BMI Calculator

The formula, and where it came from

BMI is weight / height², with weight in kilograms and height in metres. A 70 kg person who is 1.75 m tall has a BMI of 70 / (1.75 × 1.75) = 22.86. The formula was published in 1832 by the Belgian astronomer and statistician Adolphe Quetelet, who was trying to describe the body-shape distribution of a population, not diagnose individuals. For 140 years it went by the name Quetelet Index. It was renamed BMI in 1972 by Ancel Keys, who found it correlated reasonably well with skin-fold measurements of body fat and was much easier to take at scale.

The WHO categories (and their caveats)

• Under 18.5 — underweight
• 18.5–24.9 — healthy range
• 25.0–29.9 — overweight
• 30.0–34.9 — obese class I
• 35.0–39.9 — obese class II
• 40+ — obese class III

These cut-offs came from large observational studies of mostly-white European adult populations and all-cause mortality risk. The WHO's own expert consultation recommended lower thresholds for South and East Asian populations — 23 for overweight and 27.5 for obese — because the same BMI tends to correspond to more visceral fat in these groups. Japan's official cut-off is 25 for obesity. If you're of Asian descent, the standard numbers overestimate the healthy ceiling.

Why BMI is a blunt instrument

BMI cannot tell muscle from fat. Every rugby prop forward, Olympic weightlifter, and bodybuilder is "obese" by the table. Dwayne Johnson has a BMI around 34. It also cannot see fat distribution: someone who carries weight around the hips has a very different cardiovascular risk profile from someone who carries it around the waist, even at identical BMIs. Ageing bodies lose muscle mass and gain fat without the scale moving — "sarcopenic obesity" is invisible to BMI but matters clinically.

Better screening measures if you have the time and the tape: waist-to-height ratio (keep it under 0.5), waist-to-hip ratio (under 0.9 for men, 0.85 for women per WHO), and body fat percentage measured by DEXA or a good bioelectrical-impedance scale.

When BMI is still useful

For average-build adults between 20 and 65, BMI correlates well enough with body fat to be a reasonable first pass. It's cheap, it's fast, and every doctor in every country knows it. Insurers use it. Clinical trials use it. If you just want a sanity check on whether your weight is drifting into a range associated with higher health risk, BMI is fine — just don't treat a single reading as a verdict, especially if you're athletic, pregnant, under 20, over 65, or not of European descent.

Imperial conversion

The calculator accepts feet/inches and pounds and converts internally to metric for the formula. If you want to do it by hand in imperial units, the equivalent is BMI = 703 × lb / in². The 703 is there to cancel the imperial-to-metric conversion factors; it's not a biological constant, just a unit tax.

Privacy

Your height and weight never leave your browser. There is no server call; the division happens in the page you're reading. Nothing is stored in localStorage either.