Body Roundness Index (BRI) Calculator

BMI treats the body as if height and weight alone describe its shape. BRI takes a different approach, modeling the body's cross-section at the waist as an ellipse, and using how far that ellipse deviates from a circle to estimate roundness, which correlates with visceral adipose tissue and central body fat¹.

What This Calculator Measures

This estimates a BRI score, typically ranging from about 1 to 16, from your waist circumference and height. Higher values indicate a rounder body shape associated with more central adiposity, distinct from BMI, which can't distinguish where fat is distributed.

The Formula

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

Waist circumference and height must be in the same units. This eccentricity-based formula was developed specifically to estimate body fat and visceral adipose tissue from simple external measurements, without requiring imaging¹.

Worked Example

Take someone who is 178 cm tall with a 100 cm waist (about 5'10" and 39.4 in):

• Waist ÷ 2π = 100 ÷ 6.283 = 15.92

• 0.5 × height = 89.0

• (15.92 ÷ 89.0)² = 0.0320, and √(1 − 0.0320) = 0.9839

• BRI = 364.2 − 365.5 × 0.9839 = 4.6

The same person with an 86 cm waist would have a BRI of about 3.1. Because the formula works with very small differences from a circle, keep full precision until the last step if you check it by hand. The calculator shows the result to one decimal place.

How This Calculator Computes Your Result

Enter your height and waist circumference, measured just above your hip bones as described below. We calculate BRI directly from the formula. Implausible waist-to-height combinations, like a waist measurement exceeding height, are flagged as likely data entry errors rather than computing a nonsensical eccentricity value.

Your result is also placed in one of three illustrative bands: lower (under 4.5), middle (4.5 to under 6.9) or higher (6.9 and above). The cut points come from the mortality study described below, rounded to one decimal². They are not an official classification.

How to Measure Your Waist

The NIH advises standing, placing a tape measure around your middle just above your hip bones, and reading it just after you breathe out³. The datasets used to build BRI measured waist at or just above the top of the hip bone (the iliac crest)¹. Keep the tape level all the way around and snug without pressing into the skin. Whichever spot you use, measure at the same place each time so changes in BRI reflect your body rather than the tape.

Why BRI's Real-World Validity Is Still Being Established

The original 2013 paper proposed BRI as a theoretical improvement on BMI, but proposing a formula and demonstrating it predicts real health outcomes are two different things.

More recent research has started to close that gap: a large cohort study following US adults found BRI associated with all-cause mortality in a U-shaped pattern, both unusually low and unusually high BRI carried elevated risk, and used population quintiles rather than fixed cutoffs, since no standardized classification system for BRI currently exists the way it does for BMI².

The study followed 32,995 US adults from the NHANES surveys (1999 to 2018) for a median of about 10 years, during which 3,452 died. After adjusting for age, sex, race and ethnicity, education, income, smoking, drinking and family history of heart disease and diabetes, risk compared with the middle fifth looked like this²:

A hazard ratio of 1.49 means a 49% higher rate of death over the follow-up than the reference group, not a 49% chance of dying. The same study found average BRI in US adults rose from 4.80 to 5.62 between 1999 and 2018². The authors suggest BRI could be used as a screening tool, pending consistent results in other independent cohorts².

That's a meaningfully different, and generally more useful, kind of evidence than the original geometric derivation alone, it's now been tested against actual mortality data, not just theorized to correlate with body composition.

Using BRI Alongside Other Measures

BRI uses the same two measurements as the waist-to-height ratio. In fact, the formula depends only on waist divided by height, so the same ratio always gives the same BRI. A ratio of 0.50 equals a BRI of about 3.4, 0.56 about 4.5, and 0.66 about 6.9. What BRI adds is a scale built from an ellipse model, which in the original study slightly improved predictions of body fat and visceral fat compared with BMI or waist alone¹.

A low BRI is not automatically a good sign, since the lowest quintile also carried higher risk in the US cohort². Read your number as one piece of information about body shape, not a verdict on your health.