BMI, Explained: What It Actually Measures and Where It Falls Short
Where the BMI formula comes from, why it was never designed as an individual health diagnosis, and what it can and can't tell you.
Published March 3, 2026
BMI (Body Mass Index) is weight divided by height squared, a formula devised in the 1830s by a Belgian mathematician named Adolphe Quetelet, who was studying population averages, not individual health. That origin matters more than it sounds: a tool built to describe statistical patterns across large groups gets used today, constantly, to make judgments about individuals, which is a different job than the one it was built for.
What it's actually good at
At a population level, BMI correlates reasonably well with body fat percentage and with certain health risks, which is exactly why public health researchers still use it: it's cheap to calculate, doesn't require special equipment, and is good enough for spotting broad trends across thousands or millions of people.
Where it breaks down for an individual
BMI can't distinguish muscle from fat, both just add weight for a given height. A muscular athlete with low body fat can register as "overweight" or even "obese" by BMI alone, while someone with a normal BMI but very low muscle mass and high visceral fat (fat around organs, a real health risk) can register as perfectly fine.
It also doesn't account for where fat is distributed, which matters medically. Fat carried around the abdomen carries different health risks than fat carried elsewhere, and two people with identical BMI can have very different actual risk profiles because of it. Age, sex, and ethnicity all shift what a "healthy" BMI range even means, and the standard categories most calculators use are simplified defaults, not universally accurate cutoffs.
How to actually use a BMI number
Treat it as one input among several, not a verdict. It's a reasonable first-pass screening number, cheap and instant, worth checking the same way you'd check any quick estimate. For anything beyond a general sense of where you stand, a healthcare provider can look at actual body composition, family history, and other markers that BMI has no way of capturing on its own.