BMI Calculator

How to use the BMI calculator

Body Mass Index is a single number that compares your weight to your height. It's the screening measure the WHO, the NHS, and the US CDC use as a starting point for weight-related health conversations. It is not a body-fat measurement, and it doesn't distinguish fat from muscle — keep that in mind before reading anything dramatic into a single value.

1. Choose your unit system. Imperial uses feet/inches for height and pounds for weight; metric uses centimeters and kilograms.
2. Enter your height. For imperial, fill both the feet and inches fields — 5'10" is 5 feet and 10 inches, not 5.10.
3. Enter your weight in the unit the toggle is set to.
4. Read your BMI. The category label (underweight, normal, overweight, obese class I/II/III) and the color bar show where you sit on the WHO adult scale.

The formula

Metric: BMI = kg ÷ m². For imperial the tool converts first, then applies the same formula — equivalent to the 703 × lb / in² shortcut you may have seen. The WHO cutoffs are 18.5, 25, 30, 35, and 40.

Where BMI is a bad fit

BMI was designed for populations, not athletes. Muscular adults routinely register as "overweight" despite low body fat; older adults can sit in "normal" range while carrying abdominal fat that matters more. It also does not apply to children, pregnant women, or people under 20 — talk to a clinician for those cases. This tool is informational, not medical advice.