What BMI measures
Body mass index is your weight in kilograms divided by the square of your height in metres. A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. In imperial units the same value comes from weight in pounds times 703 divided by height in inches squared. The tool converts for you, so you can enter feet and inches or centimetres. BMI was devised in the 19th century as a simple population statistic, and it remains a quick screening number because it needs nothing more than a tape measure and scales.
The standard adult categories
- Below 18.5: underweight.
- 18.5 to 24.9: healthy weight range.
- 25.0 to 29.9: overweight.
- 30.0 and above: obesity (further divided into classes by health organisations).
These cut-offs are those used by the World Health Organization for adults. Some Asian populations are advised to use lower thresholds because health risks rise at lower BMI values, so local guidance may differ.
What BMI cannot tell you
BMI does not distinguish muscle from fat. A muscular athlete can land in the overweight range with very little body fat, while an older person who has lost muscle can have a "normal" BMI and still carry too much fat. It also ignores where fat is stored, and waist measurement is a useful complement. It is not designed for children and teenagers, who are assessed against age and sex charts, or for pregnant women. Treat it as a starting point for a conversation with a doctor, not as a diagnosis.
Healthy weight range
The result also shows the weights at your height that correspond to BMI 18.5 and 24.9. This is more useful than a single number because it shows a band rather than a target. If you are outside it, small, steady changes in diet and activity are generally recommended over rapid ones.
Privacy
The calculation runs in your browser, and your height and weight are neither stored nor sent anywhere. The information is general and not medical advice.