How BMI is calculated
Body mass index relates your weight to your height. It is the measure the World Health Organization uses to classify weight in adults.
For example, someone who weighs 160 lb and is 5 ft 9 in (69 in) tall has a BMI of 703 × 160 ÷ 69² = 23.6.
WHO BMI chart
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25 – 29.9 | Overweight |
| 30 – 34.9 | Obesity class I |
| 35 – 39.9 | Obesity class II |
| 40 or above | Obesity class III |
Limitations of BMI
BMI can't tell muscle from fat. A muscular person may show as overweight without excess fat, and an older adult with little muscle may show as healthy while carrying too much. It also doesn't apply to children, teens or pregnant women, who have their own charts.
For a fuller picture, pair it with waist size: over 40 in (102 cm) for men or 35 in (88 cm) for women signals higher cardiovascular risk.
Frequently asked questions
What is a healthy BMI?
The WHO considers 18.5 to 24.9 a healthy range. There's no single “perfect” value inside it; body composition and habits matter more.
Is BMI different for men and women?
No. The formula and WHO ranges are the same for adult men and women, although at the same BMI women usually carry a higher body-fat percentage.
Is BMI accurate for athletes?
Only partly. For very muscular people it overestimates fatness. Body-fat percentage or waist size is a better measure in that case.
Can I use this calculator for children?
No. Under 18, BMI is read with age- and sex-specific percentiles. Ask your pediatrician.
Results are estimates for general information only and are not a substitute for advice from a doctor, registered dietitian or qualified coach.