Health Calculators
Body roundness index calculator.
The body roundness index treats your body as an ellipse and scores how round it is from just height and waist. Enter both to get your BRI and see where NHANES put people at each BMI class. Runs in your browser; nothing is sent or stored.
Your measurements
Body roundness index
Fill the form and press Calculate BRI.
How BRI is worked out
Thomas and colleagues (Obesity, 2013) modelled the body as an ellipse. Your height is the long axis, and the short axis comes from your waist circumference divided by π. The more the ellipse looks like a circle, the rounder the body. They scored this with the ellipse’s eccentricity (0 for a circle, close to 1 for a thin line) and rescaled it to a friendlier range.
- Semi-axes: a = height ÷ 2 and b = waist ÷ (2π), both in centimetres.
- Eccentricity: e = √(1 − b² ÷ a²).
- BRI: 364.2 − 365.5 × e. The paper says this rescaling was derived only to put eccentricity on a more usable scale.
Worked example. Height 175 cm, waist 80 cm. Then a = 87.5 and b = 12.73, so e = √(1 − 0.0212) = 0.9894 and BRI = 364.2 − 365.5 × 0.9894 = 2.59. Widen the waist to 100 cm at the same height and the BRI becomes 4.8.
More worked examples
- 165 cm, waist 70 cm: BRI 2.05.
- 165 cm, waist 95 cm: BRI 4.89. A 25 cm wider waist more than doubles the score at this height.
- The height effect. Same 90 cm waist at 170 cm and at 190 cm tall: the taller person scores lower, because the ellipse is longer for the same width. That is the “height-independent” part of the design.
What BRI numbers look like in NHANES
The paper’s Table 3 reports the average BRI in each BMI class in the NHANES data. These are descriptive averages, not cut-offs for risk:
| BMI class | Men, mean BRI | Women, mean BRI |
|---|---|---|
| Under 18.5 | 1.76 | 1.95 |
| 18.5 to under 25 | 3.07 | 3.28 |
| 25 to under 30 | 4.66 | 4.96 |
| 30 to under 40 | 6.47 | 6.86 |
| 40 or over | 10.00 | 9.87 |
The authors note that BRI ranges overlap across BMI classes, so BRI does not map neatly onto BMI categories. In that dataset the lowest observed BRI was 1.00, which is the floor of the scale.
Is there a healthy BRI range?
The paper says healthy body roundness ranges were established, but it does not print a single set of numbers. Those ranges came from a formula fitted to NHANES III that predicts roundness from age, sex, height and body-fat percentage, with published healthy body-fat ranges substituted in. The result therefore depends on age, height and sex, and the authors showed it in an online tool that this site does not reproduce. For that reason this calculator does not label your BRI healthy or unhealthy, and you should be wary of any page that gives one cut-off for everyone.
BRI is a research index. In the paper it improved predictions of body-fat percentage and visceral fat only slightly compared with BMI, waist or hip circumference alone. Waist measurements vary with technique, time of day and breathing, and a small error moves the score. Use BRI to follow your own changes over time with the same measuring method, not to judge a single reading.
Common mix-ups
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Often assumedBRI has fixed healthy and unhealthy cut-offs, like BMI 25 and 30.
What the source saysThe paper derived healthy roundness ranges from a formula using age, sex, height and healthy body-fat ranges, so they vary by person. Its table of BRI by BMI class shows overlapping ranges, not clean cut-offs. [1]
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Often assumedBRI predicts body fat far better than BMI or a waist measurement alone.
What the source saysIn the 2013 paper, body roundness only slightly improved predictions of percent body fat and percent visceral fat compared with BMI, waist circumference or hip circumference. [1]
Checked against these sources on .
FAQ
What is a good body roundness index?
There is no single published cut-off. Thomas and colleagues describe healthy ranges that depend on age, sex and height, and the paper does not print them as a table. Lower values mean a leaner, narrower shape and higher values a rounder one.
How is BRI different from BMI?
BMI uses weight and height. BRI uses waist circumference and height only, so it responds to where you carry weight. In the paper BRI slightly improved predictions of body fat and visceral fat over BMI, and BRI ranges overlap across BMI classes.
Is BRI better than waist-to-height ratio?
This page cannot say. Both use waist and height. The paper compared BRI with BMI, waist and hip circumference, not with waist-to-height ratio, so any claim of one beating the other is outside what this page sourced.
How do I measure my waist for BRI?
Use a flexible tape around the waist at the same spot each time, snug but not tight, after a normal breath out. Your result moves with the method, so consistency matters more than which landmark you choose.
What BRI values does the formula produce?
The paper says the rescaling maps eccentricity onto a 1 to 20 range, and the lowest BRI seen in the NHANES data was 1.00. Very round shapes give large values, and the calculator rejects inputs that make the ellipse impossible.
Does BRI diagnose obesity or visceral fat?
No. It is a shape score that correlates with body fat and visceral fat in the study data. Only imaging or a clinical assessment measures visceral fat, and a doctor can interpret waist and weight changes for you.
Sources
Primary sources used on this page, retrieved October 2026 (2026-10-26). The eccentricity step is the standard ellipse formula applied to the semi-axes the paper describes; the rescaling constants and Table 3 values are quoted from the paper.
- 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 2013;21(11):2264-2271: the elliptical model, the abstract, and the statement that healthy roundness ranges were established (retrieved 2026-10-26)
- Thomas et al. 2013, full text (PMC): the 364.2 and 365.5 rescaling constants, the minor-axis definition from waist circumference, how the healthy ranges were derived, and Table 3 of mean BRI by BMI class (retrieved 2026-10-26)
Disclaimer
Educational estimate, not medical advice. BRI is a research index, not a diagnosis or a risk score. For questions about your weight, waist or health risks, see a qualified health professional.