Body Mass Index is the most-cited and most-misunderstood number in health. It was never designed to diagnose individuals — it was built in the 1830s by a Belgian astronomer studying populations — yet today doctors, insurers, and gym apps use it as if it were a precise measure of personal health. This guide explains exactly what BMI is, why the formula works the way it does, where it succeeds, where it fails (athletes, older adults, people of certain ancestries), and what to use alongside it for a complete picture.