Body roundness index calculator
Enter your waist and height, get your BRI.
For general information only — not medical advice. Talk with your doctor or another health professional about your own health. How we check our calculators.
What the number tells you
Weight and BMI do not say where the weight sits, and carrying fat around the middle is linked to more health problems than carrying it elsewhere. BRI tries to capture that with just a tape measure and a height: it treats the body as an ellipse and measures how far from a thin, tall ellipse the middle has become. In the NHANES study of US adults the average value rose from 4.80 in 1999–2000 to 5.62 in 2017–2018.
How it is calculated
The calculator converts everything to centimeters, divides the waist by 2π, squares it and divides by the square of half your height, and applies BRI = 364.2 − 365.5 × √(1 − that ratio). A waist that would make the ratio reach 1 (a waist of about 3.14 times your height) is impossible for the formula and is rejected.
What is the body roundness index?
The body roundness index (BRI) is a number that describes body shape from your height and waist circumference. It was proposed in 2013 by Diana Thomas and colleagues, who modeled the body as an ellipse: the rounder the middle compared with height, the higher the index. They reported that BRI predicted percent body fat and the fraction of visceral (belly) fat slightly better than BMI, waist or hip circumference alone.
What is the BRI formula?
BRI = 364.2 − 365.5 × √(1 − ((waist ÷ 2π)² ÷ (0.5 × height)²)). Waist and height must be in the same unit; the original paper's authors give centimeters. The 2π turns a circumference into a radius, and half of height is the other radius of the ellipse. The root is the eccentricity of the ellipse: a slim, tall shape has an eccentricity close to 1, which gives a low BRI, and a rounder shape lowers the eccentricity and raises the BRI.
Worked example: 34-inch waist, 69 inches tall
Convert to centimeters: waist 86.4 cm, height 175.3 cm. Divide the waist by 2π to get 13.75 cm. Then (13.75)² ÷ (87.6)² = 0.0246, and 1 − 0.0246 = 0.9754, with square root 0.9876. BRI = 364.2 − 365.5 × 0.9876 = 3.22. The waist-to-height ratio is 0.49. A 175 cm person with an 85 cm waist gets 3.09, and a 170 cm person with a 102 cm waist gets 5.43.
Is a lower BRI always better?
Not according to a large US study. In 2024, Zhang and colleagues followed 32,995 US adults from the NHANES survey (1999 to 2018) for a median of 10 years; 3,452 died. The lowest death rates were in the middle fifth of BRI, 4.45 to under 5.46. Compared with that group, people in the lowest fifth (under 3.41) had a hazard ratio of 1.25 and people in the highest fifth (6.91 or more) had 1.49, a U-shaped pattern. This is an association in survey data, not proof that changing your waist changes your risk.
What BRI is healthy?
There is no official BRI category scale from a health agency. The table on this page shows the five groups from the NHANES study, which describe where a value sits among US adults and how the death risk in that group compared with the middle group, not a diagnosis. The original 2013 paper established healthy roundness ranges for its own study samples; this site has only read the abstract, so it does not reproduce them. Ask your doctor what your waist and weight mean for you.
How is BRI different from the waist-to-height ratio?
Less than it seems. Because the BRI formula only uses the waist divided by height, two people with the same ratio get the same BRI: 85 cm at 170 cm and 100 cm at 200 cm both give 3.36; both ratios are 0.50. BRI is a smooth rescaling of the waist-to-height ratio, which carries the same information, and which NICE already grades in simple bands (healthy 0.4 to 0.49, increased risk 0.5 to 0.59, further increased from 0.6). BRI's value is the larger body of recent mortality research built on it.
How do I measure my waist?
Use a flexible tape at the same point each time, such as midway between the bottom of the ribs and the top of the hip bones, after a normal breath out, with the tape snug but not pressing into the skin. A home tape measure is less exact than a clinic measurement: for a 67-inch-tall person, one inch more or less at the waist changes BRI by about 0.29.
Common mistakes and limits
Mixing units is the usual error: the journal printed a correction to the formula in July 2024 because the original used centimeters for waist and meters for height. The calculator rejects waists under a quarter of height for this reason. BRI ignores muscle, age, sex and ethnicity; the NHANES authors note that the best range may differ between ethnic groups because body fat is distributed differently, and the survey's response rate fell over the years, so some bias is possible. BRI is a screening idea, not a diagnosis, and it does not replace blood pressure, blood sugar, lipid tests or a doctor's assessment.
Sources and related calculators
Sources: D. M. Thomas and others, “Relationships between body roundness with body fat and visceral adipose tissue emerging from a new geometrical model,” Obesity 21(11):2264–2271 (2013), abstract read; X. Zhang and others, “Body roundness index and all-cause mortality among US adults,” JAMA Network Open (2024), including its July 2024 correction to the formula units, article read; NICE guideline NG246 for the waist-to-height bands, as described on this site's waist-to-height page. Related: waist-to-height ratio calculator, waist-to-hip ratio calculator, body fat calculator, BMI calculator and Army body fat calculator.