What is the Body Roundness Index?
The Body Roundness Index is a body-shape metric that models the body as an ellipse and estimates how 'round' it is. Unlike BMI, which uses weight and height, BRI uses waist circumference and height, so it captures central fat - the type most closely associated with metabolic and cardiovascular risk.
BRI values typically range from about 1 (very lean) to 16 (very round), with most people falling between 3 and 8.
The BRI formula
BRI is derived from a geometric model of the body using the eccentricity of an ellipse. You don't need to calculate it by hand - the tool does it for you - but the formula is shown below for reference. Waist and height must be in the same units.
- BRI = 364.2 − 365.5 × √(1 − [ (waist ÷ 2π) ÷ (0.5 × height) ]² )
- Waist = waist circumference; height = standing height (same units, e.g. cm)
BRI ranges and what they mean
The approximate categories below are based on population research. A lower BRI indicates a leaner, less central-fat physique; a higher BRI indicates greater roundness and abdominal fat.
| BRI | Category |
|---|---|
| Below 3.4 | Very lean |
| 3.4–4.4 | Lean |
| 4.5–5.4 | Average |
| 5.5–6.9 | Above average |
| 7.0 and above | High |
BRI vs BMI
BMI is quick but can't distinguish fat from muscle or show where fat is stored, so it misclassifies many muscular or centrally-lean people. BRI addresses part of this by focusing on waist size relative to height, making it a better indicator of visceral fat. A large 2024 study found BRI was associated with mortality risk, fuelling interest in it as a complement to - not a replacement for - BMI.
How to lower your BRI
Because BRI is driven by waist circumference, reducing abdominal fat is the way to lower it. A modest, sustained calorie deficit, regular cardiovascular exercise, strength training to preserve muscle, adequate protein and good sleep all help shrink waist size over time. As with any single metric, use BRI alongside other measures rather than in isolation.
BRI vs waist-to-height ratio
BRI is closely related to the waist-to-height ratio, a simple metric with a memorable rule of thumb: keep your waist circumference under half your height. Both focus on central fat, but BRI applies a geometric model to turn the waist-to-height relationship into a single roundness score that's easy to track over time.
Compared with body fat percentage, BRI is far easier to measure - you only need a tape measure and your height, with no scales, calipers or scans. It won't tell you your exact fat mass, but as a quick, repeatable indicator of abdominal fat it's a practical complement to the numbers you already track.