BMI Explained: What the Number Means and When to Ignore It

BMI = weight (kg) ÷ height (m)². It was devised in the 1830s by Belgian mathematician Adolphe Quetelet as a way to describe body weight across a population, not to assess any one individual — a distinction that explains most of its modern criticism.

The reason it squares height rather than using it directly: body mass roughly scales with the square of height across a population (taller people are proportionally wider as well as taller), so dividing by height² produces a number that doesn't systematically increase or decrease with height alone. It's a statistical convenience, not a measurement of body composition.

The WHO's widely used bands for adults are: below 18.5 is underweight, 18.5–24.9 is normal weight, 25–29.9 is overweight, and 30 and above is obese.

Worked example: someone who is 172 cm (1.72 m) tall and weighs 70 kg has a BMI of 70 ÷ (1.72 × 1.72) = 70 ÷ 2.9584 ≈ 23.7 — inside the normal range, close to its upper edge.

Note the categories shift for some populations: the WHO's own guidance lists lower cutoffs for several Asian populations (overweight from 23, obese from 27.5) because health risk at a given BMI is observed to rise earlier in these groups — the same number of 23.7 that reads as 'normal' by the general cutoffs sits inside 'overweight' by the Asian-specific ones.

BMI cannot distinguish muscle from fat, only total mass relative to height. A muscular athlete and a sedentary person of the same height and weight get the identical BMI, despite very different body composition and health risk — this is the single most common real-world failure of the metric.

It's also considered less reliable for children and teenagers (who need age- and sex-specific percentile charts instead of adult cutoffs), older adults (where some muscle loss and bone density change the relationship between weight and health risk), and pregnant people (where weight gain is expected and healthy).

Because of this, most clinicians treat BMI as a fast screening step, not a diagnosis — a signal to look closer, alongside measures like waist circumference or waist-to-height ratio, rather than a verdict on its own.

Waist circumference and waist-to-height ratio track abdominal fat specifically, which correlates more directly with cardiovascular and metabolic risk than total body mass does. A common rule of thumb is keeping waist circumference under half your height.

Body fat percentage (from skinfold calipers, bioelectrical impedance scales, or DEXA scans) directly measures what BMI can only infer — useful specifically for the muscular-but-'overweight-by-BMI' case described above.