Flesch-Kincaid
A pair of widely used readability formulas — Reading Ease (0–100 score) and Grade Level — developed for the U.S. Navy in 1975.
Definition
The Flesch-Kincaid formulas are two related readability tests developed by Rudolf Flesch and J. Peter Kincaid for the U.S. Navy in 1975. They remain the most widely cited readability measures in SEO, education, and writing tools. The two formulas produce different outputs but use the same core variables: average sentence length (ASL) and average number of syllables per word (ASW).
Flesch Reading Ease (FRE) produces a score from 0 (very difficult) to 100 (very easy). Scores of 60–70 are considered standard (plain English, easily understood by 8th–9th grade readers). Scores below 30 indicate academic or very technical writing. The formula: 206.835 − (1.015 × ASL) − (84.6 × ASW).
Flesch-Kincaid Grade Level (FKGL) converts the same variables into an approximate U.S. school grade level. A score of 8.0 means the text is readable by an average 8th grader. The formula: (0.39 × ASL) + (11.8 × ASW) − 15.59. Both measures respond primarily to two levers that writers directly control: sentence length (shorter sentences → better score) and vocabulary complexity (fewer multi-syllable words → better score). Use the pair together: FKGL tells you the grade level equivalent, while FRE gives an absolute ease rating that is useful for cross-industry comparison.