The Coleman-Liau Index formula
CLI = 0.0588 × L − 0.296 × S − 15.8 (L = letters per 100 words, S = sentences per 100 words)- L: average letters per 100 words
- S: average sentences per 100 words
What it counts
Letters (A–Z, not digits or punctuation), words and sentences. More letters per word raise the grade; more sentences per 100 words lower it.
What score to aim for
Like other grade formulas, grade 7–9 suits most public web content.
Limits to keep in mind
Letter counts penalise long-but-simple words (“understanding”) and forgive short-but-rare ones. Numbers and symbols add words without letters, which can pull the score down in data-heavy text.
Where it comes from
Meri Coleman and T. L. Liau designed it in 1975 for machine scoring: counting letters was far easier for early computers and scanners than counting syllables.
Source: Coleman, M., & Liau, T. L. (1975). “A computer readability formula designed for machine scoring.” Journal of Applied Psychology, 60(2), 283–284.
How we count syllables, sentences and words, and how our numbers compare on test passages, is documented in Readability formulas explained.