The waist-to-height ratio treats the middle of the body as a single number. Diana Thomas and her colleagues at Montclair State University and the NIH asked a different question: what if the body were treated as a shape? Seen from the front, the torso is roughly an ellipse, tall in a lean person and rounder in someone who carries fat around the middle. The body roundness index (BRI) measures how close to a circle that ellipse is, using only height and waist circumference. The authors found it tracked body fat percentage and visceral fat volume.
BRI is a research index, still new, and there are no agreed clinical cut-offs. It is best used to follow your own trend and compare it with other measures.
How to use the body roundness index calculator
- Enter your height in centimetres, inches, feet or metres.
- Measure your waist at the navel, relaxed, after breathing out, and enter it in any unit.
- Read the BRI, the eccentricity of the ellipse and the waist-to-height ratio.
- Compare it over time, using the same measuring method each time.
Body roundness index formula
BRI = 364.2 − 365.5 × e
Waist and height must be in the same unit. The formula is from Thomas DM, Bredlau C, Bosy-Westphal A, et al., Relationships between body roundness with body fat and visceral adipose tissue emerging from a new geometrical model, Obesity 2013;21:2264–2271. In the model, the waist circumference is treated as the perimeter of the ellipse’s width, so (waist ÷ 2π) is its half-width, and half of height is its half-length. The constants 364.2 and 365.5 were fitted to data to put BRI on a scale of about 1 to 20.
Worked example
A man 1.78 m tall with an 0.86 m waist.
- Half-width = 0.86 ÷ 2π = 0.1369 m; half-height = 0.89 m
- Ratio² = (0.1369 ÷ 0.89)² = 0.0237
- Eccentricity = √(1 − 0.0237) = 0.9881
- BRI = 364.2 − 365.5 × 0.9881 = 3.05
A woman 1.65 m tall with a 0.76 m waist has a BRI of 2.65.
BRI by height and waist
| Height | Waist 70 cm | Waist 80 cm | Waist 90 cm | Waist 100 cm | Waist 110 cm |
|---|---|---|---|---|---|
| 160 cm | 2.3 | 3.4 | 4.6 | 6.0 | 7.6 |
| 170 cm | 1.9 | 2.8 | 3.9 | 5.2 | 6.5 |
| 180 cm | 1.5 | 2.4 | 3.4 | 4.5 | 5.7 |
How to interpret it
- Higher means rounder. A tall person with a narrow waist has a high eccentricity and a low BRI.
- It is non-linear. Changes in waist matter more at larger waists, so a 5 cm reduction lowers BRI more for a round body than for a lean one.
- It is related to waist-to-height ratio. The two move together; the Waist-to-Height Ratio Calculator gives a risk zone, while BRI gives a continuous scale.
- Research is ongoing. Some large cohort studies have linked higher BRI with greater risk of cardiometabolic disease and mortality, but results are not uniform and no cut-off is official.
Cautions and related tools
The model assumes a particular body shape and relationship between waist and height. It does not include muscle mass, age or sex, so interpretation should be cautious, especially in athletes or older adults. A measurement error of 2 cm in the waist can shift BRI noticeably, so measure twice. For body fat estimates see the Relative Fat Mass (RFM) Calculator and Body Fat Calculator, for hip-based shape see the Waist-to-Hip Ratio Calculator, and for the standard weight category see the BMI Calculator.
This page is for information only and is not medical advice. Speak to a doctor about your own health and any treatment decisions.
Sources
- Thomas DM, Bredlau C, Bosy-Westphal A, et al. Relationships between body roundness with body fat and visceral adipose tissue emerging from a new geometrical model. Obesity 2013;21:2264–2271.
- Rico-Martín S, et al. Effectiveness of body roundness index in predicting metabolic syndrome: a systematic review and meta-analysis. Obes Rev 2020;21:e13023.
Frequently asked questions
What is the body roundness index?
The body roundness index (BRI) is a measure of body shape proposed in 2013 by Diana Thomas and colleagues. It treats the body as an ellipse whose width is the waist and height is the person’s height, and calculates how elongated or round that ellipse is. Rounder bodies, with larger waists for their height, score higher.
What is a good BRI?
Thomas et al. designed the scale so that values run from about 1 for a very lean shape up to around 20 for a very round one. There are no universal cut-offs. Some cohort studies report that cardiometabolic risk rises with higher BRI, but the thresholds differ between studies and populations. Use the number as a trend marker.
How is BRI different from waist-to-height ratio?
Both use only waist and height. BRI applies a non-linear transformation that rises faster at larger waists, which makes differences between round bodies more visible. The two are strongly correlated.
Is BRI better than BMI?
The original paper found BRI predicted body fat percentage and visceral fat volume about as well as, and in some respects better than, BMI. Subsequent studies are mixed. It captures central fat, which BMI does not, but is not a diagnostic test.
Where should I measure my waist?
Around the abdomen at the level of the navel, standing relaxed, after breathing out. The same site should be used each time, and a measurement taken in the morning before eating is most consistent.