Last updated: 2026-06-22

Body Roundness Index

The Body Roundness Index (BRI) describes body shape using just two measurements: waist circumference and height. It was introduced by Diana Thomas and colleagues in 2013 as a way to capture how round or how slim a torso is, on a scale that runs roughly from 1 for a very lean build to 16 for a very round one. Because it is driven by the waist rather than by total weight, it reflects central fat — the fat stored around the abdomen — and serves as a screening figure for people interested in body composition and cardiometabolic risk.

What the index represents

BRI treats the human body as an ellipse. Picture the silhouette as an oval: its long axis is set by height and its short axis by the girth of the waist. A slim person is a long, narrow ellipse; a person carrying abdominal weight is a rounder, fatter ellipse. The index is read directly from the eccentricity of that oval — a geometric measure of how far it departs from a circle.

The formula

Here $\mathrm{WC}$ is the waist circumference and $h$ is height, both in the same unit. The waist is divided by $2\pi$ because, if the cross-section through the waist were a circle, its radius would be the circumference over $2\pi$; that radius becomes the horizontal semi-axis of the ellipse. Half the height becomes the vertical semi-axis. The square-root term is the eccentricity, and the two constants in the formula rescale it onto the familiar 1-to-16 range.

Worked example

Consider an adult with a waist of 85 cm33 in and a height of 170 cm67 in. Working in metres:

A value near 3.4 sits at the lower end of the typical adult range, consistent with a modest waist relative to height.

Interpreting the number

There is no single clinical cut-off for BRI. The bands shown alongside the result are population quintiles: the middle of the range is typical, while the highest fifth has been linked in observational studies to greater visceral fat and cardiometabolic risk. Research comparing BRI with imaging has found it tracks visceral adipose tissue and total body fat percentage somewhat more closely than body mass index, which uses weight and cannot tell where fat is stored. For an individual, the trend across several measurements usually says more than one reading against population averages.

Limits

The index is a geometric approximation, not a body-fat measurement. It assumes the torso behaves like a smooth ellipse and takes no account of muscle, bone, frame, or the difference between subcutaneous and visceral fat. It is best used alongside other measures such as the Waist-to-Height Ratio Calculator and BMI Calculator, and any health decision should rest on a professional assessment rather than a single index.