Coleman-Liau Index Calculator
The Coleman-Liau Index estimates the US school grade level needed to understand a piece of writing. Unlike Flesch-Kincaid, SMOG or Gunning Fog, it counts letters instead of syllables. That choice makes it easy for software to compute.
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What the Coleman-Liau Index measures
Meri Coleman and T. L. Liau published the formula in 1975 (“A Computer Readability Formula Designed for Machine Scoring”, Journal of Applied Psychology). As the title says, it was designed for machine scoring: a program can count letters and sentences directly, without estimating syllables.
The inputs are letters per 100 words and sentences per 100 words. The output is a US grade level.
How to interpret your Coleman-Liau score
| CLI | Label on this calculator |
|---|---|
| Up to 5 | Elementary |
| 6–8 | Middle school |
| 9–12 | High school |
| 13–16 | College |
| 17+ | Graduate |
Coleman-Liau and Flesch-Kincaid can differ by several grades on the same text, because one counts letters and the other counts syllables.
When to use the Coleman-Liau Index
- Automated scoring of many pages — it needs only letter, word and sentence counts.
- Text with unusual words or names, where estimated syllable counts are least certain.
- Cross-checking — a second reading next to a syllable-based score such as Flesch-Kincaid.
How the Coleman-Liau Index is calculated
S = (sentences / words) × 100 ← average sentences per 100 words
CLI = 0.0588 × L − 0.296 × S − 15.8
Two intermediate values: L is the average number of characters per 100 words, S is the average number of sentences per 100 words. Combine with the constants and the result is a US grade level.
Word length carries a lot of weight. Cutting average word length from 5 letters to 4 lowers the CLI by about 6 grade levels. Halving average sentence length from 20 words to 10 lowers it by about 1.5.
Coleman-Liau vs other readability formulas
ARI (Automated Readability Index) is Coleman-Liau's closest cousin: both count characters and give US grade levels. ARI gives more weight to sentence length, so text with long sentences and short words scores higher on ARI, and text with short sentences and long words scores higher on Coleman-Liau.
Compared with Flesch-Kincaid Grade Level, Coleman-Liau can differ by several grades on the same text.
SMOG and Gunning Fog count words of three or more syllables instead of letters.
Frequently asked questions
What is the Coleman-Liau Index?
A readability formula that estimates the US grade level needed to understand a text. It counts letters and sentences per 100 words instead of syllables.
What is a good Coleman-Liau score?
It depends on your readers. No standard sets a Coleman-Liau target. Lower scores suit a wider audience.
Why use Coleman-Liau instead of Flesch-Kincaid?
Coleman-Liau counts letters, which software can do exactly. Syllable counts are estimates and can be wrong for names and unusual words. The two formulas can differ by several grades on the same text, so compare like with like.
Who created the Coleman-Liau Index?
Meri Coleman and T. L. Liau published the formula in 1975 in the Journal of Applied Psychology. The paper's title says it was designed for machine scoring.
Does Coleman-Liau work for code or technical documentation?
It works on prose that contains technical terms, because it only counts letters, words and sentences. For code samples the score has no meaning.
How do I lower my Coleman-Liau score?
Use shorter words and shorter sentences. Cutting average word length from 5 letters to 4 lowers the score by about 6 grades. Halving sentence length from 20 words to 10 lowers it by about 1.5.