What is BMI (Body Mass Index)?
BMI (Body Mass Index) is a screening metric for body weight relative to height, defined as weight in kilograms divided by height in metres squared (kg / m^2). The World Health Organization (WHO) classifies adult BMI into underweight (under 18.5), normal weight (18.5 to 24.9), overweight (25 to 29.9), and obese (30 and above), with separate lower cutoffs for Asian-Pacific populations because of higher body-fat percentages at the same BMI.
Detailed definition
The Body Mass Index was originally devised by Belgian astronomer, mathematician, and statistician Adolphe Quetelet between 1830 and 1850, then called the Quetelet Index. It was rediscovered in modern epidemiology by physiologist Ancel Keys in his 1972 paper Indices of relative weight and obesity, where Keys explicitly endorsed weight/height^2 over alternatives (weight/height, weight/height^3) because it correlated best with measured body density in the cohorts he studied. WHO formally adopted the term BMI and standardised the adult cutoff categories in its 1995 Expert Committee report on physical status.
BMI is fundamentally a screening tool for use at the population level. Public health departments use it to track obesity prevalence; clinicians use it as a first check before more accurate body-composition assessment. It is not a diagnostic test. A 95 kg rugby player at 1.85 m and a 95 kg sedentary office worker at 1.85 m have the same BMI of 27.8 (overweight) but vastly different cardiometabolic risk profiles. For this reason, organisations like the American Medical Association, in 2023, formally cautioned against using BMI as the sole measure of individual health.
The Asian-Pacific cutoff variant exists because South Asian, Chinese, Japanese, and Southeast Asian populations have a different relationship between BMI and body-fat percentage. At the same BMI of 25, an Indian adult typically carries 3 to 5 percentage points more body fat than a European adult, raising diabetes and cardiovascular risk earlier on the scale. WHO's 2004 Expert Consultation in Singapore set the lower thresholds (overweight at 23, obese at 27.5), and India's Ministry of Health and Family Welfare adopted these in its 2017 obesity management guidelines.
Formula
Metric: BMI = weight (kg) / height (m)^2 Imperial: BMI = (weight in pounds x 703) / height (inches)^2 Example (metric): Weight = 70 kg, Height = 1.70 m BMI = 70 / (1.70 x 1.70) = 70 / 2.89 = 24.22 (Normal, WHO) Example (imperial): Weight = 154 lb, Height = 67 inches BMI = (154 x 703) / (67 x 67) = 108,262 / 4,489 = 24.12 (Normal)
- Weight is total body weight, measured first thing in the morning after voiding for the most reliable reading.
- Height is standing height in metres or inches, measured without shoes, heels and head touching the wall.
- The squared denominator makes the index scale-invariant for the height range of most adults (roughly 1.5 m to 2.0 m).
- For children under 18, use BMI-for-age percentiles from CDC or WHO growth charts instead of adult cutoffs.
WHO BMI categories
| Category | WHO international (kg/m^2) | WHO Asian-Pacific (kg/m^2) | Health risk |
|---|---|---|---|
| Underweight (severe) | Below 16 | Below 16 | High (malnutrition, immune suppression) |
| Underweight (moderate) | 16 to 16.9 | 16 to 16.9 | Moderate |
| Underweight (mild) | 17 to 18.4 | 17 to 18.4 | Low to moderate |
| Normal weight | 18.5 to 24.9 | 18.5 to 22.9 | Lowest |
| Overweight (pre-obese) | 25 to 29.9 | 23 to 27.4 | Increased |
| Obese class I | 30 to 34.9 | 27.5 to 32.4 | Moderate |
| Obese class II | 35 to 39.9 | 32.5 to 37.4 | Severe |
| Obese class III (morbid) | 40 and above | 37.5 and above | Very severe |
Worked example
Suppose Rahul is 32, Indian, weighs 78 kg, and is 1.72 m (5 feet 8 inches) tall. He wants to know whether he is overweight by either standard.
- Height squared: 1.72 x 1.72 = 2.9584 m^2.
- BMI: 78 / 2.9584 = 26.4 kg/m^2.
- WHO international classification: 26.4 is between 25 and 29.9, so Rahul is Overweight (pre-obese).
- WHO Asian-Pacific classification: 26.4 is between 23 and 27.4, so Rahul is also Overweight on the Asian-Pacific scale, sitting in the upper half of that band.
- Target BMI of 23 (Asian-Pacific normal): 23 x 2.9584 = 68 kg. Rahul needs to lose 10 kg to reach the high end of the normal range.
- Health add-on: measure waist circumference. Above 90 cm for Asian men adds an Indication of Abdominal Obesity per the South Asian guidelines, regardless of BMI.
Limitations and what to use alongside BMI
- Muscle vs fat. BMI cannot distinguish lean from fat mass. A bodybuilder at 1.80 m and 95 kg has a BMI of 29.3 (overweight) but a body fat percentage under 12 percent. Use body-fat percentage (skinfold, bioimpedance, or DEXA) for athletes.
