What a Readability Score Actually Measures, and What It Doesn't
Common readability formulas (like Flesch Reading Ease) calculate a score based primarily on sentence length and word length (or syllable count) — they measure surface-level structural complexity, not the actual clarity, accuracy, logical organization, or genuine comprehensibility of the writing's ideas, which is why a structurally simple sentence can still be confusing, and a longer sentence can still be genuinely clear.
Understanding exactly what these formulas count — and what they don't — is essential for using a readability score as a useful signal rather than mistaking it for a complete measure of writing quality.
What the formulas actually calculate
Most common readability formulas are built from just two structural inputs: average sentence length (words per sentence) and average word complexity (typically syllables per word) — combined through a specific formula into a single score or estimated reading grade level.
What this structural measure misses
These formulas have no way to evaluate whether an idea is logically organized, whether technical terms are properly explained, whether the writing is factually accurate, or whether a reader genuinely understands the content after reading it — a text can score as highly "readable" by these metrics while still being confusing due to poor organization or unclear reasoning, and vice versa.
Why the formulas are still genuinely useful despite this limitation
Sentence and word length are real, measurable proxies that do correlate reasonably well with actual reading difficulty in practice, especially for flagging text with unnecessarily long, complex sentences or overly technical vocabulary — the formulas are a useful, quick diagnostic signal, just not a complete or final measure of overall writing quality.