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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.

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.

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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