A rough but universal health benchmark
Body Mass Index is your weight divided by the square of your height. That's the entire formula — no lab work, no scan, just two numbers. It was designed in the 1830s by a Belgian mathematician, not a doctor, to describe population averages, not individual health.
These ranges come from population health data and are the ones most doctors and public health bodies still reference, even with the tool's known flaws.
BMI can't tell the difference between muscle and fat. A powerlifter and someone with very little muscle mass can land on the exact same number and be in completely different health situations. It also doesn't account for where fat is carried, bone density, age, or sex — all of which affect what a "healthy" weight actually looks like for a given person.
It's especially unreliable for athletes, older adults, pregnant women, and anyone outside the general adult population it was built around.
As a fast, free screening number — nothing more. It's a starting point for a conversation, not an endpoint. If your number surprises you, that's a reason to talk to a doctor, not a verdict on its own.