The Coleman-Liau index grades text without counting a single syllable. It uses letters per 100 words and sentences per 100 words instead, which makes it fully deterministic — there is no heuristic to get wrong. The passage loaded below scores 18.5.
The formula is 0.0588 × L − 0.296 × S − 15.8, where L is the average number of letters per 100 words and S the average number of sentences per 100 words. Coleman and Liau published it in 1975 specifically so that scoring could be automated by the character-counting hardware of the day.
The sentence term is negative, which surprises people. More sentences per 100 words means shorter sentences, which lowers the grade. The two terms therefore pull in opposite directions exactly as they should: long words up, short sentences down.
Below the calculator sits the l, S and the resulting grade table: 4 rows — Plain narrative at one end, Formal policy prose at the other — which is usually faster than entering a value you have seen before. It loads pre-filled, with your text at The report finds that pupils from lower-income households achieve weaker examination results at sixteen. Regional differences in provision explain only part of the variation observed., and the answer updates on every keystroke after that. Alongside the number, 4 bands label it — elementary through college and beyond — because a bare figure rarely tells you whether it is unusual. Automated Readability Index Calculator and SMOG Readability Calculator are the nearest neighbours to this one.
The formula
CLI- US grade level
L- Letters per 100 words
S- Sentences per 100 words
15.8- Fitted constant from the 1975 paper
How to use this calculator
- Paste your text into the box.
- The gauge shows the Coleman-Liau grade.
- Read L and S below to see the raw inputs.
- Compare against a syllable-based index for a second opinion.
Worked examples
The loaded research summary
It has 156 letters and digits across 25 words, so L = 624.0, and 2 sentences gives S = 8.00. The grade is 0.0588 × 624.0 − 0.296 × 8.00 − 15.8 = 18.5.
Where it disagrees with Flesch-Kincaid
On the plain narrative sample Coleman-Liau returns -0.6 while Flesch-Kincaid returns -0.3. Letter-based measures treat short multi-syllable words such as "area" as easy and long single-syllable words such as "strength" as hard, which is the opposite of what syllable counting does.
Checking the Research summary row
The reference table lists Research summary, giving Letters per 100 words: 624.0, Sentences per 100 words: 8.00, Grade: 18.5. Entering that above should reproduce the same figures — if it does not, the input is being read differently to the way the table assumes.
How to read your score
Frequently asked questions
Why avoid syllables?
Because counting them by machine is unreliable. Letters can be counted exactly, so Coleman-Liau gives the same answer in every implementation, which is a real advantage when comparing documents or tracking a score over time.
What counts as a letter here?
Letters and digits, with spaces and punctuation excluded. Including digits matters for technical text where figures carry meaning.
Why is the sentence coefficient negative?
S is sentences per 100 words, so a high S means short sentences. Short sentences make text easier, so the term subtracts from the grade. It is the same relationship as every other formula, expressed the other way round.
Which index should I trust?
Run two or three and look at the spread. If Coleman-Liau, ARI and Flesch-Kincaid all land within a grade of each other the estimate is solid. A wide spread usually means unusual vocabulary or very few sentences.
What do the bands under the Coleman-Liau grade mean?
They label the figure so it can be read without a reference point of your own: Elementary from 0 to 6; Middle school from 6 to 9; High school from 9 to 13; College and beyond from 13 upwards. The band is a reading aid only — the number itself is exact, and a value sitting just over a boundary is not meaningfully different from one just under it.
What formula does the Coleman-Liau Index Calculator use?
It computes CLI = 0.0588 × L − 0.296 × S − 15.8, where CLI is US grade level; L is letters per 100 words; S is sentences per 100 words; 15.8 is fitted constant from the 1975 paper. The same expression is printed on the page above the inputs, so the result can be reproduced by hand or in a spreadsheet.
Coleman-Liau inputs and grades for the samples
| Passage | Letters per 100 words | Sentences per 100 words | Grade |
|---|---|---|---|
| Plain narrative | 314.8 | 11.11 | -0.6 |
| Business copy | 474.1 | 7.41 | 9.9 |
| Research summary | 624.0 | 8.00 | 18.5 |
| Formal policy prose | 773.3 | 6.67 | 27.7 |
Letters counts include digits and exclude spaces and punctuation.