BMI Calculator

The one thing actually worth knowing about BMI is that it can't tell muscle from fat. That's most of the story right there. It's your weight divided by your height squared, a shortcut a Belgian mathematician cooked up in the 1830s to describe whole populations, and it knows literally nothing else about you.

So here's what that breaks. Someone who lifts seriously can come out "obese" while walking around at ten percent body fat, because muscle is heavy and BMI reads all weight the same. Flip it around, and someone who's slowly lost muscle over the years can sit dead center in "normal" while carrying more fat than is good for them, even though the BMI reads the same in both cases. If you take one thing from this page, take that: BMI measures your weight, not your health, and those two aren't the same thing.

What it's genuinely good for is a fast, no-equipment gut check, and for watching your own number drift over months rather than reading too much into any single day. That trend line is the part people ignore and shouldn't: your BMI creeping up half a point a year is a real signal, and any single reading on its own tells you very little. What it's useless for is being the last word on any one person. If your result bugs you, a tape measure around your waist will tell you more than the scale does, because where you carry fat matters more than how much you weigh, and a doctor who can see you beats either measurement.

The muscle problem is worth sitting with for a second, because it's not a rare edge case. Anyone who trains, plays a sport, or just does physical work carries more muscle than the "average person" the formula was built around, and every pound of it pushes their BMI up while doing nothing but good for their health. That's the core flaw in one line: the formula treats the healthiest weight you can add exactly like the least healthy. A number that penalizes muscle this way says more about your mass than your health.

This tool gives you the number to one decimal, the category it lands in, and the healthy weight range for your height, in metric or imperial, with the conversions handled for you. Your height and weight never leave your browser, so there's no server out there holding your body stats. It's a screening number, not medical advice, definitely not a verdict.

1. Pick metric or imperial.
2. Type your height and weight. The number updates as you go.
3. Read your BMI, the category it lands in, and the healthy weight range for your height underneath.

There is a toggle on the calculator for a second set of thresholds, and it is worth saying plainly what that is, because a lot of sites carry it and very few say where it came from.

The categories almost everyone means by BMI are the WHO adult classification, which CDC publishes identically: under 18.5, 18.5 to 24.9, 25 to 29.9, and 30 up. WHO looked specifically at whether those lines sit in the right place for Asian populations, where the same body fat and the same metabolic risk tend to show up at a lower BMI, and in 2004 it kept 25 and 30 as the international classification. The reason was that the evidence did not point at one replacement number. The BMI at which risk started climbing landed anywhere from 22 to 25 depending on which population was studied, and the high-risk point ranged from 26 to 31.

What it published instead were two public health action points, at 23 and at 27.5, meaning the BMI at which screening and intervention should begin rather than a new diagnosis. The toggle uses those, under the risk names WHO gave them, and the page says so when you switch it on. Plenty of calculators present 23 and 27.5 as an Asian overweight and obesity cut-off, which reads as a stronger claim than the source makes. Treat a result on that toggle as a prompt to look at the rest of the picture, in the way a screening flag works.

If you want something better than BMI without a lab visit, measure your waist. Better still, divide your waist by your height. Keeping that ratio under about half, meaning your waist stays less than half your height, tracks health risk more reliably than BMI does, because it catches the belly fat that actually drives the risk, which BMI is completely blind to. It takes one tape measure and about ten seconds. For someone five foot ten, that line sits at a waist of about 35 inches, and if you're over it, that's worth more of your attention than whichever category the BMI happened to drop you into.

Body-fat percentage is better again if you have a way to measure it. BMI's only real edge over any of these is that you can do it with two numbers you already know, which is exactly why it's everywhere and also exactly why it's so limited. Free and instant comes at a price, and the price is that it's rough.

None of that makes the number worthless. It makes it a flag, not a diagnosis. A flag's whole job is to say "maybe look closer here," and that's it. Let it do that much and nothing more.