BRI Calculator — Body Roundness Index

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How this is calculated

BRI was introduced by Thomas et al. in 2013 as a geometric model of the human body. The torso is approximated as an ellipse whose semi-major axis c equals half the standing height (height / 2) and whose semi-minor axis a equals the waist circumference divided by 2π (the radius of a circle with the same circumference). The eccentricity of this ellipse — √(1 − (a/c)²) — measures how far the shape deviates from a circle (eccentricity 0) towards a line (eccentricity 1). BRI rescales that eccentricity so the resulting number rises with increasing roundness rather than decreasing.

The formula simplifies to BRI = 364.2 − 365.5 × √(1 − (WC / (π × height))²) with both measurements in metres. A perfectly cylindrical body (extreme roundness) would approach 364.2, while a very lean individual sits near 1. Most healthy adults score between about 1 and 7, with published research thresholds at 3.41 (below which body fat is low) and 6.91 (above which body fat is markedly elevated).

BRI adds information over BMI because it captures body shape — specifically abdominal girth relative to stature — rather than mass alone. Studies have found BRI correlates more strongly with visceral adipose tissue (VAT) and cardiometabolic risk markers than BMI. However, it still does not account for age, sex, muscle mass, or fat distribution below the waist, and population norms vary by ethnicity. Use it as a screening hint alongside, not instead of, clinical assessment.