BMI Calculator

Body Mass Index is a single number that relates your weight to your height. Doctors and public-health bodies use it as a fast, cheap screening step — not a diagnosis, but a way to flag whether someone's weight is worth a closer look. This calculator gives you the number the instant you type, tells you which WHO category it falls in, and works out the weight range that would put you in the "normal" band for your height.

The formula

BMI is weight divided by height squared. The two unit systems use the same idea with different constants:

• Metric: BMI = weight(kg) ÷ height(m)²
• Imperial: BMI = 703 × weight(lb) ÷ height(in)²

The 703 in the imperial version isn't arbitrary — it's the conversion factor that makes pounds-and-inches produce the same number metric would. Pick your units at the top; the tool handles the conversion, so you never have to translate cm to metres or feet to inches yourself.

WHO categories (adults)

BMI	Category
below 18.5	Underweight
18.5 – 24.9	Normal
25 – 29.9	Overweight
30 and above	Obese

Alongside your result, the tool shows your healthy weight range: the low and high weights that keep BMI between 18.5 and 24.9 at your current height. That's often the more useful number in practice — instead of an abstract score, it's a concrete target like "for your height, 58–78 kg is the normal band."

Reading the number honestly

BMI's biggest weakness is that it only knows your weight and height — it has no idea what that weight is made of. Muscle is denser than fat, so a lean, heavily trained athlete can post a BMI of 27 and land in "overweight" while carrying very little body fat. The reverse happens too: someone can sit in the "normal" band while carrying excess fat and little muscle. Treat a borderline result as a prompt to look at other signals — waist measurement, body-fat percentage, how your clothes fit, blood work — rather than a verdict.

BMI is also calibrated for adults only. It should not be applied to children or teenagers (who are assessed against age-and-sex growth percentiles instead), and it isn't meaningful during pregnancy. Population differences matter as well: some health authorities use lower overweight thresholds for people of South and East Asian descent, because cardiometabolic risk can rise at a lower BMI.

A quick sanity check

Because the result updates live, it's a handy way to see how sensitive the number is. Change your height by a couple of centimetres and watch the BMI move noticeably — height is squared in the formula, so small measurement errors have an outsized effect. Measure without shoes and enter your real height rather than a rounded-up one for an accurate reading.

Everything is computed locally in your browser. Your height and weight are used only for the on-screen calculation — neither value is sent to a server or stored anywhere, and BMI, useful as it is, is a general guide, not a substitute for advice from a health professional.