Body Composition
Body Roundness Index
Computes the Body Roundness Index (Thomas 2013), which models body shape from waist and height to estimate body fatness and mortality risk.
Body Roundness Index at a glance
Rather than repeating a single reference figure, the Body Roundness Index below personalizes it: your own measurements feed the same published equation the reference ranges are built from.
Computes the Body Roundness Index (Thomas 2013), which models body shape from waist and height to estimate body fatness and mortality risk.
Nothing here is a black box - the script embedded in this page applies the published equation shown in the code with the inputs you enter, and returns the same precision you would get working it out manually. The page is written for people who re-measure over time: the result fits the same progress rhythm Macrition uses for weight and waist, so the number stays comparable with everything else you record.
About Body Roundness Index
The Body Roundness Index (BRI) was developed by Diana Thomas and colleagues and published in 2013 in the journal Obesity. The index models the human body as an ellipse whose eccentricity is determined by waist circumference and height, and it produces a single dimensionless number (typically between 1 and 15 in adults) that reflects how 'round' the body is. The motivation was to create an anthropometric index that, like BMI, is simple to measure but, unlike BMI, captures body shape (specifically abdominal adiposity) and predicts body fat percentage and cardiometabolic risk more accurately.How the formula works
The BRI is computed from waist and height (both in the same units, typically centimeters). First the eccentricity-related term x is computed as waist / (pi x height). The BRI is then 364.2 - 365.5 x sqrt(1 - x^2). The formula is derived from the geometry of an ellipse: as the waist grows relative to height, the body becomes more circular (less elliptical), the sqrt term shrinks, and the BRI increases. A perfectly cylindrical body (waist = pi x height, x = 1) would have BRI = 364.2, while a very lean body (x near 0) would have BRI near 364.2 - 365.5 = -1.3, clamped in practice to small positive values.Evidence and validation
Thomas and colleagues derived the BRI from NHANES data and showed that it correlated more strongly with body fat percentage measured by DEXA (r about 0.8) than BMI did (r about 0.7), and that it predicted cardiometabolic risk factors (blood pressure, lipids, glucose) at least as well as BMI and waist-to-height ratio. A 2022 study by Chang and colleagues in JAMA Network Open, pooling NHANES data from 1999 to 2018 with mortality follow-up, found that BRI predicted all-cause mortality with a U-shaped association, with the lowest mortality at BRI around 4.5 to 5.5 and rising risk at both low and high BRI values. The study confirmed BRI as a useful mortality-predictive anthropometric index.Interpretation
Typical BRI values are about 1 to 3 in lean young adults, 3 to 5 in middle-aged adults with average adiposity, 5 to 8 in overweight adults, and 8 or higher in obese adults. The interpretation bands used here are approximate; the 2022 JAMA Network Open analysis identified the lowest-mortality BRI range as about 4.5 to 5.5, with risk rising progressively above about 7. BRI rises with age and is generally higher in women than in men at any given age, reflecting sex differences in fat distribution.When to use it
The BRI is useful as a screening measure of body shape and adiposity that captures abdominal roundness better than BMI. It requires only waist and height measurements, needs no sex or ethnicity adjustment, and predicts body fat percentage and mortality risk more accurately than BMI. It is particularly useful in populations where BMI underestimates risk, such as South Asians and older adults with sarcopenic obesity.Limitations
The BRI, like all waist-based measures, captures abdominal adiposity but does not directly measure body composition. The geometric model of the body as an ellipse is a simplification, and the formula produces small or even slightly negative values for very lean individuals, which are clamped in practice. BRI does not distinguish subcutaneous from visceral fat, and waist measurement is subject to technique variation. The reference ranges and mortality risk thresholds are based on NHANES data and may not perfectly generalize to other populations. Despite these limitations, the BRI is a promising and increasingly used anthropometric index that complements BMI and waist-to-height ratio.
Editorial and formula review
Formula sources, population limits, and references for this body composition calculator are documented in our public methodology. Read the methodology · Medical disclaimer
Frequently asked questions
BRI uses waist and height to model body shape (eccentricity of an ellipse), while BMI uses only weight and height. BRI captures abdominal roundness and predicts body fat percentage and mortality risk more accurately than BMI, at the cost of requiring a waist measurement.
BRI values of 8 or higher generally indicate high body roundness consistent with obesity. The lowest-mortality BRI range is about 4.5 to 5.5, with risk rising progressively above about 7. BRI rises with age and is higher in women than in men at any given age.
They are highly correlated and perform similarly for predicting cardiometabolic risk and mortality. BRI has a stronger theoretical basis (geometric modeling of body shape) and correlates slightly better with DEXA body fat, while waist-to-height ratio has a simpler, more memorable 0.5 action threshold. Both are useful; the choice is often a matter of preference.