A Developer's Guide to Readability Formulas: Comparing All 8

Readability formulas have been around since the 1940s. The U.S. military commissioned them to make training manuals accessible. Insurance regulators adopted them to ensure policy language was understandable. Healthcare organizations use them to write patient-facing materials that people actually read. There are dozens of formulas, but 8 are widely used in software. They all try to answer the same question: how many years of education does someone need to understand this text? They disagree on how to answer it. This guide explains what each formula measures, how they differ, and when to use which one. All examples use textlens , which implements all 8 in a single zero-dependency package. Disclosure: I built textlens. The big picture Every readability formula is a proxy. None of them understand meaning. They count things — syllables, characters, sentence boundaries, word lists — and plug those counts into a regression equation that was calibrated against human comprehension tests decades