How BMI is calculated
BMI = 703 × weight (lb) ÷ height (in)2
A person who weighs 70 kg and is 1.70 m tall has a BMI of 70 ÷ (1.70 × 1.70) = 24.2.
WHO categories for adults
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 and above | Obesity |
What BMI can and cannot tell you
BMI is quick and free, and across large populations it correlates with health risks. For an individual it is only a rough screen. It does not measure body fat directly, and it cannot tell muscle from fat, so muscular athletes can land in the overweight range without carrying excess fat. It also does not show where fat is stored, and waist size and body composition often say more about health than BMI alone.
Who should not rely on it
- Children and teenagers. This calculator is for adults aged 18 and over. For younger people, BMI is compared with growth charts by age and sex, so ask a pediatrician.
- Pregnant women, older adults with muscle loss, and people with unusually high muscle mass.
- Some ethnic groups. Some health bodies use lower BMI cut-offs for people of Asian descent because health risks can rise at lower BMI values.
Next steps
If your result surprises you, or if you are worried about your weight in either direction, talk to a doctor or registered dietitian. They can look at blood pressure, blood tests, waist measurements and family history, none of which BMI captures. This page is general information and not medical advice.