Where the number comes from
BMI wasn't invented by a doctor. A Belgian astronomer and statistician named Adolphe Quetelet worked it out in the 1830s while trying to describe the "average man" for population studies. Weight divided by height squared turned out to sort large groups of people reasonably well, and in the 1970s researchers picked it up as a cheap screening measure. That history explains both its strength and its weakness: it was built to describe populations, and it got repurposed to judge individuals.
The rugby player problem
BMI knows two things about you: how tall you are and what you weigh. It cannot tell muscle from fat, and muscle is denser. A 5'10" rugby winger at 200 pounds of mostly muscle scores a BMI of 28.7, squarely "overweight," while carrying maybe 12% body fat. Meanwhile someone the same height at 150 pounds who never exercises can score a tidy 21.5 while carrying more fat than the athlete. The calculator isn't wrong; the categories just weren't built for either of those bodies.
The same blindness runs the other way with age. People lose muscle from their 50s onward, so an older adult's BMI can hold steady for decades while their body composition quietly shifts. A stable number isn't automatically a stable situation.
So what is it good for?
Screening, honestly. For most adults who aren't athletes, a BMI drifting from 24 toward 29 over a few years is telling you something real, and telling you for free, with no equipment beyond a scale. Doctors still start with it because it flags who might benefit from a closer look. Used that way, as a trend line and a conversation starter rather than a verdict, it earns its keep. Check yours on the BMI calculator; it takes ten seconds.
The tape measure often beats the scale
Health risk tracks less with total fat than with where fat sits. Fat around the middle, packed around the organs, is the kind linked to heart disease and type 2 diabetes. Which is why a plain tape measure around your waist at navel height is such a good check. Common guidance flags increased risk above about 40 inches (102 cm) for men and 35 inches (88 cm) for women. Another quick version: your waist should be less than half your height. A tape measure can't be fooled by muscle the way BMI can, and it catches the specific fat that matters most.
Numbers worth caring about more than either
Blood pressure. Resting heart rate. Whether you can climb three flights of stairs without stopping. How you sleep. Every one of those predicts more about your next twenty years than the second decimal of your BMI. The useful move is to check BMI and waist occasionally, notice the trend, and spend your actual effort on the things that move the trend: what you eat, how often you move, and how you sleep.
Obvious but necessary: this page is general information, not medical advice. If your numbers worry you, that's a conversation for a doctor who can see the whole picture, not a website that knows two facts about you.
Related: the BMI calculator for your number and healthy range, and the unit converter if you think in pounds but your doctor talks in kilograms.