What is BMI?
Body mass index (BMI) compares your weight with your height. It was designed to classify weight across large populations, and it is still widely used by doctors as a quick first screen.
BMI = weight (kg) ÷ height (m)²In imperial units: BMI = 703 × weight (lb) ÷ height (in)²
BMI categories for adults
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5–24.9 | Healthy weight |
| 25.0–29.9 | Overweight |
| 30.0–34.9 | Obesity class I |
| 35.0–39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
These are the World Health Organization categories for adults. For people of Asian descent, health risks tend to rise at lower BMIs, and some health authorities use lower cut-offs, such as 23 for overweight.
What BMI can and cannot tell you
BMI is quick, free and useful across large groups of people. For individuals, it has clear limits:
- It cannot tell muscle from fat. A muscular person and a person with high body fat can have the same BMI.
- It ignores where fat sits. Fat around the waist carries more health risk than fat elsewhere.
- It shifts with age. Older adults can lose muscle while their BMI stays the same.
If you lift weights, your BMI will often overstate your body fat. Use it alongside other measures rather than on its own.
Better measures to use alongside BMI
- Waist-to-height ratio: keep your waist measurement under half your height. It is simple and reflects fat around the organs.
- Body fat percentage: from a DEXA scan or a trained professional with calipers.
- Performance and health markers: blood pressure, blood tests, fitness and strength.
Research behind this calculator
- Rothman KJ (2008). BMI-related errors in the measurement of obesity. International Journal of Obesity.
- World Health Organization. A healthy lifestyle: WHO recommendations.
Last updated September 21, 2026.