The body roundness index (BRI) estimates how round your body is from your height and waist, as a score that usually falls between about 3 and 7 for adults. In a 2024 study of 32,995 US adults, the risk of death was lowest for a BRI of 4.45 to 5.45, 25% higher below 3.41 or from 5.46 to 6.90, and 49% higher at 6.91 or above.
BRI was developed in 2013 by the mathematician Diana Thomas and colleagues, who modelled the body as an ellipse to estimate total and visceral fat. Enter your height and waist to see your score and which band it falls in.
Worked example
Height: 5 ft 9 inWaist: 36 in
Body roundness index: 3.78 Low BRI
At 5 ft 9 in, a 36-inch waist gives a BRI of 3.78 — in the second-lowest fifth of US adults, a band whose risk of death in the 2024 study was not significantly different from the middle one.
Open this example in the calculator →How to use the Body Roundness Index Calculator
- Choose US or metric units and enter your height.
- Measure your waist midway between your lowest rib and the top of your hip bone, after breathing out.
- Read your body roundness index and its band.
- See the waist that would put you in the lowest-risk band at your height.
The BRI formula
BRI = 364.2 − 365.5 × √(1 − (waist ÷ 2π)² ÷ (0.5 × height)²), with waist and height in the same units. The body is treated as an ellipse whose long axis is your height and whose short axis comes from your waist: the rounder the ellipse, the higher the score. A tall, slim person scores near 1 or 2; a short person with a large waist can score over 10.
For example, at 5 ft 9 in (175.3 cm) with a 36-inch (91.4 cm) waist: 91.4 ÷ 2π = 14.55 cm; half the height is 87.63 cm; 1 − (14.55 ÷ 87.63)² = 0.9724; the square root is 0.98611; and 364.2 − 365.5 × 0.98611 = 3.78.
BRI chart (2024 US study)
| BRI | Band | Risk of death vs the middle band |
|---|---|---|
| Below 3.41 | Very low | 25% higher (HR 1.25) |
| 3.41–4.44 | Low | Not significantly different (HR 1.13) |
| 4.45–5.45 | Middle | Reference — lowest risk |
| 5.46–6.90 | High | 25% higher (HR 1.25) |
| 6.91 or more | Very high | 49% higher (HR 1.49) |
The bands are fifths (quintiles) of US adults in the National Health and Nutrition Examination Survey, 1999 to 2018, followed for a median of 10 years; the hazard ratios are adjusted for age, sex, ethnicity, education, income, smoking, drinking and family history. The relationship was U-shaped: risk rose at both ends.
Is BRI better than BMI?
BRI uses the waist, so like waist-to-height ratio it sees the abdominal fat that BMI misses, and studies have found it predicts diabetes, heart disease and mortality at least as well as BMI. It is not a different kind of information from waist-to-height ratio, though — both come from the same two numbers — and BRI has no agreed clinical cut-offs. For a simple rule, keeping your waist under half your height captures most of the same message; see the waist-to-height ratio calculator.
Why BRI made the news
The 2024 study by Xiaoqian Zhang and colleagues, published in JAMA Network Open, was widely reported as a possible replacement for BMI. Its finding was the U-shaped curve above. The journal later published a correction noting that height and waist must be entered in the same unit — which this calculator handles for you.
Frequently asked questions
What is a good body roundness index?
In the 2024 US study, the lowest risk of death was at a BRI of 4.45 to 5.45, with risk not significantly higher from 3.41 to 4.44. Below 3.41 and from 5.46 up, risk was 25% higher; at 6.91 or more, 49% higher.
How is body roundness index calculated?
BRI = 364.2 − 365.5 × √(1 − (waist ÷ 2π)² ÷ (0.5 × height)²), with waist and height in the same units. It needs only a tape measure.
Is BRI better than BMI?
It accounts for waist size, which BMI ignores, and predicts health risks at least as well. But it has no official cut-offs yet. Most guidelines still use BMI together with a waist measure such as waist-to-height ratio.
What is an average BRI?
For US adults the middle fifth runs from 4.45 to 5.45. Most adults fall between about 3 and 7.
Why is a very low BRI linked with higher risk?
Probably because a very low BRI includes people who are underweight, frail or ill, not because being lean is harmful. A similar pattern is seen with low BMI.
Sources
- Thomas DM et al. Relationships between body roundness with body fat and visceral adipose tissue emerging from a new geometrical model. Obesity 2013;21:2264–2271 — Obesity
- Zhang X et al. Body Roundness Index and All-Cause Mortality Among US Adults. JAMA Netw Open 2024;7(6):e2415051 — JAMA Network Open
- Overweight and obesity management (NG246), 2025 — National Institute for Health and Care Excellence (UK)
- Ashwell M, Gibson S. Waist-to-height ratio as an indicator of “early health risk”. BMJ Open 2016;6:e010159 — BMJ Open
For general information only — not a diagnosis or a substitute for advice from a doctor. How we calculate.