What Is BMI?
Body Mass Index (BMI) is a numerical value calculated from your height and weight. It serves as a simple screening tool used to categorise adults as underweight, normal weight, overweight, or obese. Developed by Belgian mathematician Adolphe Quetelet in the 19th century, BMI is now used worldwide in clinical practice and public health research as a quick, low-cost proxy for body fatness.
The World Health Organization (WHO) endorses BMI as the primary tool for tracking overweight and obesity trends at a population level. At the individual level, however, BMI has meaningful limitations that make it an imperfect measure of health — which is why it should always be interpreted alongside other clinical information.
How to Calculate Your BMI
The BMI formula is simple: divide your weight in kilograms by the square of your height in metres.
- Formula: BMI = weight (kg) ÷ height (m)²
- Example: 70 kg ÷ (1.75 m)² = 70 ÷ 3.0625 = 22.9
This result of 22.9 falls in the Normal weight category. The calculation is identical for men and women, which is itself one of BMI's limitations — women naturally carry a higher body fat percentage than men at the same BMI. For children and teenagers, BMI must be interpreted using age- and sex-specific percentile charts rather than the fixed adult thresholds.
BMI Categories
The WHO defines the following standard BMI ranges for adults aged 18 and over:
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Increased (malnutrition, bone loss) |
| 18.5–24.9 | Normal weight | Lowest risk |
| 25.0–29.9 | Overweight | Mildly increased |
| 30.0–34.9 | Obesity class I | Moderately increased |
| 35.0–39.9 | Obesity class II | Severely increased |
| 40.0 and above | Obesity class III | Very severely increased |
For people of South Asian, East Asian, or Middle Eastern descent, the WHO recommends lower thresholds: overweight at BMI = 23, obesity at BMI = 27.5. These populations tend to have higher central fat deposition at a given BMI compared to European populations.
Important Limitations of BMI
BMI is a useful population-level screening tool, but it has well-documented limitations at the individual level that are important to understand before drawing health conclusions:
- It cannot distinguish fat from muscle. Athletes and highly trained individuals often have a high BMI due to lean muscle mass despite having low body fat. A professional rugby player at 100 kg and 180 cm has a BMI of 30.9 — classified as Obesity I — despite minimal body fat.
- It ignores fat distribution. Visceral (abdominal) fat is far more metabolically harmful than subcutaneous fat. Waist-to-height ratio is a stronger predictor of cardiovascular risk than BMI alone.
- It varies by ethnicity. Optimal BMI thresholds differ by up to 3–5 points between ethnic groups, and fixed universal cut-offs can misclassify risk in some populations.
- It does not capture fitness level. A sedentary person at BMI 22 may carry far greater metabolic risk than an active person at BMI 27. Cardiorespiratory fitness is one of the strongest predictors of all-cause mortality.
For a more complete picture of body composition, use BMI alongside our TDEE Calculator to understand your calorie needs, and consider consulting a healthcare professional for clinical interpretation. Use the BMR Calculator to find your base metabolic rate, or calculate your daily protein needs with our Protein Intake Calculator.
Scientific References: World Health Organization. (2000). Obesity: preventing and managing the global epidemic. WHO Technical Report Series, No. 894. — Keys A, et al. (1972). Indices of relative weight and obesity. J Chronic Dis. 25(6–7):329–343.