What is BMI?
Body Mass Index (BMI) estimates body fat from your weight relative to your height. It's a quick screening number used to flag whether your weight sits in a range linked to higher or lower health risk.
It's calculated the same way worldwide, which makes it useful for comparing across large populations. Below is the exact formula in both unit systems.
The formula
Metric
BMI =
weight (kg)height (m)²
Imperial
BMI = 703 ×
weight (lb)height (in)²
BMI categories
| Category | BMI range |
|---|---|
| Underweight | Below 18.5 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Moderately obese | 30.0 – 34.9 |
| Severely obese | 35.0 – 39.9 |
| Very severely obese | 40.0 and above |
What BMI doesn't tell you
- Muscle vs fat. Muscle is denser than fat, so athletes can read as "overweight" while being lean.
- Fat distribution. BMI ignores where fat sits — waist measurement is a better signal of metabolic risk.
- Age, sex & ethnicity. Healthy ranges differ across groups; BMI uses one cut-off for all adults.
- Children & pregnancy. Standard BMI doesn't apply — different charts are used.