Coleman-Liau Readability Index Calculator
The Coleman-Liau Index (CLI), published by Meri Coleman and T. L. Liau in the Journal of Applied Psychology in 1975, is a readability formula that needs no syllable counting — it uses only letters per 100 words (L) and sentences per 100 words (S), and returns a US K-12 grade level. It is widely used by English writing teachers, ESL / TOEFL / IELTS students, automated text-analysis pipelines, SEO and copy auditors. Compared with Flesch-Kincaid Reading Ease, CLI is more robust on OCR text and scraped corpora because it does not need to syllabify words. The formula is calibrated for English; it does not apply to Chinese, Japanese, Korean or other non-alphabetic scripts.
Please enter English text with at least one word and one sentence-ending mark (./?/!).
Coleman-Liau Index (US grade level)
12
US grade scale: 1 = 1st grade, 12 = high-school senior, 13+ = college.
Audience
—
Total letters
0
Words
0
Sentences
0
Letters per 100 words (L)
—
Sentences per 100 words (S)
—
Formula
CLI = 0.0588 × L − 0.296 × S − 15.8 L = (letters / words) × 100, S = (sentences / words) × 100
The Coleman-Liau Index, published by Meri Coleman and T. L. Liau in 1975, uses only letter and sentence counts — no syllable splitting required. That makes it easier to automate (and more robust on OCR / scanned text) than Flesch-Kincaid. The output maps to the US K-12 grade scale; the formula is calibrated for English and does not apply to Chinese or other non-alphabetic scripts.
Formula
CLI = 0.0588 × L − 0.296 × S − 15.8 L = (letters / words) × 100 S = (sentences / words) × 100 The index maps to the US K-12 grade scale: 1 = 1st grade, 12 = high-school senior, 13+ = college.
- · Quick reference: simple children's books and primers score ≈ 1–5; mainstream newspapers (New York Times, SCMP) ≈ 8–11; Wikipedia main articles ≈ 10–14; academic papers and legal contracts ≈ 14–18+.
- · CLI is sensitive to long words (more letters per word) — swapping "use" for "utilise" instantly raises the grade level. It is also sensitive to long sentences — fusing two sentences raises the grade. So the canonical "plain English" moves are exactly: shorter words and shorter sentences.
- · Versus Flesch-Kincaid: CLI needs no syllable counting and is more robust to OCR / scraped text, but the two scores correlate at R² > 0.85 in practice and are usually interchangeable. Citing both lends credibility.
- · For text with no terminal punctuation at all (headlines, tweets, UI labels) the tool treats the whole passage as one sentence to keep the denominator non-zero; short samples (< 30 words) can swing widely, so sample ≥ 50 words for stable readings.
- · CLI captures structural complexity (word and sentence length) only — it does not measure conceptual abstraction, logical density or required background knowledge. A low CLI does not guarantee easy content, and vice versa.
- · References: Coleman M & Liau TL (1975), Journal of Applied Psychology 60: 283–284; DuBay WH (2007), The Classic Readability Studies; the textstat and readability-python open-source packages; US Plain Writing Act of 2010.
Frequently asked
Which is more accurate — Coleman-Liau or Flesch-Kincaid?
Both were calibrated on different reader populations in the 1970s, so neither is "absolutely" more accurate. Flesch-Kincaid (Kincaid et al. 1975, US Navy edition) used Navy training-manual data on adult enlisted readers; Coleman-Liau, published the same year, used college-student samples and a more general corpus. Empirically the two scores usually fall within ±1 grade of each other on the same passage (R² > 0.85). CLI's practical edge is automation: (1) no syllable dictionary; (2) more robust to OCR and unnormalised web scrapes; (3) very fast to compute. For rigorous analysis you should compute CLI plus Flesch-Kincaid, Gunning Fog and SMOG and look for agreement — when they agree the rating is reliable; when they disagree it tells you which structural feature (word length, sentence length, polysyllable density) is the outlier.
Does a high CLI mean the writing is "bad"?
No. A high CLI only means a higher reading level is needed to decode the text fluently — it does not judge quality. Legal contracts, academic papers and technical specifications (RFCs, IEEE standards) routinely score 14+ for a good reason: they trade readability for precision and unambiguity. "Good writing" depends on your audience: children's books should be ≤ 5; popular science ≈ 8; annual reports, policy white papers and news analysis ≈ 11; legal, scientific and technical writing ≥ 14. If you write a customer support email at CLI 16 you have overshot; if you write a PhD-peer paper at CLI 8 you have oversimplified. Readability formulas are alignment metrics, not quality verdicts.
Why does Chinese / Japanese text produce NaN or wildly high scores?
Because the Coleman-Liau formula is built around the Latin alphabet and whitespace-separated words. (1) Chinese, Japanese and Korean use CJK ideographs, which fall outside the Latin range — the tool counts almost no "letters". (2) Chinese is unsegmented — a whole sentence often collapses to one whitespace token, sending L to absurd values. (3) CJK sentence terminators are 、 。 ? rather than . ! ?. The result is NaN or wildly inflated scores. To measure Chinese readability, use a formula designed for Chinese (e.g. the Yang & Lin Chinese Readability Index, the CKIP character-frequency index, or the Tsinghua Chinese Readability Pool); do not transplant English formulas. This tool supports English only.
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