What BMI Does and Doesn’t Tell You

BMI is popular because it only needs a scale and a tape measure. That simplicity is also its weakness: it can't tell fat from muscle, and it treats a 25-year-old athlete the same as a sedentary 70-year-old of the same height and weight.

What it misses

• Body composition. Two people with the same BMI can have very different body fat percentages. A muscular person is often classified "overweight" despite low body fat.
• Where fat is carried. Fat around the waist carries more health risk than fat elsewhere, and BMI can't see the difference — waist circumference or waist-to-hip ratio can.
• Age and muscle loss. Older adults naturally lose muscle. A BMI in the "healthy" range can hide low muscle mass (sarcopenia) that's its own health concern.
• Pregnancy. BMI categories aren't meant to be applied during pregnancy.

What to check alongside it

Waist circumference, waist-to-hip ratio, and (where available) a body-fat-percentage measurement all add information BMI can't provide on its own. None of these — including BMI — are a diagnosis; they're screening numbers to discuss with a doctor if you're concerned, not a verdict.

None of this means BMI is useless — for most adults, at a population level, it correlates reasonably well with health risk, which is exactly why it's still the standard first screen. It's just not the last word for any one individual. Back to the BMI calculator.