Readability Checker — Flesch Reading Ease & Grade Level

Readability formulas turn three things you can count — how long your sentences are, how many syllables your words carry, and how many letters they use — into a single number that estimates how hard the text is to read. This tool tokenises your text into sentences, words, letters, and syllables, then applies six formulas verbatim from the papers that defined them.

Sentences are split on ., !, and ? (and, in strict mode, ; and :). Decimal points like 3.14 and common abbreviations such as “Dr.” are masked first so they don't split a sentence by mistake. Words are runs of letters and digits. Syllables use a deterministic vowel-group heuristic: count contiguous vowel clusters, drop a silent trailing “e” and most silent “-ed”/“-es” endings, keep the sounded “-le”, and never count fewer than one per word. A “complex” (polysyllabic) word has three or more syllables.

With W = words, St = sentences, Sy = syllables, C = letters, and Cx = complex words:

• Flesch Reading Ease = 206.835 − 1.015·(W/St) − 84.6·(Sy/W)
• Flesch-Kincaid Grade = 0.39·(W/St) + 11.8·(Sy/W) − 15.59
• Gunning Fog = 0.4·[(W/St) + 100·(Cx/W)]
• SMOG = 1.0430·√(Cx·(30/St)) + 3.1291  (needs St ≥ 3)
• Coleman-Liau = 0.0588·(100C/W) − 0.296·(100St/W) − 15.8
• ARI = 4.71·(C/W) + 0.5·(W/St) − 21.43

Reading Ease runs from roughly 0 (very difficult) to 100 (very easy) and maps to the bands Flesch published in 1948. The five grade-level formulas each estimate a school year; the consensus tile reports their median, which is more stable than any single formula. Coleman-Liau and ARI are useful because they count letters instead of syllables, so they cross-check the syllable-based scores. As an internal check, the tool also recovers the words-per-sentence and syllables-per-word inputs from the two Flesch scores by solving their two-equation system and confirms they match the measured values — the readability equivalent of proving a sum two different ways.

• Flesch, R. (1948). A new readability yardstick. Journal of Applied Psychology 32(3).
• Kincaid, J.P. et al. (1975). Derivation of New Readability Formulas. Research Branch Report 8-75, US Navy.
• Gunning, R. (1952). The Technique of Clear Writing. McGraw-Hill — Fog Index.
• McLaughlin, G.H. (1969). SMOG grading — a new readability formula. Journal of Reading 12(8).
• Coleman, M. & Liau, T.L. (1975). A computer readability formula for machine scoring. Journal of Applied Psychology 60(2).
• Smith, E.A. & Senter, R.J. (1967). Automated Readability Index. AMRL-TR-66-220.

The formula coefficients on this page were last cross-checked against their original sources on 2026-06-05. Each is reproduced verbatim in the tool's data module, alongside hand-worked examples used to verify the implementation.