BMI is the most-cited health metric in the world — and one of the most misunderstood. Knowing what it does and doesn't measure makes it useful instead of misleading.
The formula
BMI = weight (kg) ÷ height (m)². Imperial: 703 × weight (lb) ÷ height (in)². Categories: under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30+ obese.
Where BMI breaks
It can't distinguish muscle from fat, so athletes register as 'overweight'. It ignores body-fat distribution, which matters more for cardiovascular risk than total weight.
Better complements
Waist circumference, waist-to-hip ratio, and body-fat percentage give a much sharper picture together than BMI alone.