Calculate your Body Roundness Index (BRI) from your waist and height, get your body-shape category, and see where you land on the health-risk ranges.
Body Roundness Index Formula
The calculator models your body as an ellipse and returns its roundness. To find your BRI from a waist and height, it uses:
BRI = 364.2 - 365.5 * sqrt(1 - ((WC / (2*pi))^2 / (0.5 * H)^2))
In the “Waist size for a target BRI” mode it rearranges the same equation to solve for the waist that would produce a chosen BRI at your height:
WC = pi * H * sqrt(1 - ((364.2 - BRI) / 365.5)^2)
Variables:
- BRI is the Body Roundness Index, a unitless number that usually falls between about 1 and 16
- WC is your waist circumference
- H is your height
- pi is the constant 3.14159
- WC and H must be in the same unit. The calculator converts centimeters, inches, feet and inches, or meters to meters before applying the formula
The default mode returns your BRI, the body-shape category it falls in, your waist-to-height ratio, and a note on where that BRI sits for health risk. The reverse mode is useful for goal setting: pick a BRI you want to reach and it tells you the waist measurement that matches it at your current height. The waist-to-height ratio shown alongside the result is simply WC divided by H, a related shape measure many people already track.
BRI Ranges and Health Risk
The first table maps BRI values to body-shape categories. The second table is the part most calculators leave out: it links BRI to all-cause mortality using a 2024 study in JAMA Network Open that followed 32,995 US adults. That study found a U-shaped pattern, meaning both very low and very high BRI carried higher risk than the middle of the range, and it reported that the average BRI in the US rose from 4.80 to 5.62 between 1999 and 2018.
| BRI value | Body-shape category | What it suggests |
|---|---|---|
| Under 3.4 | Very lean | Low central fat; a very low value can also reflect low muscle mass |
| 3.4 to 4.45 | Lean to below average | Slim waist relative to height |
| 4.45 to 5.46 | Average | Typical adult range; lowest observed mortality |
| 5.46 to 6.9 | Above average | More central fat; worth watching |
| 6.9 and up | High roundness | High central fat linked to higher metabolic risk |
| BRI group (fifth of the population) | All-cause mortality vs the middle group |
|---|---|
| Lowest, BRI under 3.4 | About 25% higher (hazard ratio 1.25) |
| 3.4 to 4.45 | Modestly higher |
| Middle, 4.45 to 5.46 | Reference, lowest risk |
| 5.46 to 6.9 | Modestly higher |
| Highest, BRI 6.9 and up | About 49% higher (hazard ratio 1.49) |
Read the second table as association, not a personal verdict. It shows that in a large population the risk curve bottomed out in the middle, so aiming for the lowest possible BRI is not the goal.
Example Problems
Example 1: Find BRI from waist and height.
You have a waist of 100 cm (1.0 m) and a height of 170 cm (1.7 m). The waist term is 1.0 / (2 * pi) = 0.159 m, and half your height is 0.85 m.
BRI = 364.2 – 365.5 * sqrt(1 – (0.159^2 / 0.85^2)) = 364.2 – 365.5 * sqrt(0.9649) = 364.2 – 365.5 * 0.98232 = 5.16. A BRI of 5.16 sits in the average range.
Example 2: Find the waist for a target BRI.
You are 175 cm (1.75 m) tall and want a BRI of 4.5. Using the reverse formula, WC = pi * 1.75 * sqrt(1 – ((364.2 – 4.5) / 365.5)^2) = 5.498 * sqrt(1 – 0.9685) = 5.498 * 0.1774 = 0.976 m, or about 97.6 cm (38.4 in).
Frequently Asked Questions
What is a good Body Roundness Index?
In the 2024 US cohort the lowest all-cause mortality fell in the middle of the range, roughly a BRI of 4.5 to 5.5. Both much lower and much higher values carried more risk, which is why the relationship is described as U-shaped. Treat a mid-range BRI as the target rather than the smallest number you can reach, and use the result as one screening signal alongside blood pressure, blood sugar, and lipids.
Is BRI better than BMI?
BRI and BMI answer different questions. BMI compares weight to height and cannot tell where fat sits, while BRI uses waist and height, so it reflects central and visceral fat that drives much of the metabolic risk. Research has found BRI tracks body-fat percentage and abdominal fat more closely than BMI. It is newer and less validated than BMI, so it is best used together with other measures, not as a diagnosis.
How do I measure my waist for BRI?
Stand up and wrap a tape measure around your bare waist at the level of your navel, or at the midpoint between the bottom of your lowest rib and the top of your hip bone. Keep the tape snug but not pressing into the skin, keep it level all the way around, and read the number after breathing out normally. Measuring at the same spot each time makes your BRI comparable from month to month.