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The Coleman–Liau Readability Formula (also known as The Coleman–Liau Index) is a readability assessment test by linguists Meri Coleman and T. L. Liau to calculate the usability of a text. Coleman said he created the formula as one way to help the U.S. Office of Education calibrate the readability of all textbooks for the public school system.
Similar to the Automated Readability Index, but unlike most other grade-level predictors, the Coleman–Liau relies on characters (alphabetical letters) instead of syllables per word. Instead of using syllable/word and sentence length indices, Coleman and Liau believed that computerized assessments understand characters more easily and accurately than counting syllables and sentence length.
The formula is as follows:
The Coleman-Liau Readability Formula
$$5.88 \times AWL + 29.6 \times \frac{n_{st}}{n_{w}} – 15.8$$
AWL = Average Word Length | nST = # of sentences | nW = # of words
AWL: Count the total number of (upper/lowecase) letters in the text, divide by the total number of words, and then multiply by 100.
nST: Count the total number of sentences in the text, divide by the total number of words, and then multiply by 100.
Rationale Behind the Coefficients
The coefficients in the formula (0.0588 and 0.296) were determined through empirical analysis—examining various texts and their associated readability levels. The constant of 15.8 at the end adjusts the score to align the result with U.S. grade levels. A score of 8.5 means a student in 8-9th grade can understand the text with little difficulty.
Meri Coleman and T. L. Liau developed this formula in 1975 to calculate (by computer) samples of hard-copy text, instead of manually hard-coding the text. Unlike syllable-based readability indicators, it does not require you to analyze the characters that create the words (such as syllable counts)—only their length in characters. According to Coleman, “There is no need to estimate syllables since word length in letters is a better predictor of readability than word length in syllables.”
Limitations of the Calculation
While the calculation is simple and easy to compute, it’s limited in these areas:
- Language Sensitivity: The formula assumes certain characteristics of English, such as word and sentence lengths. It’s difficult to apply it to non-English languages.
- Ignores Vocabulary Complexity: The Coleman-Liau Index doesn’t consider how complex or simple words are in a text; it focuses only on letter count.
- Sentence Complexity: The index doesn’t consider complex sentences. A text with simple sentences but with complex concepts may yield a misleading score.
As natural language processing (NLP) and artificial intelligence (AI) continue to advance, there may be opportunities to augment or refine the Coleman-Liau Index with more nuanced analysis.
Our Readability Scoring System lets you score your text using the Coleman-Liau Index.
