FatCalc

Body Roundness Index (BRI) Calculator

Use this calculator to find your Body Roundness Index (BRI), a measure of body shape calculated from your waist and height. A 2024 study of nearly 33,000 US adults found that BRI tracks the risk of death from any cause, and this calculator shows which of the study's five BRI ranges you fall in.

BRI Calculator
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Waist: Just above the top of your hip bone (iliac crest), level all the way around. Measure at the end of a normal exhalation.
Height: Stand straight against a wall, without shoes, and measure from the floor to the top of your head.

What Is the Body Roundness Index?

The Body Roundness Index was introduced in 2013 by Diana Thomas and colleagues as a way to describe body shape with a single number. It treats the body as an ellipse, with your height as the long axis and your waist as the distance around it. The rounder that ellipse is, the higher your BRI.

The idea behind it is the same one behind waist-to-height ratio: fat carried around the middle, especially the visceral fat packed around the organs, is far more closely tied to heart disease and type 2 diabetes than fat stored elsewhere. In the original study, which compared body measurements against DXA scans and MRI, BRI predicted both total body fat and visceral fat slightly better than BMI or waist circumference alone.

How BRI Is Calculated

The formula uses only your waist and height, measured in the same unit:

BRI = 364.2 − 365.5 × √(1 − (waist ÷ 2π)² ÷ (0.5 × height)²)

The square root is the eccentricity of the ellipse, a number from geometry that describes how far a shape is from a perfect circle. A tall, slim body is a long, narrow ellipse; a body with a larger waist for its height is a rounder one, and its BRI is higher.

Example: If you are 5'10" (177.8 cm) tall with a 36 inch (91.4 cm) waist, your BRI is 3.63, in the Low range. The Elevated range would begin at a waist of about 42.1 inches (106.9 cm).

BRI and Mortality: The 2024 Study

BRI drew wide attention in 2024, when researchers published an analysis in JAMA Network Open of 32,995 US adults from the National Health and Nutrition Examination Survey (NHANES), surveyed between 1999 and 2018 and followed for a median of about 10 years. They split participants into five equal groups by BRI and compared the risk of death from any cause in each group with the middle one, after adjusting for age, sex, race and ethnicity, education, income, smoking, drinking, and family history of heart disease and diabetes.

The pattern was U-shaped. People in the highest fifth, with a BRI of 6.91 or more, had a 49% higher risk of death than the middle group, and those in the fourth fifth had a 25% higher risk. The lowest fifth also had a 25% higher risk, which the authors link to malnutrition and muscle loss in very slim people rather than to a slim waist itself. The study also found that average BRI among US adults rose from 4.80 to 5.62 over the 20 years it covered.

The U-shape is not unique to that study. A 2025 study that followed Japanese adults for a median of 13 years found the same pattern, and so did the preliminary pooled analysis of earlier studies included in the same paper.

These five ranges are what this calculator places you in. They are mortality bands from one large US study, not clinical cut-points, and like any study of this kind they show an association rather than proof of cause and effect.

BRI and the "Half Your Height" Rule

BRI is calculated entirely from your waist-to-height ratio, so a higher ratio always means a higher BRI. A ratio of 0.5, the well-known "keep your waist under half your height" boundary, works out to a BRI of about 3.36.

That means the middle BRI group, where the 2024 study found the lowest mortality, corresponds to a waist-to-height ratio of about 0.56 to 0.60, above the 0.5 boundary. The two do not actually disagree. The BRI bands describe how the risk of death varied across the US population, where the average waist is large, while the 0.5 boundary comes from research on the risk of heart disease and diabetes: a 2010 review of 78 studies from 14 countries found 0.5 to be a suitable boundary for both men and women. A waist under half your height remains a sound target, and nothing in the BRI research suggests that a lean person should try to gain waist.

BRI Compared With Other Measures

  • BMI: Uses weight and height, so it cannot tell muscle from fat or show where fat is stored. BRI replaces weight with waist size, so a muscular person with a high BMI can still have a low BRI.
  • Waist-to-height ratio: Ranks people in exactly the same order as BRI. A 2020 meta-analysis found waist-to-height ratio slightly better than BRI at detecting metabolic syndrome, though the difference was not statistically significant.
  • A Body Shape Index (ABSI): Also uses waist size but measures it relative to what is expected for your weight and height, so it needs a scale as well as a tape measure. In the same meta-analysis, BRI outperformed ABSI, BMI and waist-to-hip ratio for detecting metabolic syndrome.

How to Measure

Use a flexible, non-stretchable measuring tape. Remove bulky clothing or measure over thin, form-fitting garments, and stand straight with your feet together.

For waist: Find the top of your hip bone (the iliac crest) at your side and wrap the tape around your waist just above it, keeping it level all the way around. Keep it snug without compressing the skin, don't pull your stomach in, and read the tape at the end of a normal breath out. This is the site used in the NHANES surveys the BRI ranges come from.

