BMI — weight ÷ height² — was devised in the 1830s by a Belgian astronomer studying population averages. It was never designed to judge individuals, and using it that way produces famous absurdities: most elite rugby players are "obese".
What BMI Cannot See
- Composition: muscle is ~18% denser than fat; a lean 90kg athlete and a sedentary 90kg office worker share a BMI and nothing else
- Distribution: visceral fat around organs drives metabolic risk far more than fat on hips — invisible to a scale
- Age: healthy older adults benefit from slightly higher BMIs; muscle loss can mask fat gain at a "stable" number
- Ethnicity: risk thresholds differ — NICE applies lower cut-offs (23 overweight, 27.5 obese) for South Asian, Chinese and Black African populations
Better Single Checks
Waist-to-height ratio: keep your waist under half your height — outperforms BMI at predicting cardiometabolic risk in meta-analyses and needs one tape measure. Waist circumference alone works too: elevated risk above 94cm (men) / 80cm (women).
The Fair Verdict
For most non-athletic adults, BMI is a reasonable first screen — cheap, standard, trend-trackable. Treat it as one dial on the dashboard alongside waist, fitness, blood pressure and bloods, not the verdict. A number invented before the telephone deserves exactly that much authority.