Type your height and weight above — kilograms and centimetres, or pounds with feet and inches — and your body mass index appears instantly with the WHO adult category it falls into. This BMI calculator is free, needs no signup, and runs entirely in your browser; the exact formula and official sources are shown lower on this page so you can check the math yourself.
How to use
- Pick your units — the metric/imperial toggle converts your entries both ways, so you can switch at any time without retyping.
- Enter your weight in kilograms or pounds.
- Enter your height in centimetres, or as feet plus inches.
The result updates as you type, and the address bar updates with it — copy the URL to save your numbers or compare readings over time.
What the number actually measures
BMI is a ratio, not a body scan: weight divided by height squared. The index dates back to the 19th-century statistician Adolphe Quetelet and was adopted for health screening in the 1970s, when researcher Ancel Keys found it the best of the simple height-weight formulas available. It was designed as a fast, equipment-free way to screen populations for weight ranges associated with higher health risks, and for that job it works well — it correlates reasonably with body-fat percentage across whole populations and costs nothing to measure. The WHO adult cut-offs are the ones this calculator shows: below 18.5 is underweight, 18.5–24.9 is the healthy range, 25–29.9 is overweight, and 30 or above is classed as obesity. The category label always matches the rounded number you see, so a computed 24.96 displays as 25.0 and is labelled overweight — no silent mismatch between the figure and the label.
Where BMI misleads — and who it misleads
Because it only sees mass, BMI cannot distinguish a kilogram of muscle from a kilogram of fat. Strength athletes and rugby players routinely land in the overweight band with low body fat; conversely, older adults who have lost muscle can sit in the healthy band while carrying more fat than the number suggests. Fat distribution matters too — abdominal fat carries more metabolic risk than the same weight on hips and thighs, and BMI is blind to the difference. Risk thresholds also vary between populations: a WHO expert consultation identified additional public-health action points at 23 and 27.5 for many Asian populations. If your BMI sits near a boundary, a waist measurement and a conversation with a clinician tell you far more than a second decimal place would.
If you are using BMI as a starting point for managing your weight, the natural next question is energy: our TDEE calculator estimates the calories you burn in a day, using the same show-the-formula approach as this page.
Limits
This tool implements the WHO adult classification and nothing more. It is not valid for children and teenagers (they use age- and sex-specific percentiles), during pregnancy, or as a diagnostic instrument for any condition. BMI is one screening signal among many — blood pressure, waist circumference, fitness, and family history all shift the picture. Treat an out-of-range result as a prompt to talk to a healthcare professional, not as a verdict.
Everything here is computed on your device — browse all our calculators, which show their formulas and sources the same way, or our file-based tools, which are private in exactly the same way.