A readability index that maps a text to a U.S. school grade level using two surface features: average sentence length and average word length in syllables. Developed by J. Peter Kincaid and colleagues for the U.S. Navy in 1975, originally to assess the readability of technical training manuals. It is the most widely deployed grade-mapped readability formula and the default in Microsoft Word, Google Docs, and most consumer readability checkers.
A score of 8.0 means a text is judged readable by an average U.S. eighth-grader. The formula is a sibling of Flesch's earlier Reading Ease score (1948); Kincaid kept Flesch's two predictors, recalibrated the coefficients, and shifted the output from a 0–100 ease scale to a grade-level scale that practitioners could read at a glance.
The strength is calibration discipline. Two short, well-defined inputs make the score reproducible across raters and tools, and the grade-level output communicates instantly to non-specialists.
The limits matter for ELT use. Average sentence length and syllable count are weak proxies for the things that actually make text hard for L2 readers: low-frequency vocabulary outside the AWL or CEFR band, dense nominalisation, anaphoric resolution across long distances, and rhetorical structures unfamiliar to the candidate's L1 academic culture. A text in highly technical English can score FKGL 9 because its sentences are short, while sitting well above C1 in topical demand. A narrative passage can score FKGL 12 because the author writes long sentences while remaining accessible to a B1 reader.
For test passage sourcing FKGL is a useful first filter and a poor final arbiter. Pair it with a discourse-aware tool such as Coh-Metrix and with human CEFR rating before locking a passage into a bank.