A readability index that maps a text to a 0–100 ease scale using two surface predictors: average sentence length and average syllables per word. Higher scores mean easier text. Published by Rudolph Flesch in 1948 in the Journal of Applied Psychology paper A New Readability Yardstick, and the parent of the more familiar Flesch-Kincaid Grade Level (Kincaid et al. 1975), which kept Flesch's two predictors and re-expressed them on a US-grade scale.
Flesch derived the coefficients by regressing the two predictors against the McCall-Crabbs Standard Test Lessons in Reading, the standard 1925 corpus of grade-calibrated reading passages. The constant 206.835 is the regression intercept; the negative coefficients ensure that longer sentences and longer words push the score down.
Flesch published a verbal description of each 10-point band, and these bands have stuck despite decades of revision attempts:
Scores can fall below 0 or rise above 100 in extreme cases. Reader's Digest sits around 65, the Harvard Law Review around 30, the King James Bible around 85.
The strength is communicative directness. A 0–100 ease score is intuitive in a way that a grade-level number never quite is, and Flesch's verbal bands give writers a target without requiring familiarity with US grade conventions. This is why FRE is the readability number embedded in Microsoft Word's spell-check pane and in most consumer writing tools.
The limits are the same as every surface-feature formula. FRE has no sensitivity to vocabulary frequency, anaphoric distance, topical demand, or rhetorical structure. A text with short sentences and short syllables can score 70 and still be hard for an L2 reader who lacks the schema the writer assumed; a long-sentence narrative can score 50 and remain accessible. For ELT use FRE is a coarse first filter, useful in combination with discourse-level measures and human CEFR judgement, never as a sole arbiter.
The two indices share the same predictors and the same regression base, so their rankings of texts are nearly identical. They disagree only in scaling: FRE is a continuous 0–100 ease scale, FKGL is a US-grade scale, and a text that is "easy" by Flesch's bands corresponds to a grade level by Kincaid's recalibration. Citing both on the same text is largely redundant. They drift apart most visibly on extreme texts: very long sentences with very short words, or the reverse.
Inter-tool variance on FRE typically runs 1–5 points across implementations on identical text — almost entirely from differences in syllable estimation. See Text Metric Implementation Variance for the mechanisms.