Body mass index is one division problem: mass ÷ height squared. It is cheap, fast, and genuinely useful for comparing large groups. It also cannot tell muscle from fat, cannot see where tissue sits, and was never built to describe an individual. Compute it, then split it apart and watch it misclassify.
One number, two hidden halves
BMI and what it missesmove the sliders to begin
Body mass82 kg
Height178 cm
Body fat, measured22%
This slider is the information BMI never receives. It would come from a method that actually measures tissue — DXA, air-displacement plethysmography, a multi-compartment model. Move it and watch BMI refuse to budge.
The calculation, both ways
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Body mass index—
WHO band, adults 20+—
Splitting that number in two
Fat-free mass—
Fat mass—
Fat-free mass index (FFMI)—
Fat mass index (FMI)—
FFMI + FMI—
Selected body
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WHO band it lands in—
Fat-free mass index—
Fat mass index—
Rank by BMI vs rank by fat mass—
BMI is a screening ratio for populations, not a diagnosis and not a description of a person. The WHO bands shown here apply to adults aged 20 and over; children and adolescents are assessed against age-and-sex percentile charts instead, and the bands are not interpretable during pregnancy or with limb loss, large fluid shifts, or significant edema or ascites. Nothing here is medical or dietary advice, no food is labelled good or bad, and no number on this page says anything about what a body should look like.