Body Roundness Index (BRI)
Page updated: Evidence reviewed: What this means
Body Roundness Index (BRI) is calculated from waist circumference and standing height using the Thomas et al. (2013) ellipse-based formula. A lower value corresponds to a narrower waist relative to height; a higher value corresponds to a rounder body profile.
These percentiles were derived from NHANES 2021-2023 microdata for 5,718 US adults aged 20 and over after excluding participants recorded as pregnant. We publish the recent cycle alone because 71 of 130 cells differed by more than 0.15 BRI units in a pooled 2015-2023 sensitivity table. The methodology page documents the formula, exclusions, weights, and sensitivity check.
BRI has no single widely accepted clinical cut-off, so the tables describe the observed population distribution rather than diagnostic or risk categories.
How to Perform This Test (Protocol)
- Equipment
-
- Flexible non-stretch anthropometric tape
- Stadiometer or other accurate height measure
- Protocol Steps
-
- Stand upright with feet together, arms relaxed at the sides, and abdomen relaxed.
- Measure waist circumference at the high point of the iliac crest (the top edge of the hip bone), with the tape horizontal and not compressing the skin. Record at the end of a normal expiration to the nearest 0.1 cm. This is the site used by the NHANES Anthropometry Procedures Manual; it is not the same as the WHO midpoint-between-ribs-and-iliac-crest site.
- Measure standing height with a stadiometer to the nearest 0.1 cm.
- Convert both measurements to metres, then compute BRI = 364.2 − 365.5 × √(1 − (WC/(2π))² / (0.5×Ht)²).
- Scoring
BRI maps waist circumference relative to height onto an ellipse-based scale. Lower values correspond to a narrower waist relative to height, and higher values to a rounder profile. The percentile chart shows where a measurement made with the NHANES waist protocol sits in this US adult distribution.
- Notes
The percentiles on this page use the NHANES iliac-crest waist site; the WHO midpoint waist site produces systematically different waist values and therefore different BRI values, and is not directly comparable to these norms.
Data source: NHANES (CDC), derived BRI percentiles About this study
Reference population: US adults aged 20 and over in NHANES 2021-2023 after excluding participants recorded as pregnant, nationally representative
Body Roundness Index (BRI) Norms Chart by Age and Sex (index)
| Age | Sex | Percentile | ||||
|---|---|---|---|---|---|---|
| 5th | 25th | 50th | 75th | 95th | ||
| 20-24 | Male | 1.78 | 2.53 | 3.39 | 5.08 | 7.62 |
| Female | 1.91 | 2.64 | 3.46 | 4.78 | 8.96 | |
| 25-29 | Male | 2.16 | 3.05 | 4.21 | 5.53 | 7.97 |
| Female | 2.21 | 3.02 | 4.06 | 6.55 | 11.20 | |
| 30-34 | Male | 2.11 | 3.36 | 4.39 | 5.74 | 9.48 |
| Female | 2.28 | 3.53 | 5.19 | 7.35 | 11.69 | |
| 35-39 | Male | 2.80 | 3.80 | 5.04 | 6.27 | 8.74 |
| Female | 2.40 | 3.77 | 4.77 | 7.11 | 11.47 | |
| 40-44 | Male | 2.94 | 3.72 | 4.86 | 6.10 | 7.59 |
| Female | 2.56 | 3.60 | 5.00 | 6.98 | 9.85 | |
| 45-49 | Male | 2.83 | 4.08 | 5.24 | 7.28 | 11.52 |
| Female | 2.83 | 3.87 | 5.37 | 7.66 | 10.64 | |
| 50-54 | Male | 2.82 | 4.22 | 5.62 | 6.55 | 10.24 |
| Female | 3.09 | 4.19 | 5.76 | 7.69 | 10.52 | |
| 55-59 | Male | 3.05 | 4.22 | 5.31 | 6.91 | 10.25 |
| Female | 2.89 | 4.39 | 6.30 | 8.10 | 10.75 | |
| 60-64 | Male | 3.03 | 4.16 | 5.29 | 6.57 | 9.26 |
| Female | 3.02 | 4.32 | 5.73 | 7.62 | 10.33 | |
| 65-69 | Male | 3.28 | 4.40 | 5.41 | 6.75 | 9.70 |
| Female | 2.95 | 4.46 | 5.90 | 7.84 | 10.99 | |
| 70-74 | Male | 3.37 | 4.59 | 5.90 | 7.05 | 9.56 |
| Female | 2.98 | 4.55 | 5.99 | 7.74 | 10.81 | |
| 75-79 | Male | 3.22 | 4.78 | 5.95 | 7.17 | 9.68 |
| Female | 3.32 | 4.64 | 5.88 | 7.55 | 10.12 | |
| 80+ | Male | 3.58 | 4.65 | 5.42 | 6.63 | 8.42 |
| Female | 2.90 | 4.55 | 5.71 | 7.37 | 9.18 | |
What to expect by age group
Among US adults aged 30-34, the middle 50% spans 3.36 to 5.74 for men and 3.53 to 7.35 for women. Median BRI generally rises from early adulthood into later life, but not in every successive age bracket. Female medians are higher in 10 of the 13 brackets; male medians are higher at ages 25-29, 35-39, and 75-79. These are distribution labels, not clinical risk categories.
