BMI Explained: Formula, Categories and Limits
Body mass index (BMI) compares your weight with your height. It is a quick screening number, not a diagnosis.
The formula
- Metric: BMI = weight in kg ÷ (height in meters)2
- US units: BMI = 703 × weight in pounds ÷ (height in inches)2
Example: 70 kg and 1.75 m gives 70 ÷ 3.0625 = 22.9. In US units, 154 lb at 68 in gives 703 × 154 ÷ 4,624 = 23.4.
Adult categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal weight |
| 25 to 29.9 | Overweight |
| 30 and above | Obesity |
What BMI cannot tell you
- It does not separate muscle from fat, so very muscular people can score high.
- It does not show where fat is stored.
- Age, sex and ethnicity can change what a healthy range looks like.
- These categories are for adults, not children and teenagers.
Use BMI as one input and talk to a health professional about your own situation. This page is general information, not medical advice.