Dale Chall Readability Calculator

Dale-Chall: 0.1579·(difficult% ) + 0.0496·(words/sentences) [+ 3.6365 if > 5%]

When Edgar Dale and Jeanne Chall published the formula in 1948, they leaned on a list of ~3,000 words familiar to American 4th-graders. Anything that falls outside that list gets flagged as "difficult". A revised list in 1995 widened the coverage and brought the vocabulary up to date. The score itself is raw = 0.1579·PDW + 0.0496·ASL, where PDW is the percentage of difficult words and ASL is the average sentence length, and you tack on 3.6365 whenever PDW goes past 5%. Take 100 words, 5 sentences and 15 difficult ones: PDW=15, ASL=20, so 0.1579·15 + 0.0496·20 + 3.6365 ≈ 7.0, which lands at grades 9–10, or "fairly difficult". The brackets translate to grade levels this way: ≤ 4.9 is grade 4 or below, 5.0–5.9 is grades 5–6, 7.0–7.9 is grades 9–10, and 9.0+ reaches college.

Applications

It helps people pick primary-school textbooks and reading material that matches a grade, prepare easy-to-read editions and adapted classics for adult literacy programs, weigh up which titles a children's library should buy, check government text against plain-language rules (US Plain Writing Act), and screen news copy aimed at low-literacy audiences.

FAQ

Why a word list instead of syllables? How familiar a word is tells you more about whether someone will understand it than counting syllables does. A 4th-grader handles "elephant" (3 syllables) more easily than "zenith" (2). The list reflects how vocabulary is actually picked up.

What about inflections? If a word is on the list, its plural, past tense, -ing and -ly forms count as familiar too. Proper nouns, numerals and common abbreviations are usually treated as familiar as well.

Why is the +3.6365 constant added only above 5%? It comes out of the empirical regression. Below the 5% mark the formula reads texts as harder than they are, and the constant rescales the curve so easy texts don't end up with inflated grades.

Is there a Portuguese version? There's no official PT-BR Dale-Chall. The nearest things you'll find are the Flesch BR adaptation (Martins et al., 1996) and the Coh-Metrix-Port readability suite.