Readability Formula for those in third grade or below, and to use the Dale-Chall Readability Formula for those in fourth grade or above.

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