Coleman-Liau vs Flesch-Kincaid: The Letter-Count Formula
The Coleman-Liau Index counts letters instead of syllables. See the formula with the right units, a worked example, and where it helps.
Most readability formulas count syllables. The Coleman-Liau Index counts letters. That one change removes the least reliable step in automated scoring.
Where it came from
Meri Coleman and T. L. Liau published the formula in 1975, in a paper titled "A computer readability formula designed for machine scoring." The abstract explains the reason. Existing programs relied on routines that "estimate number of syllables, usually by counting vowels." Their formula used two inputs a machine could count directly: "letters per 100 words and sentences per 100 words."
The formula, with the right units
The commonly published form is:
Grade = 0.0588 × L − 0.296 × S − 15.8
- L is the average number of letters per 100 words.
- S is the average number of sentences per 100 words.
The units matter. L and S are rates per 100 words, not raw counts. If you plug in the raw number of letters and sentences, the result comes out far too low.
A worked example
Take this sentence:
Our platform automates customer data aggregation and enrichment workflows for SaaS teams.
It has 12 words, 77 letters and 1 sentence.
- L = 77 ÷ 12 × 100 = 641.7
- S = 1 ÷ 12 × 100 = 8.33
- Grade = 0.0588 × 641.7 − 0.296 × 8.33 − 15.8 = 19.5
This site's calculator gives the same result. Flesch-Kincaid scores the same sentence at 12.7. Both say it is hard. They disagree on how hard, which is normal for a single sentence.
Why letters can be steadier than syllables
English spelling does not show syllables clearly, so software has to guess. Simple counters count groups of vowels. That method misses on some common words. It reads "every" and "business" as three syllables, where a dictionary gives two. Better counters, including the one on this site, add rules and a list of exceptions, but none is perfect.
A letter count has no such problem. "Business" has eight letters in every tool.
What letters do not fix
- Word and sentence counts still vary. Tools differ on hyphenated words, numbers and abbreviations. Our calculator counts "well-being" as one word. Some tools count two.
- It measures the same things. Coleman-Liau still reflects only word length and sentence length. It cannot tell whether the ideas are clear.
- Short text is unreliable. One long word can swing the score of a single sentence by several grades.
- It was built on English. Do not assume the grade means the same thing in another language.
Which tools show it
Microsoft Word reports two scores: Flesch Reading Ease and Flesch-Kincaid Grade Level. Both use syllables. This site's calculator shows Coleman-Liau next to five other formulas, so you can see where they agree.
How to use it
Use Coleman-Liau as a cross-check. If Flesch-Kincaid and Coleman-Liau are close, the syllable count is probably not distorting your score. If they are far apart, look at the words your tool may be miscounting. Our post on syllable counting errors covers the common cases, and the guide to all six formulas explains what each one measures.
How we checked
The history and the two inputs come from the paper's abstract. We could not read the full 1975 paper, so the three coefficients are given as they are commonly published, and as this site's calculator uses them. The example was scored on 9 October 2026.
Frequently asked questions
Why does the Coleman-Liau Index count letters instead of syllables?
It was built in 1975 for machine scoring. A machine can count letters exactly, but it has to estimate syllables. The authors used letters per 100 words and sentences per 100 words as the two inputs.
What is the Coleman-Liau formula?
The commonly published form is 0.0588 × L − 0.296 × S − 15.8. L is the average number of letters per 100 words. S is the average number of sentences per 100 words. The result is a US grade level.
Is Coleman-Liau more accurate than Flesch-Kincaid?
Not necessarily. It avoids syllable-counting errors, but it measures the same two things: word length and sentence length. The two formulas often differ by a few grades on the same text. Use one formula consistently when you compare drafts.