Body roundness index calculator
The body roundness index describes how round the midsection is, using only waist circumference and height. Enter both to get your BRI. Higher values mean a rounder body shape.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Pick metric (centimetres) or US units (inches, feet).
- Enter your waist circumference and your height.
- Read your body roundness index. The ellipse eccentricity and the waist-to-height ratio are shown for reference.
The index is from Thomas DM, Bredlau C, Bosy-Westphal A, et al., “Relationships between body roundness with body fat and visceral adipose tissue emerging from a new geometrical model”, Obesity 21(11), 2013, pp. 2264–2271:
BRI = 364.2 − 365.5 × √(1 − ((waist ÷ 2π)² ÷ (0.5 × height)²))
Waist and height must be in the same unit, so the result does not depend on whether you use centimetres or inches. The calculator converts inches to centimetres (1 in = 2.54 cm) before it applies the formula.
Worked example
Waist 80 cm, height 175 cm. The waist divided by 2π is 12.73 cm and half the height is 87.5 cm. The squared ratio is 0.02117, so 1 − 0.02117 = 0.97883 and its square root is 0.98936. Then BRI = 364.2 − 365.5 × 0.98936 = 2.59.
Good to know
BRI is a research measure based on a geometric model, not a diagnosis, and this page gives no health category for it. The waist measurement matters a great deal: a change of a few centimetres moves the result noticeably, so measure the same way each time. For a simpler ratio, see the waist-to-height tool.
Frequently asked questions
What is the body roundness index?
BRI is a number calculated from waist circumference and height. It was proposed by Thomas and colleagues in 2013, who modelled the body as an ellipse and used its eccentricity to describe how round or lean the body shape is.
What is a good BRI?
There is no official cut-off. A lower value means a leaner, narrower midsection and a higher value a rounder one. Studies compare groups of people rather than give a personal target, so use the number to track change over time and talk to a doctor about what it means for you.
How do I measure my waist for BRI?
Measure with a soft tape around the bare midsection, halfway between the lowest rib and the top of the hip bone, after a normal breath out. Keep the tape level and snug but not pressing into the skin.
How is BRI different from BMI?
BMI uses weight and height and cannot tell where the weight is carried. BRI uses waist and height, so it reflects the shape of the midsection. Neither number is a diagnosis.
Why do I get an error for a large waist?
The formula takes a square root of one minus the squared ratio of the waist radius to half the height. If the waist is so large that this goes below zero, no ellipse exists, which usually means a typing slip or mixed units.