BMI calculator
The Trefethen index, 1.3 × weight ÷ height^2.5, was proposed in 2013 as a correction for the way plain BMI overstates tall people and understates short ones. It is shown for comparison, not as a replacement — no health guidance uses it.
Bands are the World Health Organization thresholds for adults: under 18.5, 18.5 to 24.9, 25 to 29.9, and 30 or above. They are not adjusted for age, sex or build, and different cut-offs are used for some Asian populations.
BMI is a screening figure for populations, not a diagnosis for a person. It cannot tell muscle from fat, and it reads high for athletes and low for older people who have lost muscle. Talk to a doctor before acting on it.
Body mass index is weight in kilograms divided by height in metres squared. The World Health Organization treats 18.5 to 24.9 as the healthy range for adults, below 18.5 as underweight, 25 to 29.9 as overweight and 30 or above as obese. Someone 170 cm and 70 kg has a BMI of 24.2.
How to work out your BMI
BMI was devised in the 1830s by Adolphe Quetelet as a way of describing populations, and it was never intended to assess an individual. It uses height squared while bodies scale closer to a cube, which is why it drifts high for tall people and low for short ones. It also cannot distinguish muscle from fat: a well-trained athlete routinely scores as overweight while being nothing of the sort.
Questions
Between 18.5 and 24.9 for adults, by the WHO definition. The range of weights that produces that at your height is shown under the result.