Readability Score Calculator

The Flesch Reading Ease score compresses two surface features of text into a single number: average sentence length in words and average word length in syllables. The formula is 206.835 minus 1.015 times words per sentence minus 84.6 times syllables per word. The coefficients are regression weights, fitted against passages whose difficulty had been established by comprehension testing, not derived from theory. The syllable term carries roughly five times the weight of the sentence-length term at typical values, which reflects the finding that polysyllabic vocabulary predicts comprehension difficulty more strongly than sentence length does. Scores are calibrated so that 90 to 100 corresponds to material a fifth grader can read, 60 to 70 to standard eighth- to ninth-grade prose, 30 to 50 to college-level text, and below 30 to professional or academic writing. The companion Flesch-Kincaid Grade Level formula, 0.39 times words per sentence plus 11.8 times syllables per word minus 15.59, rescales the same two inputs onto a US school grade. Applied targets are well documented: general-audience web copy is usually aimed at 60 to 80, and the National Institutes of Health and other health agencies advise that patient-facing material sit at or below a sixth-grade level, which corresponds to a Flesch score of roughly 70 to 80. The limitations are severe and worth stating plainly. The formula cannot detect whether sentences are logically ordered, whether a term is defined before use, whether the passage is in the active voice, or whether it is factually coherent - text scrambled word by word scores identically to the original. Syllable counting is itself approximate, since automated counters apply vowel-cluster heuristics that misfire on names, acronyms, and irregular spellings. Short samples are unstable; at least 100 words, and preferably several hundred, are needed before the averages settle.

Readability measurement began in the 1920s American effort to grade schoolbooks objectively. Bertha Lively and Sidney Pressey published an early vocabulary-based index in 1923, and Mabel Vogel and Carleton Washburne followed in 1928 with the Winnetka formula, the first to use multiple regression against measured comprehension. Rudolf Flesch, an Austrian-born refugee who took a doctorate at Columbia University under the reading researcher Lyman Bryson, refined this line of work and published A New Readability Yardstick in the Journal of Applied Psychology in 1948, introducing the Reading Ease score in its modern two-variable form. The same year Edgar Dale and Jeanne Chall published their competing formula based on a list of familiar words, and Robert Gunning introduced the Fog Index for newspaper writing in 1952. The grade-level variant came out of military training research. J. Peter Kincaid and colleagues recalibrated Flesch's formula in 1975 under a US Navy contract, producing a grade-level scale intended to match enlisted personnel against the reading demands of technical manuals; the work was issued as a Navy research branch report and the formula was subsequently adopted across the Department of Defense. Flesch himself moved into public advocacy, and his readability test was written into law in several US states, which since the 1970s have required insurance policies to meet a minimum Flesch Reading Ease score before they may be sold. Inclusion of the Flesch and Flesch-Kincaid scores in Microsoft Word from the 1990s onward made them the most widely computed readability measures in existence, and the US Plain Writing Act of 2010 extended the same plain-language expectation to federal agency documents.