How this instrument works
The Body Roundness Index (BRI) treats the body as a simple ellipse and asks how round that ellipse is, using only waist circumference and height. Thomas, Bredlau, Bosy-Westphal and colleagues introduced it in a 2013 paper in the journal Obesity, pooling data with DXA-measured body fat and MRI-measured visceral adipose tissue to test how well the geometric figure tracked each. A rounder ellipse — a wider midsection relative to height — produces a higher BRI, and the study found that BRI tracked visceral fat specifically, not just overall body fat, more closely than BMI did in the same subjects.
BRI sits alongside two other waist-and-height-based indices this site covers, and each models the body differently. A Body Shape Index (ABSI) divides that circumference by BMI raised to the two-thirds power and by the square root of height, so it needs a weight measurement and is built to isolate midsection size from BMI rather than to model overall shape. The Body Adiposity Index (BAI) leaves that measurement out entirely and instead estimates percent fat from hip circumference and height. BRI is the only one of the three built as an explicit ellipse-geometry model, and the only one aimed specifically at visceral fat rather than a body-shape z-score or a percent-fat estimate.
A single BRI reading is best read against population percentiles rather than one fixed universal cutoff — the original study reported ranges tied to its own reference population, and later work has applied percentile bands to different groups rather than settling on one worldwide threshold. Treat the figure as a screening estimate of central body fat to track alongside waist circumference and BMI, not as a replacement for imaging or a clinician's assessment.
- Enter Waist circumference, measured at navel level, in centimetres or inches.
- Enter Height in centimetres, metres, inches, or feet — the equation works internally in metres.
- Read the Body Roundness Index; the working block shows the ratio and square-root terms before the final subtraction.
Worked example — same height, three waist sizes
An 85 cm waist on a 1.75 m frame: dividing by 2π gives 0.85 ÷ 6.2832 ≈ 0.1353, squared to about 0.0183. Half the height squared is 0.875² = 0.7656, and dividing gives 0.0183 ÷ 0.7656 ≈ 0.0239. Subtracting from 1 leaves 0.9761, whose square root is about 0.9880. Multiply by 365.5 to get roughly 361.10, then subtract from 364.2: the Body Roundness Index lands at about 3.09.
Keep the same 1.75 m height and widen the waist to 100 cm: dividing by 2π now gives ≈ 0.1592, squared ≈ 0.0253, divided by 0.7656 ≈ 0.0331, leaving 0.9669 under the square root, which comes out near 0.9833. Multiplying by 365.5 gives about 359.41, and 364.2 minus that is roughly 4.80 — a larger measurement on the same frame raises BRI.
Narrow the waist instead to 70 cm on the same 1.75 m height: dividing by 2π now gives ≈ 0.1114, squared ≈ 0.0124, divided by 0.7656 ≈ 0.0162, leaving 0.9838 under the square root, close to 0.9919. Multiplying by 365.5 gives about 362.53, and 364.2 minus that leaves a BRI of roughly 1.67 — the slimmer measurement pulls the figure down well below the 85 cm result.
Questions
What does a Body Roundness Index score actually measure?
How round the body is when modeled as an ellipse using just waist circumference and height, which the 2013 study by Thomas and colleagues found tracks visceral adipose tissue — the fat packed around abdominal organs — more closely than BMI does. A higher BRI reflects a wider waist relative to height and is associated with more central fat, though it is a geometric estimate rather than a direct measurement of fat mass.
How is BRI different from ABSI or BAI?
All three combine a waist or hip measurement with height, but they model the body differently. ABSI (A Body Shape Index) needs a weight measurement too, since it divides that circumference by BMI to the two-thirds power, and is built to isolate midsection size from BMI. BAI (Body Adiposity Index) uses hip circumference and height instead, to estimate percent body fat. BRI is the only one built as an explicit ellipse-geometry model, and it targets visceral fat specifically rather than a shape z-score or a percent-fat figure.
What BRI value counts as high?
There is no single fixed cutoff the way there is for a BMI of 30. The original study and later applications interpret BRI against percentiles within a reference population, so the same numeric value can sit at different percentiles depending on age, sex, and the population used for comparison. Read a BRI figure as a relative marker of central fat, not an absolute pass-or-fail threshold.
Does a larger waist always raise BRI even if height changes too?
A larger waist raises BRI for a given height, but height moves the figure in the opposite direction — a taller person with the same measurement gets a lower BRI, since the formula divides by a term based on half the height squared. The three worked figures on this page hold height fixed at 1.75 m specifically to isolate that effect; changing both measurements at once moves the result along both dimensions.
Can BRI replace a DEXA scan or an MRI for measuring visceral fat?
No — BRI is a geometric estimate derived from two tape measurements, not a direct scan. The 2013 study validated it against DXA and MRI measurements in a research sample, but for an individual, a raised BRI is a reason to discuss waist size and overall risk with a clinician, not a substitute for imaging when imaging is what's actually needed.
References
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.