Flesch-Kincaid
The most widely used readability formula. Combines average sentence length and average syllables per word into either a 0–100 Reading Ease score or a US school grade level. Developed in 1975 by Rudolf Flesch and J. Peter Kincaid for the US Navy.
Flesch-Kincaid is a pair of readability formulas developed in 1975 by Rudolf Flesch and J. Peter Kincaid at the U.S. Navy. The pair includes the Flesch Reading Ease score (0–100, higher is easier) and the Flesch-Kincaid Grade Level (a US school grade, e.g. "8.0" means an 8th-grader can read it). Both formulas combine the same two inputs — average sentence length and average syllables per word — and are derived from the original Flesch Reading Ease score Flesch published in 1948 for the publishing industry.
Flesch-Kincaid became the default readability check inside Microsoft Word, Google Docs, WordPress, and most content management systems because the formulas are simple, fast, and easy to validate. Yoast, Hemingway Editor, Grammarly, and every other major writing tool report at least one of the two Flesch-Kincaid metrics. Ahrefs' 2025 AI search study found a small but positive correlation (0.115 for ChatGPT, 0.113 for Perplexity) between Flesch Reading Ease and being cited in AI-generated answers, which has made the formula more useful in the AI search era.
The formulas are calibrated for English and were validated against comprehension data on adult readers — not children. That means the "grade level" output is best read as "an average US adult can read this at this grade level," not "this is the grade where students learn to read it." The grade number works well for relative comparisons (is this harder or easier than my last post?) and worse as an absolute reading-age guarantee. Critically, Flesch-Kincaid does not measure coherence, factual accuracy, or whether the writing is actually engaging — only whether the sentences and words are short and simple.
For content marketing and SEO, the practical targets are: general-audience web content aims for Flesch Reading Ease 60–70 (FK grade 7–9), B2B / professional copy aims for FRE 30–60 (FK 9–13), and consumer / casual content aims for FRE 70–80 (FK 5–7). Numbers above 13 typically mean the audience is academic, legal, or specialist; numbers below 5 may feel oversimplified for an expert audience.