How BMI is calculated
Body Mass Index expresses weight relative to height using a simple ratio. It does not measure body fat directly but correlates well with it at a population level.
Example: a 70 kg adult who is 172 cm (1.72 m) tall has a BMI of 70 ÷ 1.72² ≈ 23.7 kg/m².
Normal values & classification
| BMI (kg/m²) | Category |
|---|---|
| < 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese Class I |
| 35.0 – 39.9 | Obese Class II |
| ≥ 40.0 | Obese Class III (severe) |
These are the WHO global cutoffs for adults. Some populations (e.g. South Asian ancestry) have higher cardiometabolic risk at lower BMI thresholds — many guidelines suggest lowering the "overweight" cutoff to 23 kg/m² in these groups.
Clinical use
BMI is a quick population-level screening tool for weight-related health risk (type 2 diabetes, hypertension, cardiovascular disease). It is not diagnostic for an individual: it cannot distinguish fat mass from muscle mass, doesn't account for fat distribution (visceral vs. subcutaneous), and is less accurate in athletes, older adults with sarcopenia, and pregnancy. Waist circumference or waist-to-hip ratio are useful adjuncts when body composition matters clinically.
Frequently asked questions
What is a normal BMI?
A BMI between 18.5 and 24.9 kg/m² is classified as normal weight by WHO criteria for adults. Below 18.5 is underweight, 25–29.9 is overweight, and 30+ is obese.
How is BMI calculated?
BMI = weight in kilograms divided by height in metres squared (kg/m²). It's a simple ratio, not a direct measurement of body fat.
Is BMI accurate for muscular or very athletic people?
Not reliably — BMI can't distinguish muscle mass from fat mass, so a muscular person can show a "high" BMI despite having low body fat. Waist circumference or body composition testing are better in this group.
What BMI is considered overweight?
25.0–29.9 kg/m² is classified as overweight under the standard WHO cutoffs; some guidelines lower this threshold for South Asian populations given their higher cardiometabolic risk at the same BMI.
Does a healthy BMI range differ by ethnicity?
Yes — several guidelines recommend lower BMI cutoffs for South Asian and other Asian populations (e.g. overweight starting at 23 kg/m² rather than 25) because cardiometabolic risk rises at lower BMI values in these groups.
References
- World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894, 2000.
- WHO Expert Consultation. Appropriate body-mass index for Asian populations. Lancet. 2004;363(9403):157–163.
- CDC. About Adult BMI. cdc.gov/healthyweight/assessing/bmi