Body Mass Index (BMI)

Adolphe Quetelet, a Belgian astronomer turned statistician, proposed $m/h^2$ in 1832 while trying to describe l'homme moyen, the average man. He was building a statistical portrait of populations, and he said so. The ratio was renamed "body mass index" in 1972 by Ancel Keys, who tested it against several rivals and found it the least bad proxy for body fatness in large samples. Neither man proposed it as a measurement of a person.

That history matters because of what the number cannot see. BMI knows your mass and your height and nothing else, so it cannot distinguish muscle from fat, cannot tell where fat sits, and cannot account for bone density, ancestry or age. A lean, heavily muscled athlete and a sedentary person of the same height and mass get identical scores. The squared height also treats bodies as though they scale in two dimensions when they scale in roughly three, which nudges tall people upward and short people downward for no biological reason.

What it is good for is what it was built for: comparing groups, tracking a population over decades, and flagging a range worth a closer look. A BMI is a prompt for a conversation with a clinician, never a diagnosis on its own.