A statistician, not a doctor, devised the index
The formula now called body mass index traces back to Adolphe Quetelet, a Belgian mathematician and statistician. In the 1830s, he used measurements of height and weight to study patterns across populations. He called the measure the Quetelet Index; it was a tool for describing groups, not a test designed to diagnose an individual patient.
The calculation divides a person’s weight in kilograms by the square of their height in meters. Squaring height helps account for the way body weight tends to rise as people get taller, making comparisons across people of different heights more useful.
How a population measure entered medicine
The index’s purpose shifted over time. In 1972, American physiologist Ancel Keys and colleagues used the name “body mass index” in research comparing ways to relate weight to height. The convenient calculation offered clinicians and researchers a simple way to sort large groups into categories and track patterns without specialized equipment.
That convenience also explains its limits. BMI uses only height and weight; it cannot distinguish muscle from fat or show where fat is carried. It can flag a pattern that may merit attention, but it does not directly measure a person’s health. A formula built to summarize populations became a common clinical shorthand—useful for screening, but only one piece of an individual assessment.
