BMI (Body Mass Index) is a number calculated from your height and weight that provides a rough indicator of body fatness at a population level. It's widely used in public health because it requires only two easy measurements. Its limitations are equally worth knowing.
How BMI Is Calculated
BMI = Weight (kg) ÷ Height (m)²
Example: 68 kg, 1.70 m tall BMI = 68 ÷ (1.70 × 1.70) = 68 ÷ 2.89 = 23.5
The Standard Categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese (Class I) |
| 35.0 – 39.9 | Obese (Class II) |
| 40.0 and above | Severely obese (Class III) |
What BMI Can Tell You
At a population level, BMI correlates with body fat percentage and with risk of conditions like type 2 diabetes, cardiovascular disease and hypertension. For large-scale public health research and general screening, it's useful precisely because it's simple and requires no special equipment.
What BMI Cannot Tell You
It doesn't distinguish fat from muscle. A competitive rugby player and a sedentary person of the same height and weight have the same BMI, despite very different body compositions and health profiles.
It doesn't show fat distribution. Where fat is located matters — visceral fat (around the organs, measured by waist circumference) carries different health risks than subcutaneous fat (under the skin).
South Asian populations may face higher risk at lower BMIs. Research suggests that people of South Asian descent (including Pakistanis) may have higher health risks at BMI levels classified as "healthy" for European populations. Some health bodies recommend using lower thresholds for South Asian patients.
Complementary Measures
If BMI is concerning or ambiguous, additional measures provide more information:
- Waist circumference: over 94 cm (men) or 80 cm (women) is associated with increased health risk.
- Waist-to-hip ratio
- Body fat percentage (requires measurement beyond height and weight)
Use BMI Calculator for quick calculation. For any health concerns, a doctor or healthcare professional's assessment is always more informative than any single number.
BMI for Children
The standard BMI ranges above apply only to adults (over 20 years old). For children and teenagers, BMI is age and sex-specific (called BMI-for-age percentiles) because healthy body composition changes as children grow. A doctor or paediatrician calculates child BMI differently. For adults using BMI Calculator, the result is a starting point for a health conversation, not a diagnosis. Interpret it alongside other measures and with a healthcare professional's guidance.
