The Coleman Liau Index estimates the US school grade needed to understand a text, using only letters, words, and sentences. Unlike syllable-based formulas, it needs no pronunciation judgement, which makes it easier to compute by hand.
How it works
The formula is CLI = 0.0588 x L – 0.296 x S – 15.8, where L is the average number of letters per 100 words and S is the average number of sentences per 100 words. Letters means letters only, not spaces or punctuation. The result rounds to the nearest whole number and reads as a grade level. Scores below 1 are reported as very easy, and scores above 16 are college level and beyond.
Step by step
- Pick a sample of at least 100 words, ending at a sentence boundary.
- Count words, sentences, and letters in the sample.
- Compute L: letters / words x 100.
- Compute S: sentences / words x 100.
- Apply the formula: 0.0588 x L – 0.296 x S – 15.8.
- Round to the nearest whole number for the grade level.
Worked example
A sample has 150 words, 10 sentences, and 700 letters. L = 700 / 150 x 100 = 466.7. S = 10 / 150 x 100 = 6.67. CLI = 0.0588 x 466.7 – 0.296 x 6.67 – 15.8 = 27.44 – 1.97 – 15.8 = 9.67, which rounds to 10, about tenth-grade level. A sample with shorter words, say 620 letters for the same 150 words, would score about 7, showing how word length drives this index.
Common mistakes
- Counting spaces as letters, which inflates L and the score.
- Using a sample far below 100 words, where one sentence swings S wildly.
- Forgetting to scale L and S to per-100-words before applying the formula.
- Treating the grade as exact rather than an estimate that varies by tool and sample.
Try it with the free tool
You can follow every step above by hand, but the free tool below does the same calculation instantly. Try your own numbers in it, then check one result by hand so you trust the method as well as the answer. Open the full tool here: open the tool.
Checklist before you finish
- Write down every input number first, with its unit, before you calculate anything.
- Work through the steps in order, and do not skip the unit conversions.
- Check the answer against the worked example above: is it the right size, and does the direction make sense?
- Round only at the end, and keep the unrounded figure for any follow-up calculation.
Practice with your own numbers
Pick three cases of your own and work each one by hand. First, try a simple round-number case you can check mentally. Second, try a realistic case with awkward numbers, the kind you will actually meet. Third, try an edge case, such as a very small or very large input, and confirm the answer still behaves sensibly. If any case gives a surprising result, re-check the units and the formula step by step before you trust it.
If your answer looks wrong
Most errors come from a small set of causes. Check that you used the same units throughout, that you divided where the formula divides and multiplied where it multiplies, and that you applied any percentage as a decimal. Check the order of operations: powers and brackets come before multiplication and division, which come before addition and subtraction. Finally, test the method on the worked example in this guide. If your method reproduces the example answer, the method is right and the inputs deserve a second look.
Quick tips
Take two or three samples from different parts of a long text and average the scores. If your audience needs plain language, aim for a score around 8 or below, and shorten words and sentences together.