BMI calculator

What BMI is

Body mass index is your weight in kilograms divided by the square of your height in metres. Adolphe Quetelet devised it in the 1830s as a way to describe populations, not individuals, and it has been doing that job ever since. The World Health Organization bands are the ones printed above the calculator.

Its virtue is that it needs only two numbers everyone knows and that it has decades of epidemiology behind it. At the level of a country, BMI distributions track health outcomes well. That is what it was built for.

What BMI cannot see

It cannot tell muscle from fat. A rower and a sedentary person of the same height and weight get the same BMI, which is plainly wrong about their health.

It cannot tell where the fat is, and for metabolic health that is the more important omission. Fat around the organs behaves differently from fat under the skin: it is the deposit most tightly linked to insulin resistance, and BMI is blind to it. That is how someone lands in the healthy band and still has raised fasting insulin.

It also shifts with age and body composition, and it was derived from European populations. A WHO expert consultation on Asian populations, whose risk rises at lower BMI values, decided not to redefine the international cut-offs and instead added four extra action points on the same scale; NICE goes further and applies lower overweight and obesity thresholds to several ethnic groups outright. The exact figures and both sources are in the line under the scale.

Use the tape measure instead

For the question "is my body composition raising my metabolic risk?", the waist-to-height ratio answers better than BMI, from a tape measure and your height. NICE recommends it alongside BMI for exactly this reason. It catches the normal-BMI, high-visceral-fat pattern that BMI declares healthy.

If you want the metabolic picture rather than the shape, the risk test combines both measurements with five other factors, and HOMA-IR measures the process directly from a blood test.

When BMI does not apply at all

It is not valid in pregnancy. It is not valid for children and adolescents, who need age-and-sex percentile charts rather than fixed bands. It is unreliable in athletes and anyone with high muscle mass, in older adults who have lost muscle, in people with amputations, and in anyone with significant fluid retention.

Used as a single number to judge one person's health, it is a poor instrument. Used as one input among several, it is fine — which is how the risk test uses it.