BMI: What the Number Means and Where It Fails

What BMI is

BMI is weight in kilograms divided by height in metres squared. A person 1.75 m tall weighing 70 kg has a BMI of 22.9. The WHO categories are: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, 30 and above obese.

It was devised in the 1830s by Adolphe Quetelet, a Belgian statistician who was describing populations, not diagnosing individuals. He said so explicitly. The measure was renamed 'body mass index' in 1972 by Ancel Keys, who found it the best of the available height-weight ratios for population studies — and who also warned against using it on individuals.

Why the square, and why it is a compromise

Mass scales with volume, which scales with the cube of a linear dimension, so a geometrically similar person should scale as height cubed. Real human bodies do not scale geometrically: we get proportionally wider rather than taller. The exponent that best fits observed data sits between 2 and 3, and 2 was chosen because it made the arithmetic possible in 1832.

The practical consequence is a known bias: BMI slightly overestimates fatness in tall people and underestimates it in short people. It is a small effect, but it is systematic rather than random.

Where it fails, specifically

Muscular people. BMI cannot distinguish muscle from fat. A 1.80 m rugby player at 95 kg scores 29.3 — 'overweight' — at 12% body fat. Roughly half of NFL players are classified obese by BMI.

Older adults. Muscle mass declines with age while fat rises, so an unchanged BMI can conceal a substantially worse body composition. Several large studies find the lowest mortality in over-65s at a BMI of 25–27, above the 'healthy' band.

Different ancestries. Health risk appears at lower BMI thresholds in South Asian and East Asian populations, which is why the WHO publishes lower Asian cut-offs (23 for overweight, 27.5 for obese) and the UK's NICE guidance uses them.

Anyone very short or very tall, because of the exponent problem above.

What to use alongside it

Waist circumference is the single most useful addition: over 94 cm for men or 80 cm for women signals raised risk regardless of BMI. Waist-to-height ratio — keep your waist under half your height — is simpler still and performs at least as well in most studies. Body fat percentage adds the composition BMI cannot see.

None of this makes BMI useless. It is free, takes five seconds, and at population scale it tracks health outcomes well. It is a screening tool that flags a question, not a measurement that answers one.

Check yours in context

The BMI calculator shows your healthy weight range alongside the number, and the waist-to-hip ratio calculator adds the distribution measure BMI cannot capture.