What this calculator does
The body roundness index models the body as an ellipse and asks how round that ellipse is. A tall person with a narrow waist approaches a thin ellipse and scores low; a shorter person with a wide waist approaches a circle and scores high.
It responds sharply to waist. At 175 cm, a waist of 70 cm gives 1.67 and a waist of 105 cm gives 5.43, more than three times as much for a 50 per cent larger measurement.
The formula
The waist circumference is converted to an equivalent radius and compared against half the height, giving the eccentricity of an ellipse. The result is rescaled so typical values land roughly between 1 and 16.
| Term | Meaning |
|---|---|
| Body roundness index | A geometric measure of body shape from waist and height. |
| Eccentricity | How far an ellipse departs from a circle, which the index is built on. |
| Visceral fat | Fat around the abdominal organs, which BRI is intended to track. |
The inputs explained
| Field | What to enter |
|---|---|
| Waist circumference (cm) | Waist circumference in centimetres, measured at the level of the navel. |
| Height (cm) | Height in centimetres. |
When to use it
Assessing shape rather than mass
BRI uses no weight at all, so it separates shape from overall size.
Tracking waist change over time
It responds to waist reduction even when weight is unchanged.
Comparing against BMI
Two people with identical BMI can have very different BRI values.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How sharply does waist move the index?
The same height at three waist measurements.
| Waist circumference | Body roundness index | Note |
|---|---|---|
| 70 cm | 1.67 | Higher BRI has been associated with higher visceral fat and cardiometabolic risk in population studies; it is not a diagnosis. |
| 85 cm | 3.09 | Higher BRI has been associated with higher visceral fat and cardiometabolic risk in population studies; it is not a diagnosis. |
| 105 cm | 5.43 | Higher BRI has been associated with higher visceral fat and cardiometabolic risk in population studies; it is not a diagnosis. |
Questions
What BRI value is high?
There is no universally agreed threshold. Studies typically work in population quintiles rather than fixed cut-offs, which limits how much an individual value means on its own. Higher has been associated with higher cardiometabolic risk in those studies.
Why does weight not appear?
Because the model describes shape rather than mass. It asks how round the body is at the waist relative to its height, and weight adds nothing to that particular question.
Is it better than BMI?
It captures something BMI cannot, namely where weight is carried, and several studies find it predicts cardiometabolic outcomes at least as well. It has nothing like BMI's accumulated reference data, which limits its practical use.
How should waist be measured?
At the level of the navel, standing relaxed, at the end of a normal breath out, with the tape snug but not compressing. The index is sensitive to waist, so an inconsistent measurement moves the result noticeably.
For waist adjusted for BMI instead, see the ABSI calculator. For a fat percentage estimate from the same measurement, see the relative fat mass calculator.