Body
Body mass index
BMI screens whole populations well and individuals poorly. It is weight over height squared — it cannot tell muscle from fat, and it says nothing about where that mass sits.
Result
Breakdown
Worth knowing
- BMI is not valid during pregnancy, for under-20s, or for highly muscular people — it reads dense muscle as excess weight.
- Adding a waist measurement gives you waist-to-height ratio, a better single predictor than BMI.
How this is calculated
BMI = kg / m². Classification follows the WHO international cut-offs, or the WHO Asian-Pacific cut-offs when selected. Healthy weight range is the BMI band inverted back to kilograms at your height.
Common questions
Why does BMI call me overweight when I don't look it?
BMI is weight over height squared, and nothing in that formula can tell muscle from fat. Dense muscle reads as excess weight, which is why athletes and regular lifters are routinely misclassified. If your waist-to-height ratio is under 0.5, the BMI figure is probably telling you about your muscle, not your health.
Why are the Asian-Pacific cut-offs lower?
Cardiometabolic risk starts rising at a lower BMI in South and East Asian populations, so the WHO publishes action points at 23 and 27.5 rather than 25 and 30. The standard cut-offs were derived largely from European populations. Neither set is more correct in the abstract — they are calibrated to different reference groups.
Can I use this for my child?
No. Children are assessed with BMI-for-age percentiles against CDC or WHO growth references, because a healthy BMI changes year by year through growth. An adult BMI category applied to anyone under 20 is meaningless.
Is BMI valid during pregnancy?
No. Use your pre-pregnancy BMI with the pregnancy weight gain calculator instead — that is the number the IOM guidelines are keyed to.