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What Is BMI?
Published Apr 17, 2026
BMI (Body Mass Index) is a numerical value derived from a person's weight and height. It is widely used by healthcare professionals as a quick screening tool to categorise people into weight ranges that may correlate with health risks.
How BMI Is Calculated
The formula is the same worldwide — only the units differ:
| Units | Formula |
|---|---|
| Metric | BMI = weight (kg) ÷ height² (m²) |
| Imperial | BMI = 703 × weight (lb) ÷ height² (in²) |
Example (metric): A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9
BMI Categories (WHO)
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity (Class I) |
| 35.0 – 39.9 | Obesity (Class II) |
| 40.0 and above | Severe obesity (Class III) |
Different thresholds apply in some countries. For example, health authorities in several Asian countries use a lower "overweight" cut-off of 23.0 because research shows that metabolic risks occur at lower BMI values in those populations.
Limitations of BMI
BMI is useful at population level but has well-known shortcomings when applied to individuals:
- Does not distinguish fat from muscle. A highly muscular athlete may have a BMI in the "overweight" range while carrying very little body fat.
- Ignores fat distribution. Visceral fat (around the abdomen) carries more risk than subcutaneous fat, but BMI cannot detect the difference.
- Age and sex variation. Older adults tend to have more fat at the same BMI; women generally carry more fat than men at equal BMI.
- Ethnic differences. Metabolic risk profiles differ across ethnic groups at the same BMI.
Clinicians often supplement BMI with waist circumference, body fat percentage (DEXA, bioelectrical impedance), or waist-to-hip ratio.
Healthy BMI vs Healthy Body
A BMI in the "normal" range does not guarantee good health, and a BMI outside that range does not guarantee poor health. Regular physical activity, a balanced diet, adequate sleep, and not smoking matter alongside any weight-related metric.
Use the BMI Calculator to find your BMI and paired healthy-weight range.