What is BMI and how is it calculated?
Body Mass Index (BMI) is calculated by dividing a person's body weight in kilograms by the square of their height in metres (kg/m²). The formula was developed by Belgian mathematician Adolphe Quetelet in the 19th century and was later adopted by the World Health Organization (WHO) as a population-level screening tool. The WHO classifies BMI below 18.5 as underweight, 18.5-24.9 as normal weight, 25.0-29.9 as overweight, and 30.0 or above as obese, with further obesity sub-classes at 35 and 40.
BMI's primary strength is its simplicity: it requires only a scale and a measuring tape, making it practical for large-scale epidemiological studies and routine clinical screening. The WHO and most national health agencies continue to recommend BMI as a first-line screening measure precisely because the measurements are standardised, inexpensive and reproducible. However, the WHO also acknowledges that BMI is a screening tool, not a diagnostic measure, and that interpretation must consider additional clinical context.
What is BRI and how is it different?
Body Roundness Index (BRI) was introduced by Diana Thomas and colleagues in a 2012 paper published in the journal Obesity. The formula uses waist circumference and height to model the human torso as an ellipse, producing a value that describes how 'round' a person's body shape is. A higher BRI value indicates a more circular cross-section (greater roundness and central adiposity), while lower values reflect a more elongated body shape. The mathematical formula is: BRI = 364.2 - 365.5 x sqrt(1 - ((WC / 2pi) squared / (0.5 x H) squared)), where WC is waist circumference and H is height, both in metres.
Because BRI incorporates waist circumference, it provides information about central adiposity -- the accumulation of fat around the abdomen -- which BMI cannot capture. Visceral fat, stored around internal organs in the abdominal cavity, is associated with higher cardiometabolic risk than subcutaneous fat stored elsewhere in the body. Research published after the Thomas et al. 2012 paper has examined whether BRI outperforms BMI at predicting metabolic outcomes, with several studies reporting stronger associations between BRI and markers such as blood pressure, fasting glucose and triglycerides.
Side-by-side comparison: BMI vs BRI
The table below summarises the key practical and scientific differences between BMI and BRI. Neither index is a definitive measure of body fatness or health; each has distinct strengths and limitations depending on the clinical or research context.
| Feature | BMI | BRI |
|---|---|---|
| Inputs required | Weight, height | Waist circumference, height |
| Original purpose | Population screening (Quetelet) | Body shape modelling (Thomas et al. 2012) |
| Captures central fat? | No | Yes (via waist circumference) |
| Distinguishes fat from muscle? | No | No |
| WHO classification system? | Yes (established) | No (research-defined bands) |
| Clinical adoption | Universal | Emerging / research settings |
| Best for | Population-level screening | Visceral fat and cardiometabolic risk estimation |
| Validated ethnicity adjustments? | Yes (WHO Asian cut-offs) | Limited evidence to date |
Visceral fat: why abdominal obesity matters
Visceral fat is metabolically active adipose tissue stored within the abdominal cavity, surrounding organs such as the liver, pancreas and intestines. Unlike subcutaneous fat located beneath the skin, visceral fat releases free fatty acids and pro-inflammatory substances directly into the portal circulation, which may contribute to insulin resistance, dyslipidaemia and elevated cardiovascular risk. The WHO and major cardiology societies recognise waist circumference as a practical proxy for visceral fat accumulation.
Because BRI incorporates waist circumference, it can differentiate two individuals with identical BMIs who have very different fat distributions. For example, an athlete with high muscle mass may have an elevated BMI but a low BRI reflecting a relatively small waist, while a sedentary individual with normal BMI but significant abdominal fat may have an elevated BRI. This distinction is clinically relevant because the latter pattern -- sometimes described as 'normal-weight obesity' or 'metabolically obese normal weight' in the research literature -- may carry elevated cardiometabolic risk despite a normal BMI.
Limitations of both measures
BMI does not account for age, sex, ethnicity, fitness level or body composition. A muscular athlete and a sedentary person of the same height and weight will receive identical BMIs despite very different health profiles. The WHO has acknowledged that lower BMI cut-offs may be more appropriate for populations of Asian descent, recommending consideration of action points at BMI 23 (overweight) and 27.5 (obese) for these groups.
