What Is BMI, and What It Cannot Tell You
BMI is a screening tool, not a diagnosis. Here is what the number means, where it works, and where it misleads.
2026-07-14 · 5 min read
The formula
BMI divides weight in kilograms by height in meters squared. It was designed to describe populations, not to diagnose individuals, and it remains the fastest screening measure available in a clinic.
Where it breaks down
BMI cannot distinguish muscle from fat, or say where fat is stored. Muscular athletes routinely score as overweight; older adults with low muscle mass can score as healthy while carrying substantial visceral fat.
Better together
Pair BMI with waist circumference or a waist-to-height ratio below 0.5. Two cheap measurements together give a far better picture than either alone.