BMI (Body Mass Index)

Body mass index is calculated by dividing weight (kg) by height squared (m2). For example, a person who is 170 cm tall and weighs 75 kg has a BMI of 75 / (1.70 x 1.70) = 26.0. BMI classifications are: underweight below 18.5, normal weight 18.5-24.9, overweight 25-29.9, obesity class I 30-34.9, class II 35-39.9, and class III or morbid obesity 40 or above.

BMI is a population-level tool originally developed in the 1800s for statistical purposes. Its greatest weakness is that it doesn't distinguish between muscle and fat mass. A muscular athlete can be overweight on the BMI scale without any real health risk, while an inactive person can be normal weight but unhealthy in terms of body composition (so-called 'skinny fat').

BMI also doesn't indicate where fat is distributed. Visceral fat accumulating around the waist is considerably more dangerous for health than fat in the hips and thighs. This is why waist circumference is a better individual health indicator than BMI. For women, a waist circumference over 80 cm and for men over 94 cm indicates elevated risks.

Practical tip: use BMI as a rough starting point, but don't let it alone define your goals. Combine it with waist circumference, body composition measurements, and how you feel. BMI is closely related to the concepts of overweight, obesity, and body composition.

Last updated: March 30, 2026