- Fat distribution. Two people with the same BMI can have very different visceral (organ-surrounding) fat. Visceral fat is the main driver of insulin resistance and cardiovascular risk. Waist-to-hip ratio (above 0.90 men, 0.85 women is high risk per WHO) and waist circumference are more sensitive.
- Age. Older adults (above 65) lose lean mass (sarcopenia) and may stay at the same BMI while body composition worsens. The optimal BMI range for elderly skews upward to about 25-27 in several large cohort studies.
- Sex. Women carry more body fat at the same BMI than men. Sex-specific body-fat targets are healthier metrics: roughly 21-32 percent body fat for women, 10-22 percent for men under 40 (American Council on Exercise).
- Ethnicity. South Asian, Chinese, and other populations have higher cardiometabolic risk at lower BMI, hence the Asian-Pacific cutoffs. Pacific Islander and African heritage populations may have higher healthy BMI ranges due to higher lean mass.
- Pregnancy and child growth. BMI is not validated for pregnant women, infants, or children. Use BMI-for-age percentile charts instead for under-18s.
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Frequently asked questions
What is BMI and what is the formula?
BMI, or Body Mass Index, is a population-level screening metric for body weight relative to height. The formula is BMI = weight (kg) / height (m)^2. In imperial units it becomes BMI = (weight in pounds x 703) / height (inches)^2. For example, a person who weighs 70 kg and is 1.70 m tall has a BMI of 70 / (1.70 x 1.70) = 24.2, which falls in the WHO 'normal weight' range of 18.5 to 24.9.
What are the WHO BMI categories?
The WHO categorises adult BMI as: Underweight below 18.5 (severe under 16, moderate 16 to 16.9, mild 17 to 18.4); Normal weight 18.5 to 24.9; Overweight 25 to 29.9 (also called pre-obese); Obese class I 30 to 34.9, class II 35 to 39.9, class III 40 and above (severe or morbid obesity). These cutoffs have been the international standard since the 1995 WHO Expert Committee report and remain unchanged.
Why are BMI cutoffs different for Asian populations?
The 2004 WHO Expert Consultation found that Asian populations carry higher percentages of body fat and a higher cardiometabolic risk at lower BMI values than European populations. The Asian-Pacific cutoffs are therefore: overweight 23 to 27.4 (instead of 25 to 29.9) and obese 27.5 and above (instead of 30 and above). India's Ministry of Health uses 23 as the overweight cutoff and 25 as the obesity cutoff in its 2017 obesity guidelines.
What are the limitations of BMI?
BMI is a screening tool, not a diagnosis. It does not distinguish muscle from fat, so muscular athletes are often misclassified as overweight or obese (Arnold Schwarzenegger at his peak weighed 107 kg at 1.88 m, giving a BMI of 30.3). It does not capture fat distribution; visceral fat around the abdomen is a much greater health risk than subcutaneous fat at the same total BMI. It also does not adjust for age, sex (women carry more body fat at the same BMI), ethnicity (as the Asian-Pacific cutoffs show), or pregnancy. Always combine BMI with waist circumference, body fat percentage, or DEXA scan for a clearer picture.
What is a healthy BMI range?
For most adults of European descent, the WHO considers 18.5 to 24.9 the healthy range, with the lowest all-cause mortality typically observed near 22 to 24 in large epidemiological studies (Global BMI Mortality Collaboration, Lancet 2016, over 10 million participants). For Asian-Pacific adults, the equivalent healthy range is roughly 18.5 to 22.9. Above 25 (or 23 for Asian-Pacific), risks for type 2 diabetes, hypertension, cardiovascular disease, and certain cancers rise progressively.
Is BMI accurate for kids and elderly?
Adult BMI categories do not apply to children. The CDC uses BMI-for-age percentile charts (under 5th = underweight, 5th-85th = healthy, 85th-95th = overweight, 95th and above = obese) instead of absolute thresholds. For elderly adults (65 and above), several studies suggest the optimal BMI range shifts up to about 25 to 27, because moderate body reserve protects against sarcopenia and faster recovery from illness. Always interpret BMI with age and clinical context.
Sources and further reading
- World Health Organization (1995), Physical status: the use and interpretation of anthropometry, WHO Technical Report Series 854. The official source of the adult BMI cutoffs.
- WHO Expert Consultation (2004), Appropriate body-mass index for Asian populations, Lancet 363(9403): 157-163 - established the lower Asian-Pacific cutoffs.
- Global BMI Mortality Collaboration (2016), Body-mass index and all-cause mortality: individual-participant-data meta-analysis of 239 prospective studies in four continents, Lancet 388(10046): 776-786.
- India Ministry of Health and Family Welfare (2017), Standard Treatment Guidelines for Management of Obesity, AIIMS/MoHFW.
- CDC, About Adult BMI - US public-health interpretation and BMI-for-age charts for children.