Body Mass Index (BMI) is the most widely used screening number for weight relative to height. It is easy to calculate and needs only a scale and tape measure for height, which is why clinics, insurers, and research studies still quote it. It is also a blunt instrument: it cannot tell muscle from fat, and it says nothing about where fat is stored.
The standard formula is BMI = weight (kg) ÷ height (m)². In imperial units that is weight in pounds × 703 ÷ height in inches squared. Our Body Indexes calculator also offers an alternative exponent formula (kg ÷ m^2.5) proposed by Nick Trefethen, which slightly reduces the penalty for very tall people compared with the traditional index.
The World Health Organization groups adults roughly as follows:
These cut-offs are population averages for health risk, not personal diagnoses. Athletes with low body fat can land in “overweight” because muscle is dense. Older adults may sit in the “normal” band while carrying excess abdominal fat.
Body Roundness Index (BRI) uses waist circumference and height. The idea is to approximate how circular a person’s cross-section is — abdominal fat changes that shape more than weight alone. A higher BRI generally aligns with greater central adiposity. It is still an estimate, but pairing BMI with BRI can flag cases where weight looks fine while waist size does not.
Someone who is 175 cm tall and weighs 80 kg has BMI = 80 ÷ 1.75² ≈ 26.1 (overweight on WHO scales). If their waist is 90 cm, BRI will be higher than for a peer with the same BMI but a 78 cm waist — reflecting different fat distribution.
WHO BMI classification; Thomas et al., BRI derivation (2013); Trefethen alternative BMI proposal. See also our Ideal Weight Range tool for height-based weight bands.
Last updated: June 2026