For height: Stand barefoot against a wall with your heels, buttocks, and upper back touching the wall. Look straight ahead and have someone mark the highest point of your head. Measure from the floor to this mark.

Limitations of BRI

  • It cannot tell visceral fat from the subcutaneous fat just under the skin.
  • The mortality ranges come from US adults aged 20 and over, and may not carry over exactly to other populations or to children and teenagers. Typical values differ between countries: in the Japanese study above, the median BRI was about 3.5 to 3.7, well below the US average.
  • It adds little beyond waist-to-height ratio, since it is calculated from the same two measurements.
  • Like all body measurements, it is a screening figure, not a diagnosis.

Frequently Asked Questions about BRI

The Body Roundness Index (BRI) is a body shape measure introduced by Diana Thomas and colleagues in 2013. It models the body as an ellipse, with your height as its long axis and your waist as its circumference, and expresses how round that ellipse is as a single number, where higher means rounder. In the original study, BRI predicted total body fat and visceral fat slightly better than BMI or waist circumference.
In a 2024 study of 32,995 US adults, the lowest risk of death from any cause was in the middle fifth of the population, with a BRI of 4.45 to 5.45. Risk was 25% higher above 5.46 and 49% higher at 6.91 or above. Risk was also 25% higher below 3.41, which the authors link to malnutrition and muscle loss in that group.
BMI uses only weight and height, so it cannot tell muscle from fat or show where fat is stored. BRI uses waist and height instead, which makes it a measure of fat carried around the middle, the kind most closely linked to heart disease and type 2 diabetes. A muscular person with a high BMI can have a low BRI, and a person with a normal BMI can have a high one.
BRI is calculated entirely from your waist-to-height ratio, so the two always rank people in the same order: a higher ratio always means a higher BRI. What BRI adds is a different scale and the mortality ranges from the 2024 NHANES study. Both are useful screening figures, and a 2020 meta-analysis found waist-to-height ratio slightly better than BRI at detecting metabolic syndrome, though the difference was not statistically significant.
The study found a U-shaped pattern: the lowest fifth of BRI had a 25% higher death rate than the middle fifth. The study's authors note that a very low BRI can go along with malnutrition, fatigue and muscle loss, much like the similar pattern seen with very low BMI, rather than a slim waist being harmful in itself. A low BRI is not a reason to try to gain weight, but unexplained weight loss is worth raising with a doctor.
No. The BRI formula uses only waist and height, and the ranges this calculator uses were set from the whole study population rather than separately for men and women. The study included adults aged 20 and over, so the ranges are not meant for children or teenagers.
Find the top of your hip bone (the iliac crest) at your side and wrap the tape around your waist just above it, level all the way around. Measure at the end of a normal breath out, keep the tape snug without compressing the skin, and don't pull your stomach in. This is the site used in the NHANES surveys the BRI ranges come from.
Because your height doesn't change, the only way to lower your BRI is to reduce your waist. There is no way to spot-reduce belly fat, but regular aerobic exercise, resistance training, a diet built on whole foods with fewer refined carbohydrates, adequate sleep and limiting alcohol all reduce abdominal fat over time.

Caution: This calculator is intended for educational purposes only, and is not for medical use. A healthcare provider can put your result in the context of your wider health picture.

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References

  1. D.M. Thomas and colleagues introduced the Body Roundness Index in Obesity (2013), modeling the body as an ellipse and showing that body roundness predicts total body fat and visceral fat measured by DXA and MRI. View
  2. A 2024 cohort study by X. Zhang and colleagues in JAMA Network Open followed 32,995 US adults from NHANES (1999–2018) and found a U-shaped association between BRI and all-cause mortality, which is the source of the five BRI ranges and hazard ratios used in this calculator. View
  3. A 2020 systematic review and meta-analysis by S. Rico-Martín and colleagues in Obesity Reviews found that BRI detects metabolic syndrome better than BMI, waist-to-hip ratio and ABSI, and on par with waist circumference and waist-to-height ratio. View
  4. A 2025 cohort study by K. Murakami and colleagues in The Journal of Nutrition followed Japanese adults for a median of 13.3 years and, together with a preliminary meta-analysis, found the same U-shaped association between BRI and all-cause and cardiovascular mortality. View
  5. A systematic review by L.M. Browning, S.D. Hsieh, and M. Ashwell in Nutrition Research Reviews (2010) examined 78 studies of waist-to-height ratio as a predictor of cardiovascular disease and diabetes, concluding that 0.5 could serve as a suitable universal boundary value. View
  6. The U.S. Centers for Disease Control and Prevention's NHANES Anthropometry Procedures Manual describes the waist measurement used in the surveys the BRI ranges come from: taken just above the top of the right hip bone (iliac crest), at the end of a normal breath out. View