Body Roundness Index (BRI) Calculator

The Body Roundness Index (Thomas et al., 2013) models the body as an ellipse rather than relying on weight and height alone the way BMI does. Waist circumference is treated as the circumference of a circle (giving a radius of waist ÷ 2π), and half your height is treated as that ellipse's semi-major axis. The eccentricity of that ellipse — how far it is from a perfect circle — is e = √(1 − (waist ÷ 2π)² ÷ (height ÷ 2)²), which is then mapped onto the published BRI scale with BRI = 364.2 − 365.5 × e.

Because e is a ratio of two squared lengths, it's dimensionless: as long as waist and height are entered in the same unit (which the metric/imperial toggle on this page keeps consistent), the result doesn't change based on which unit you use — unlike formulas such as the U.S. Navy body fat method, whose coefficients are calibrated to a specific unit.

BRI was developed as a way to capture body shape and central fat distribution that BMI can't — two people with identical BMI can have very different waist-to-height proportions, and BRI is sensitive to exactly that difference. There is no single, universally standardized cutoff table for interpreting a BRI score (published cutoffs vary by study population, sex, and age), so this calculator reports the index itself rather than a risk category — treat a given value as most useful for tracking your own change over time.