What BMI measures — and what it doesn't
BMI divides weight by height squared: kilograms ÷ meters². It was designed as a quick population-level indicator of whether people are underweight, normal weight, overweight or obese — and that is all it is. It does not measure body fat directly, and it cannot tell muscle from fat.
A muscular athlete can easily show a BMI over 25 and still be healthy; an older adult with low muscle mass can show a "normal" BMI while overweight in body fat terms. For your own health, BMI is a starting point — combine it with waist measurement, activity level and a conversation with a professional.
WHO categories for adults
- Under 18.5 — underweight
- 18.5–24.9 — normal
- 25–29.9 — overweight
- 30 and above — obese