Coleman-Liau Index Calculator

The Coleman-Liau Index is a readability formula that estimates the U.S. school grade level needed to understand a passage of English. It was published by Meri Coleman and T. L. Liau in 1975. Unlike Flesch-Kincaid or SMOG, it relies on letter counts rather than syllable counts, which makes it easier to compute by machine. A result of 9.0 means the text reads at roughly a ninth-grade level.

The formula is CLI = 0.0588 times L minus 0.296 times S minus 15.8. Here L is the average number of letters per 100 words, calculated as letters divided by words times 100, and S is the average number of sentences per 100 words, calculated as sentences divided by words times 100. Longer words raise the grade, while shorter sentences (a higher S) lower it slightly.

Counting letters is unambiguous, while counting syllables needs a pronunciation rule or dictionary and can vary between tools. Coleman and Liau designed their formula for machine scoring, so they used the character count of words as a stand-in for word difficulty. This makes the index fast and consistent, though it can read very differently from syllable-based formulas on text with many short but uncommon words.

Both report an estimated U.S. grade level, but they measure word difficulty differently: Coleman-Liau uses letters per word, while Flesch-Kincaid uses syllables per word. On typical prose the two land within a grade or two of each other. They can diverge on text that is heavy in long-but-simple words or short-but-technical terms, so checking more than one formula gives a fuller picture.

Aim for at least a few full sentences, ideally 100 words or more. Very short samples make the averages jumpy, since a single long word or extra period can swing the grade. For a long document, scoring several representative passages and comparing the grades is more telling than scoring one short snippet.

Yes. The raw formula is unbounded, so very short, simple text can produce a negative grade and dense academic prose can exceed grade 16. This calculator shows the exact raw grade and also maps a clamped 0 to 16 value to a reading-level band, where 16 represents a college-graduate level. Treat the grade as an estimate of reading difficulty, not an exact measurement.