Body Roundness Index (BRI) Calculator

178 cm tall with an 86 cm waist. The ellipse’s semi-major axis is a = 1.78 ÷ 2 = 0.89 m. Taking the waist as a circle, its radius is b = 0.86 ÷ 2π = 0.1369 m. Then (b ÷ a)² = 0.023651, the eccentricity is e = √(1 − 0.023651) = 0.98810, and BRI = 364.2 − 365.5 × 0.98810 = 3.05. At 75 kg the BMI is 23.7.

The same BMI with two different shapes. Two men are 180 cm and 81 kg, a BMI of exactly 25.0. One has an 80 cm waist: b = 0.1273 m, e = 0.98994, BRI = 2.38. The other has a 100 cm waist: b = 0.1592 m, e = 0.98424, BRI = 4.46. BMI cannot tell them apart; BRI can.

The common mistake: mixing units. A 34 in waist with a height entered as 165.1 cm gives b = 5.41 against a = 82.55, an eccentricity of 0.99785 and a BRI of −0.51, which is impossible (the lowest BRI Thomas and colleagues saw in US survey data was 1). With both in inches, 34 in and 65 in, the BRI is 3.80. Using the waist as a diameter instead of a circumference (b = 0.43 m) is the other slip; it gives a BRI of 44.2.

The body roundness index was introduced in 2013 by Diana Thomas of Montclair State University and colleagues including Steven Heymsfield, in the journal Obesity (21(11):2264–2271). They modelled the body as an ellipse, with height as the long axis and the waist (as a circle) giving the width, and borrowed eccentricity, the measure of how far an ellipse is from a circle that Johannes Kepler used for planetary orbits in 1609. Because human eccentricities all sit just below 1, they rescaled it to BRI = 364.2 − 365.5 × e, a range of roughly 1 to 20.

Using pooled data with body fat measured by DXA and visceral fat by MRI, they found BRI predicted percentage body fat and visceral fat slightly better than BMI, waist or hip circumference alone. Interest grew in 2024, when Zhang and colleagues followed 32,995 US adults from the National Health and Nutrition Examination Survey (JAMA Network Open 7(6):e2415051). Average BRI rose from 4.80 to 5.62 between 1999 and 2018, and the link with death from any cause was U-shaped: risk was 25% higher below 3.4 and 49% higher at 6.9 or above, compared with the middle group of 4.5 to 5.5.

That study shows an association in one population; it does not set a clinical cut-off. The simpler waist-to-height ratio carries the same shape information and does have published boundaries.

This calculator works out the body roundness index from your height and waist with the published Thomas (2013) formula, shows each step from the ellipse axes to the eccentricity, and draws your body ellipse to scale next to the BRI scale from the 2024 US study, so the number has some context. Add your weight to see BMI alongside it. Heights can be typed in centimetres, metres, inches or feet and inches, waists in centimetres or inches, and weights in kilograms, pounds or stones.

Everything runs in your browser; nothing you enter is sent anywhere. For general information only — not medical advice. Talk to a doctor or other qualified professional about your own health.

Related tools: Waist-to-Height Ratio Calculator, BMI Calculator, and Body Surface Area Calculator.