BMI (Body Mass Index)
body-metricA screening tool calculated as weight(kg) / height(m)^2.
Definition
A screening tool calculated as weight(kg) / height(m)^2. Categories: underweight (<18.5), normal (18.5-24.9), overweight (25-29.9), obese (30+). Does not directly measure body fat.
What Is Body Mass Index?
Body Mass Index (BMI) is a numerical value derived from a person's weight and height, used as a proxy for body fatness and to screen for weight-related health risks at the population level. Developed by Adolphe Quetelet in the 19th century, BMI remains one of the most widely used screening tools in clinical medicine and public health due to its simplicity and low cost.
How BMI Is Calculated
The formula is straightforward:
- Metric: BMI = weight (kg) ÷ height (m)2
- Imperial: BMI = 703 × weight (lb) ÷ height (in)2
Standard classification for adults (WHO):
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
For Asian populations, some health authorities use lower thresholds (overweight at BMI 23, obese at BMI 27.5) because cardiometabolic risk increases at lower BMI values in these groups.
Limitations and Clinical Relevance
BMI does not measure body fat directly and cannot distinguish between fat mass and lean mass. A muscular athlete may register as "overweight" or "obese" despite very low body fat, while an older sedentary person with normal BMI may carry excess visceral fat. BMI also does not indicate fat distribution, which is a strong independent predictor of metabolic disease. Despite these limitations, BMI remains a useful first-line population screening tool because it correlates reasonably well with body fat levels across large groups and predicts risk of type 2 diabetes, cardiovascular disease, hypertension, and certain cancers. It should always be interpreted alongside other measurements such as waist circumference, body fat percentage, and metabolic blood markers.