Understanding BMI
One number, two centuries old.
What Body Mass Index measures, where it came from, and the places it quietly fails.
The formula.
BMI is weight in kilograms divided by height in metres squared. That's it. A 70 kg adult who is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) ≈ 22.9. Imperial input converts to the same number through the constant 703 — pounds divided by inches-squared, times 703, because 703 happens to be the unit-cancelling factor between (lb/in²) and (kg/m²).
BMI = mass (kg) ÷ height (m)²
Where the bands come from.
The thresholds — under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30 and above obese — are not laws of biology. They were settled by a 1995 World Health Organisation expert consultation that pooled mortality and morbidity data from large-population studies, mostly North American and European. Cut-points are statistical conveniences: the point at which all-cause mortality risk begins to rise meaningfully in the underlying cohort. They are useful for population-level public-health work; they were never designed as individual diagnostic boundaries.
Why height is squared.
Adolphe Quetelet, the 19th-century Belgian astronomer who first proposed the ratio, observed empirically that adult body weight scales roughly with the square of height across a population — not the cube you'd expect from a uniform-density solid. People don't get proportionally wider as they get taller; bone, organ and limb mass grow more slowly than length. Squaring height was the simplest expression that flattened the height-weight relationship enough to be useful as a single number.
A worked example.
A 1.78 m, 82 kg adult. Square the height — 1.78 × 1.78 = 3.1684. Divide weight by that — 82 ÷ 3.1684 ≈ 25.9. The number lands just inside the WHO's "overweight" band, but a long marathon runner with that same height and weight is mostly lean tissue, and a sedentary office worker with the identical reading carries more adipose mass. BMI cannot distinguish those two bodies. That's the headline limitation of the measure.
1.78 m, 82 kg adult
BMI = kg ÷ m²
Square the height first; divide weight by the result.
82 ÷ (1.78 × 1.78) = 82 ÷ 3.1684
= 25.9
5'10" (70 in), 180 lb adult
BMI = (lb ÷ in²) × 703
703 cancels imperial units into the same metric ratio.
(180 ÷ 70²) × 703 = (180 ÷ 4900) × 703
= 25.8
Where it fails quietly.
BMI can't tell muscle from fat: bodybuilders and rugby forwards routinely land in the "obese" range while carrying single-digit body fat. It systematically over-reads risk in shorter people and under-reads it in taller people, because the squared denominator is empirical, not exact. It performs differently across ancestral populations — South Asian bodies tend to carry higher visceral fat at lower BMIs, prompting some clinical guidelines (Singapore, the UK NICE 2013 update) to lower the action thresholds for those groups. And it gives nothing useful for children, pregnant people, athletes, the elderly with sarcopenic body composition, or anyone with limb amputation.
What it's still good for.
Despite all the caveats, BMI persists because it costs nothing, requires no equipment, and is reproducible by anyone with a measuring tape and a scale. At a population level — comparing one cohort's metabolic risk to another's, tracking a country's trajectory over decades, screening a thousand patients for who might warrant further investigation — it remains a workable instrument. The mistake is treating it as a verdict on an individual body. Use it as a starting question, not an answer.
What to measure alongside it.
The cheapest single companion measure is waist circumference: it captures visceral fat, which is the metabolic risk factor BMI was indirectly trying to point at. The WHO's action thresholds are 94 cm (men) and 80 cm (women) for elevated risk, 102 cm and 88 cm for high risk. Waist-to-height ratio works as a rule of thumb across populations — keep waist below half your height. For a fuller picture, body-fat percentage measured by calipers, bio-impedance or DEXA will tell you what BMI structurally cannot.