Coleman–Liau Index: readability from letters, not syllables
Coleman–Liau estimates a school grade from letters per word and sentences per 100 words. Counting letters is exact, which makes the score steady from one tool to the next.
Most readability formulas need a syllable count, and syllables are where software stumbles. Meri Coleman and T. L. Liau took a different route in 1975. Their formula measures word length in letters. A computer can count letters perfectly, so it never has to guess where one syllable ends and the next begins.
The formula
Coleman–Liau = 0.0588 × L − 0.296 × S − 15.8
where:
- L is the average number of letters per 100 words
- S is the average number of sentences per 100 words
Both measures are scaled to 100 words, so you can work from any sample length. Divide letters by words and multiply by 100 to get L. Do the same with sentences to get S.
Notice the sign on S. More sentences per 100 words means shorter sentences, so S lowers the grade. Longer words raise L and raise the grade.
The weights tell you what matters most. One extra letter per word, on average, adds 100 letters per 100 words and so adds 5.88 grades. One extra sentence per 100 words takes off only 0.296 of a grade. Word length dominates this formula.
A worked example
Here is a stiff request and a plain one.
Before: Kindly forward your commentary at your earliest convenience. Every submission receives individual consideration.
The grade shown with each example is Flesch–Kincaid. The arithmetic below is Coleman–Liau, so expect different numbers.
Counts: 13 words, 98 letters, 2 sentences.
- L = 98 ÷ 13 × 100 ≈ 753.85
- S = 2 ÷ 13 × 100 ≈ 15.38
- 0.0588 × 753.85 ≈ 44.33, and 0.296 × 15.38 ≈ 4.55
- 44.33 − 4.55 − 15.8 ≈ 24.0
After: Please send us your feedback by Friday. We read every reply.
Counts: 11 words, 48 letters, 2 sentences.
- L = 48 ÷ 11 × 100 ≈ 436.36
- S = 2 ÷ 11 × 100 ≈ 18.18
- 0.0588 × 436.36 ≈ 25.66, and 0.296 × 18.18 ≈ 5.38
- 25.66 − 5.38 − 15.8 ≈ 4.5
The Before averages about 7.5 letters per word; the After, about 4.4. That gap accounts for most of the difference of nearly 20 grades. The After also adds a deadline, which the formula can’t see but the reader will appreciate.
As with any formula, two-sentence samples give exaggerated numbers. Score a few paragraphs for a figure you can use.
Why letters suit computers
Counting syllables in written English is guesswork. Silent e’s, vowel pairs, and endings like “-ed” all trip up rule-based counters, and even dictionaries disagree on words like “every” and “fire.” Counting syllables goes through the trouble spots.
Counting letters has no such problem. “Strengths” has nine letters, whatever your accent. Coleman and Liau designed their formula for machine scoring, and the letter count is what makes that work:
- It is exact. Every tool that counts letters the same way gets the same L.
- It is fast. No dictionary lookups, no pronunciation rules.
- It is consistent. Two tools that agree on what a word and a sentence are will produce the same score. That is rarely true of syllable-based formulas.
Where letters mislead
Letters and syllables usually rise together, but not always.
| Word | Letters | Syllables |
|---|---|---|
| strengths | 9 | 1 |
| through | 7 | 1 |
| idea | 4 | 3 |
| area | 4 | 3 |
| radio | 5 | 3 |
A text full of words like “through” and “thought” scores a little harder under Coleman–Liau than under a syllable formula. A text full of words like “idea” and “area” scores a little easier. Over a few hundred words these quirks mostly cancel out, but they explain why Coleman–Liau and Flesch–Kincaid rarely agree to the decimal.
Coleman–Liau also shares every blind spot of the other formulas. Short jargon (“API,” “KPI,” “SLA”) looks easy. Familiar long words (“everybody,” “television”) look hard. Meaning, order, and layout don’t register at all. Readability scores explained covers those limits.
Coleman–Liau and ARI
The Automated Readability Index uses the same idea: characters per word instead of syllables. The two differ in how they treat sentence length. ARI uses words per sentence and gives it noticeably more weight. Coleman–Liau uses sentences per 100 words and leans almost entirely on word length.
How to lower a Coleman–Liau grade
Since letters per word dominate, the most effective fix is plain vocabulary:
- Prefer the shorter of two words that mean the same thing: “send,” not “forward”; “help,” not “assistance”; “about,” not “approximately.” Our plain words list has many more pairs.
- Turn long abstract nouns back into verbs. “Consideration” becomes “consider.” See nominalizations.
- Cut padded phrases. “At your earliest convenience” is “soon,” or better, a date.
Splitting long sentences helps too, but less here than with other formulas. Do it for the reader, not for the score.
Try it on your own text
Scores
- Flesch Reading Ease–
Counts used
Hardest sentences
- Paste a few sentences to see which are hardest.