Body mass index (BMI) is a calculation based on the height and weight of humans to determine if their weight is healthy. To calculate BMI use the following formula where weight (W) is in kilograms and height (H) is in meters:

BMI = W / H^{2}

For the average adult, the ideal BMI is somewhere in the range of 18.5 to 24.9. For children aged between 2 and 18, the body mass index takes into account gender and age.

According to the NHS BMI results are described in the following way:

- below 18.5 – you're in the underweight range
- between 18.5 and 24.9 – you're in the healthy weight range
- between 25 and 29.9 – you're in the overweight range
- between 30 and 39.9 – you're in the obese range

BMI should not be confused with BSA (Body Surface Area), which is a different measurement.