BMI is a useful, free population screen — but it measures weight relative to height, not body composition. For two groups in particular, that blind spot matters.
BMI can't tell muscle from fat. Because muscle is dense, a lean, muscular athlete often lands in the "overweight" or even "obese" BMI band despite low body fat. The number is technically correct and practically meaningless for them — they're heavy because of muscle, not excess fat. For this group, a body-fat measurement or how they look and perform tells the real story. See BMI vs body fat.
The opposite problem appears with ageing. Older adults often lose muscle and gain fat while their weight — and therefore BMI — stays the same. A "normal" BMI can hide low muscle mass (sarcopenia) and a higher proportion of body fat than the number suggests. BMI also doesn't capture where fat sits; abdominal fat carries more risk, which is why waist measurement adds useful information.
For the large middle of the population — adults of average build who aren't highly muscular or very old — BMI remains a reasonable, zero-cost first screen, and it tracks meaningful weight change over time well. Its strength is that it's hard to get wrong and needs no equipment.
If you're muscular or older, pair BMI with a body-fat estimate and a waist measurement, and weight your judgement toward those. Our calculator shows BMI with WHO categories and accepts a body-fat figure to switch BMR to the lean-mass Katch-McArdle formula — a better fit when composition is the issue.
BMI measures weight for height and can't distinguish muscle from fat. Muscular athletes commonly read as overweight despite low body fat. A body-fat measurement is more informative for them.
It can miss risk. Older adults may lose muscle and gain fat at the same weight, so a normal BMI can hide low muscle mass and higher body fat. Pair it with body-fat and waist measurements.