Readability Score
A numerical score, typically Flesch Reading Ease (0–100) or Flesch-Kincaid Grade Level (US school grade), that estimates how easy a piece of text is to understand based on sentence length and word complexity.
A readability score is a number that estimates how easy a piece of text is to understand. The most widely used scores are Flesch Reading Ease (0 to 100, higher = easier) and Flesch-Kincaid Grade Level (a US school grade, e.g. "8.0" means an 8th-grader can read it). All readability scores are derived from the same two inputs: average sentence length and average word complexity — usually syllable count, sometimes character count. The formulas have been peer-reviewed for 50+ years and validated against actual adult comprehension data, but they are statistical estimates, not comprehension tests.
The history goes back to 1948, when Rudolf Flesch published the original Reading Ease formula for the publishing industry; in 1975, J. Peter Kincaid and Flesch extended it with the Grade Level variant for the US Navy's technical manual program. Both formulas are still the global default — Microsoft Word, Google Docs, WordPress, Yoast, Hemingway, and Grammarly all report at least one of them. The five-formula "complete readability suite" (Flesch Reading Ease, Flesch-Kincaid Grade, Gunning Fog, SMOG, Coleman-Liau, plus the Automated Readability Index) covers every variant used in academic, publishing, and government contexts.
The target score depends entirely on the audience. The widely cited baseline is grade 7–9 for general-audience web content (Flesch Reading Ease 60–70), with consumer / casual content aiming for grade 5–7 (FRE 70–80) and B2B / professional content aiming for grade 9–13 (FRE 30–60). A 2021 Portent study of 5.8 million pages found the average page ranking in Google's top 30 scores 51–53 on the Flesch Reading Ease scale — equivalent to 10th–12th grade reading level — so the "plain English" target is more aspirational than typical.
Readability scores are not a Google ranking factor. John Mueller confirmed in a 2018 Webmaster Hangout that Google does not score pages on Flesch-Kincaid. But the indirect effects are real: low-scoring content (very difficult) gets abandoned more often, which hurts dwell time and bounce rate — two behavior signals Google does use. Ahrefs' 2025 AI search study found a positive correlation between Flesch Reading Ease and citations from ChatGPT (0.115) and Perplexity (0.113), so for AI search, clarity helps measurably. The right way to use a readability score is as a sanity check that your content matches your audience, not as a single "SEO score" to chase.
Common pitfalls: readability scores are unreliable for short text (less than 100 words gives noisy averages; less than 30 sentences makes SMOG especially unstable). The formulas are calibrated for English — non-English text gives misleading numbers. And "grade level" is the grade where an average US adult can read it, not the grade where students learn to read it. A 5th-grade Hemingway novel is normal for adult readers, not childish.