The Coleman-Liau Index is a readability formula developed by linguists Meri Coleman and T. L. Liau in 1975. It estimates the years of education needed to understand a text. Unlike Flesch-Kincaid or SMOG, Coleman-Liau uses letter count per 100 words and sentence count per 100 words—no syllable counting required.

The formula is: CLI = 0.0588 × L - 0.296 × S - 15.8, where L is letters per 100 words and S is sentences per 100 words. We count letters (alphanumeric characters), words, and sentences, then apply this formula. The result maps to U.S. grade levels from kindergarten through college.

A Coleman-Liau score of 6–8 corresponds to 6th–8th grade reading level, often ideal for general audiences. Scores of 9–12 indicate high school level, and 13+ indicates college level. Coleman-Liau typically produces lower grade estimates than ARI and Flesch-Kincaid for technical documents.

Coleman-Liau uses letters per 100 words and sentences per 100 words, while Flesch-Kincaid counts syllables and ARI uses characters per word with a different formula. Coleman-Liau often produces lower grade estimates than ARI for technical text. All three avoid syllable counting—Coleman-Liau and ARI use character/letter counts.

Use Coleman-Liau when you need a character-based readability measure that often produces more conservative (lower) grade estimates than ARI or Flesch-Kincaid. It works well for technical documentation and automated readability checks. No syllable analysis is needed.