Medical AIBody mass index (BMI) calculator
BMI is weight in kilograms divided by height in metres squared. It is a quick population screen for weight status, not a measure of body fat or health on its own. For people of South Asian and other Asian ancestry, risk rises at lower BMIs, so the lower WHO Asia-Pacific cut points are shown alongside the standard bands.
How to measure each input
- Weight
- Measure on a calibrated scale, ideally with light clothing and shoes removed. Use kilograms; if you weigh in pounds, divide by 2.205.
- Height
- Measure standing, without shoes, looking straight ahead. Use centimetres; if you measure in inches, multiply by 2.54. The calculator converts to metres internally.
- Choosing the right cut points
- For patients of South Asian, Chinese, or other Asian ancestry, interpret against the WHO Asia-Pacific action points (overweight from 23, obesity from 27.5) or the Misra 2009 Asian-Indian consensus (overweight 23 to 24.9, obesity >= 25), not only the standard WHO bands.
Interpretation
| Band | Meaning |
|---|---|
| Underweight (< 18.5) | Below the WHO normal range. May reflect undernutrition, an eating disorder, or another illness, and warrants assessment of intake and cause rather than reassurance alone. |
| Normal weight (18.5 to 24.9) | Within the WHO normal range for most populations. Note: for people of Asian ancestry, cardiometabolic risk already begins to rise from a BMI of about 23, so see the Asia-Pacific category above. |
| Overweight (25.0 to 29.9) | WHO overweight (pre-obesity). Associated with increased cardiometabolic risk. For Asian populations this risk threshold is reached earlier, from a BMI of about 23. |
| Obesity class I (30.0 to 34.9) | WHO obesity class I. Consider waist circumference, comorbidities, and a structured weight-management plan. |
| Obesity class II (35.0 to 39.9) | WHO obesity class II. Substantially elevated risk of type 2 diabetes, cardiovascular disease, and other complications; intensified management is usually indicated. |
| Obesity class III (>= 40) | WHO obesity class III (severe obesity). High complication risk; comprehensive management including consideration of pharmacotherapy or bariatric referral is often warranted. |
Pitfalls, exclusions and caveats
- BMI does not distinguish muscle from fat or describe fat distribution. Muscular athletes can be misclassified as overweight, and people with low muscle mass (including many older adults) can have normal BMI yet high body fat.
- Lower cut points apply to Asian populations. The 2004 WHO expert consultation identified public-health action points at 23.0, 27.5, 32.5, and 37.5 kg/m^2, and recommended a 27.5 kg/m^2 trigger for lifestyle intervention in South Asian and Chinese populations. The Misra 2009 Asian-Indian consensus defines overweight as 23 to 24.9 and obesity as 25 or above.
- BMI is a screen, not a diagnosis. It does not capture metabolic health; pair it with waist circumference, blood pressure, lipids, and glucose where weight is a clinical concern.
- It is not valid as a standalone tool in pregnancy, in children and adolescents (use age- and sex-specific charts), or in people with significant oedema, ascites, or amputations.
- Standard WHO bands were derived largely from European-ancestry populations and can under- or over-estimate adiposity-related risk in other groups.
BMI = weight (kg) / height (m)^2. WHO bands: underweight < 18.5, normal 18.5 to 24.9, overweight 25.0 to 29.9, obesity class I 30.0 to 34.9, class II 35.0 to 39.9, class III >= 40. WHO Asia-Pacific action points: overweight >= 23, obesity >= 27.5. Misra 2009 Asian-Indian consensus: overweight 23 to 24.9, obesity >= 25.Frequently asked
Why are the cut-offs lower for Asian people?
At any given BMI, people of South Asian and other Asian ancestry tend to carry more body fat (especially abdominal fat) and face higher cardiometabolic risk than European-ancestry populations. The 2004 WHO expert consultation therefore set lower action points (overweight from 23, obesity from 27.5), and the 2009 Misra consensus defines obesity in Asian Indians as a BMI of 25 or above.
Is a normal BMI a guarantee of good health?
No. BMI does not measure body fat distribution or metabolic health. Someone with a normal BMI can still have a high waist circumference, high blood pressure, or abnormal glucose and lipids, particularly in Asian populations. Use BMI alongside other measures.
Can I use BMI for my child or during pregnancy?
No. Adult BMI categories do not apply to children and adolescents (use age- and sex-specific growth charts) or to pregnancy. The thresholds here are for non-pregnant adults.
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