Readability measures the difficulty of a text. Modern readability formulas combine two main factors: sentence length (longer is harder) and word complexity (more syllables or characters per word is harder). Common formulas include Flesch Reading Ease, Flesch–Kincaid Grade Level, Gunning Fog, and SMOG.
Readability scores are heuristics, not measurements — they predict difficulty but don't account for jargon, prior knowledge, or layout.