Body weight alone says little about health, so several indexes combine weight, height and circumferences. They are screening estimates, not diagnoses. The worked example below uses an adult 175 cm tall who weighs 70 kg, with an 85 cm waist and 98 cm hips. Compute them with the BMI Calculator, Waist-to-Height Ratio Calculator, Waist-to-Hip Ratio Calculator, Relative Fat Mass Calculator and Body Roundness Index Calculator.
The five indexes
| Index | Formula | Worked example |
|---|---|---|
| BMI | kg / m² |
70 / 1.75² = 22.9 |
| WHtR | waist / height |
85 / 175 = 0.486 |
| WHR | waist / hip |
85 / 98 = 0.867 |
| RFM | 64 − 20 × height / waist |
64 − 20 × 175 / 85 = 22.8 % |
| BRI | 364.2 − 365.5 × √(1 − ((waist / 2π)² / (0.5 × height)²)) |
3.09 |
What each one captures
- BMI uses only weight and height, so it cannot tell muscle from fat or show where fat is stored.
- Waist-to-height ratio reflects central fat; a common rule is to keep the waist under half your height (a ratio below 0.5).
- Waist-to-hip ratio compares abdominal and hip fat; the thresholds differ by sex.
- Relative fat mass (RFM) estimates body-fat percentage from height and waist alone. The formula differs by sex (here for a man; for a woman, add 12).
- Body roundness index (BRI) models the body as an ellipse and scores how round it is; higher means more central fat.
Limits and good practice
- Measure the waist the same way each time (midway between the lowest rib and the hip bone, after a normal exhale).
- None of these were built for children, pregnancy, or very muscular or very short people; norms vary between populations.
- Trends over time are more informative than a single number.
- See BMI (body mass index) and Waist-to-hip ratio for definitions and ranges.