BMI calculator
Body mass index in metric or imperial, with an honest note on its limits.
Educational only — BMI is a population screening tool, not a diagnosis.
Healthy weight
BMI 18.5 to 24.9
kg/m³ — scales better at extreme heights
How it works
Body mass index divides weight by the square of height. It was devised by Adolphe Quetelet in the 1830s as a statistical tool for characterising populations — he explicitly warned against applying it to individuals.
BMI = weight(kg) / height(m)²
= 703 × weight(lb) / height(in)² Where it fails
- Muscle. Muscle is denser than fat, so athletes routinely score “overweight” or “obese” while carrying very little fat.
- Distribution. Visceral fat around the organs carries far more risk than the same mass on hips and thighs. BMI cannot see the difference; waist-to-height ratio can.
- Height. The square in the denominator undercounts tall people and overcounts short ones, because bodies scale closer to a cube. The ponderal index above corrects for this.
- Ethnicity. Risk thresholds differ meaningfully across populations. The UK's NICE guidance, for example, lowers the overweight threshold for people of South Asian, Chinese, Black African and African-Caribbean background.
- Age. Children and teenagers need age-and-sex-specific percentile charts, not these adult categories.
Better questions
Waist-to-height ratio (keep your waist under half your height) predicts metabolic risk better than BMI and needs only a tape measure. Blood pressure, fasting glucose, lipid panel and cardiorespiratory fitness all tell you more about health than any weight-derived number.
Sources
Category thresholds follow the World Health Organization adult BMI classification, as used by the CDC. These are the international standard cut-offs; several national bodies apply lower thresholds for specific populations.