BRI, while theoretically superior for capturing body shape, has its own limitations. Waist circumference measurement is subject to inter-observer variability; different measurement protocols (at the navel, at the narrowest point, or at the midpoint between the lowest rib and the iliac crest) produce different values. BRI also does not distinguish subcutaneous from visceral fat directly -- it uses waist circumference as a proxy. Additionally, BRI thresholds and classification bands are not yet standardised by major health organisations in the way that WHO BMI thresholds are, which limits its direct clinical application.
Which measure should be used?
Current clinical guidelines from the WHO, NHS and major obesity medicine societies recommend using BMI as a primary screening tool alongside waist circumference as a complementary measure. Waist-to-height ratio and waist-hip ratio are also widely used to assess central adiposity. BRI integrates waist and height into a single index and may be a useful research or screening tool, but it has not yet replaced BMI in routine clinical guidelines.
For individuals wanting to understand their body composition more fully, clinicians may combine BMI with waist circumference, waist-hip ratio, or more direct body composition measures such as dual-energy X-ray absorptiometry (DEXA). No single index captures the full complexity of body composition and metabolic health; context, clinical judgement and a full assessment are always required for health decision-making.
Häufig gestellte Fragen
Is BRI more accurate than BMI?
BRI incorporates waist circumference, which makes it a better predictor of central adiposity and visceral fat than BMI alone. Research following the Thomas et al. 2012 publication has reported stronger associations between BRI and some cardiometabolic risk markers compared to BMI. However, BRI is not yet endorsed by major health organisations as a replacement for BMI, and both measures fail to distinguish fat mass from lean mass. Whether BRI is 'more accurate' depends on the specific outcome being evaluated.
What is a healthy BRI score?
Based on the Thomas et al. 2012 framework and subsequent research, BRI values in roughly the 3.41 to 5.46 range are generally considered lean to average. Values above 6.91 are associated with higher body roundness and potential cardiometabolic risk. However, standardised BRI classification cut-offs have not been adopted by major health organisations, so interpretation should be done in consultation with a healthcare professional rather than relying on general thresholds alone.
Can BMI be normal but BRI be high?
Yes. A person with normal BMI (18.5-24.9 kg/m²) but a relatively large waist circumference in proportion to their height can have an elevated BRI. This pattern -- sometimes described as 'metabolically obese normal weight' in the research literature -- may be associated with excess visceral fat despite a normal BMI. This is one reason researchers have proposed BRI as a complementary metric to BMI when assessing cardiometabolic risk.
Does BMI apply equally to all ethnic groups?
BMI thresholds were derived primarily from European population studies. The WHO and health agencies in several countries recommend lower action points for people of Asian descent, where overweight risk may begin at BMI 23 kg/m² and obesity risk at BMI 27.5 kg/m², compared with the standard thresholds of 25 and 30 respectively. This reflects differences in body fat distribution and cardiometabolic risk at given BMI levels across ethnic groups.
What is visceral fat and why does it matter?
Visceral fat is adipose tissue stored inside the abdominal cavity, surrounding organs such as the liver and pancreas. It is metabolically active and releases substances that may contribute to insulin resistance and cardiovascular risk. Waist circumference is a widely used clinical proxy for visceral fat. The WHO defines abdominal obesity as a waist circumference above 88 cm in women and above 102 cm in men based on European-origin population data, with lower thresholds recommended for Asian populations.
Should I track BMI, BRI, or waist circumference?
Health agencies such as the WHO and NHS recommend tracking BMI alongside waist circumference as complementary screening tools. BRI can be calculated from the same two measurements used for waist-to-height ratio and may offer additional information about body shape, but it is not yet a standard clinical measure. Using multiple indicators rather than any single index gives a more complete picture of body composition, and a healthcare professional can advise on which measures are most relevant for an individual's situation.
Quellenangaben
- World Health Organization. "Obesity: Preventing and Managing the Global Epidemic." WHO Technical Report Series 894. WHO, 2000.
- 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(11):2264-2271.
- World Health Organization. "Waist Circumference and Waist-Hip Ratio: Report of a WHO Expert Consultation." WHO, 2008.
- WHO Expert Consultation. "Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies." The Lancet, 2004;363(9403):157-163.
- NHS. "What is the body mass index (BMI)?" NHS, 2023. www.nhs.uk
- Browning LM, Hsieh SD, Ashwell M. "A systematic review of waist-to-height ratio as a screening tool for the prediction of cardiovascular disease and diabetes: 0.5 could be a suitable global boundary value." Nutrition Research Reviews, 2010;23(2):247-269.