What BMI measures
Body mass index compares your weight with your height. It's quick, free and widely used as a first screening number. It does not measure body fat directly, and it can't tell muscle from fat.
BMI = weight (kg) ÷ height (m)²BMI = 703 × weight (lb) ÷ height (in)²
Adult BMI categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 and above | Obesity |
These are the standard categories used by the CDC and the World Health Organization for adults aged 20 and over.
Worked example
At 5 feet 9 inches (69 inches) and 165 pounds: 703 × 165 ÷ 69² = 115,995 ÷ 4,761 = 24.4, which is in the healthy range. For that height, a BMI of 18.5 to 24.9 corresponds to about 125 to 169 pounds.
Limits of BMI
- Muscle: athletes with a lot of muscle can have a high BMI and little body fat.
- Age and sex: older adults tend to carry more fat than younger adults at the same BMI, and women tend to carry more than men.
- Ancestry: some health bodies suggest lower cut-offs for people of Asian descent, because health risks can start at a lower BMI.
- Children and teens: BMI for ages 2 to 19 is compared against growth charts by age and sex, so these adult categories don't apply.
BMI is a starting point for a conversation with a doctor, not a diagnosis. This page is for general information and is not medical advice.