A BMI calculator that goes past a single number: it shows your body mass index, places it on a colour-coded category scale, and works out the healthy weight range for your exact height — so you can see not just where you are, but how far you are from the healthy band and in which direction. It accepts both metric (centimetres and kilograms) and imperial (feet/inches and stones/pounds), and every figure updates the moment you type.
How it works
Body mass index is a simple ratio of weight to height, designed to flag whether someone is likely under-, over- or healthily-weighted for their size. The formula is:
BMI = weight (kg) ÷ height (m)²
When you enter imperial units, the tool first converts them to SI: pounds become kilograms (1 lb = 0.45359237 kg) and inches become centimetres (1 in = 2.54 cm), with stones expanded as 14 lb each and feet as 12 in each. Because both unit systems collapse to the same metres-and-kilograms figures, you always get an identical BMI.
The result is matched against the WHO adult bands — underweight, healthy weight, overweight, and obese classes I–III — and a marker is dropped onto a gradient bar that runs from BMI 15 to 40. To turn the abstract number into something actionable, the tool also inverts the formula: it multiplies the healthy BMI bounds (18.5 and 25) by your height squared to give the weight range that would keep you in the green zone, then reports how many kilograms or pounds you are above or below it. Expand “Show the working” to see every intermediate step, including the squared height and the range maths.
Worked example
Take someone 175 cm tall weighing 72 kg. Height squared is 1.75 × 1.75 = 3.0625 m². BMI = 72 ÷ 3.0625 = 23.5, which lands in the healthy weight band. The healthy range for that height is 18.5 × 3.0625 to 25 × 3.0625 = 56.7 kg to 76.6 kg, so at 72 kg they sit comfortably inside it with about 4.6 kg of headroom before the overweight threshold.
Switch the same person to imperial — 5 ft 9 in and 11 st 5 lb — and the calculator returns the very same BMI of 23.5, with the healthy range shown as roughly 8 st 13 lb to 12 st 1 lb. The numbers never depend on which unit you prefer.
Formula note: BMI uses metric units internally; the kg/m² value is unitless across systems once converted. Categories follow WHO adult thresholds and are not adjusted for age, sex, ethnicity, muscle mass or frame size — treat the output as a screening signal, not a diagnosis.
Every calculation happens in your browser. Nothing you enter is uploaded or stored.