| Age | Males | Females |
|---|---|---|
| 20-24 | 2.53 to 5.08 | 2.64 to 4.78 |
| 25-29 | 3.05 to 5.53 | 3.02 to 6.55 |
| 30-34 | 3.36 to 5.74 | 3.53 to 7.35 |
| 35-39 | 3.80 to 6.27 | 3.77 to 7.11 |
| 40-44 | 3.72 to 6.10 | 3.60 to 6.98 |
| 45-49 | 4.08 to 7.28 | 3.87 to 7.66 |
| 50-54 | 4.22 to 6.55 | 4.19 to 7.69 |
| 55-59 | 4.22 to 6.91 | 4.39 to 8.10 |
| 60-64 | 4.16 to 6.57 | 4.32 to 7.62 |
| 65-69 | 4.40 to 6.75 | 4.46 to 7.84 |
| 70-74 | 4.59 to 7.05 | 4.55 to 7.74 |
| 75-79 | 4.78 to 7.17 | 4.64 to 7.55 |
| 80+ | 4.65 to 6.63 | 4.55 to 7.37 |
Detailed Breakdowns
Select an age group and sex below for detailed percentile charts and distribution labels.
Frequently Asked Questions
What is Body Roundness Index?
BRI is a body-shape index introduced by Thomas et al. (2013). It models the body outline as an ellipse using waist circumference and height. The formula is BRI = 364.2 − 365.5 × √(1 − (WC/(2π))² / (0.5×Ht)²), with WC and height in metres. Lower values correspond to a narrower waist relative to height, and higher values to a rounder profile.
How does BRI differ from BMI or waist-to-height ratio?
BMI relates weight to height and does not describe where mass is carried. Waist-to-height ratio (WHtR) uses the same two inputs as BRI but divides waist by height directly. BRI applies a nonlinear ellipse-based transform, so its numeric scale and upper-tail spacing differ from WHtR.
Why do female BRI medians tend to sit above male medians?
In this NHANES dataset, the female median is higher in 10 of 13 age brackets; the male median is higher at ages 25-29, 35-39, and 75-79. BRI depends on waist relative to height, so it should not be interpreted as a ranking of waist circumference alone. The tables describe the observed sex-specific distributions but do not establish why they differ.
Why use the 2021-2023 cycle alone rather than pooling earlier NHANES cycles?
A sensitivity check compared the 2021-2023 cycle with a duration-adjusted 2015-2023 pool. Of 130 sex, age, and percentile cells, 71 differed by more than 0.15 BRI units. The median absolute difference was 0.160, and the largest was 1.90 at male ages 45-49 P95 (11.52 versus 9.62). The differences could reflect change over time, sampling variation, and amplification by the nonlinear formula; the comparison cannot separate those contributors. We therefore publish the recent cycle and retain the pool only as a sensitivity analysis.
Are there clinical cut-offs for BRI?
There is no single widely accepted clinical BRI threshold. Published studies use different cut-offs for different populations and outcomes, so a percentile on this page should not be treated as a diagnosis or universal risk boundary.
Where does the data come from?
These percentiles were calculated from the CDC's NHANES 2021-2023 public microdata. The analysis includes 5,718 adults aged 20 and over with complete waist and height measurements after excluding participants recorded as pregnant. The methodology page gives the full derivation.