BMI (Body Mass Index): What It Means and Its Limitations

Body Mass Index (BMI) is calculated by dividing a person's weight in kilograms by the square of their height in metres (kg/m²). It was developed in the 19th century by Belgian statistician Adolphe Quetelet as a population-level statistical tool and was later adopted in clinical medicine as a simple, low-cost screening measure.

The World Health Organization (WHO) classifies BMI as follows for adults: below 18.5 = underweight; 18.5–24.9 = normal weight; 25–29.9 = overweight; 30 and above = obese (with sub-categories of obesity at 30–34.9, 35–39.9, and 40+). Some guidelines suggest lower cutoffs for Asian populations, as health risks may occur at lower BMI values.

BMI is useful as a population screening tool but has important limitations. It does not distinguish between fat mass and muscle mass — a muscular athlete may have a high BMI without excess fat. It does not account for fat distribution (visceral abdominal fat is more metabolically harmful than subcutaneous fat). It does not reflect fitness level, bone density, or metabolic health. Waist circumference, waist-to-hip ratio, and body fat percentage measurement provide complementary information. In clinical practice, your doctor uses BMI alongside other findings — not as a standalone diagnosis.