Plateknow Your BMI and nothing else — no “normal”, no “overweight”, no coloured band. The number is yours; what it means about you is not something a web page knows.
That is the target. The hard part is the other half — knowing what you actually ate.
Download freeHow BMI is calculated
Weight in kilograms divided by height in metres squared. Adolphe Quetelet devised it in the 1830s to describe the average build of populations, not to judge an individual.
What BMI does not tell you
- Muscle from fat. A rower and a sedentary person of the same height and weight get the same figure.
- Where the weight sits. BMI cannot see waist size, which carries its own information about health risk.
- Children and teens. Under 18, BMI is read against age-and-sex growth percentiles, not adult numbers. This calculator is for adults.
- Pregnancy. The figure is not interpretable, and the standard thresholds are not written for it.
- Which population you belong to. The WHO notes the link between BMI and health risk differs between populations, and several countries use lower thresholds for people of Asian descent.
Is BMI calculated differently for women and men?
No — the adult formula is the same for both. It cannot see that two bodies of the same height and weight can differ in muscle and fat.
Does BMI change with age?
For adults the formula has no age input. For children and teenagers BMI is read against age-and-sex percentiles, which this calculator does not do.
Where can I find the BMI categories?
The WHO and the NHS publish the standard thresholds. This page gives the ratio only, with no label attached.
Quetelet A., Sur l’homme et le développement de ses facultés (1835) — the index and its population-level intent. World Health Organization guidance on body mass index, including its note that the association between BMI and health risk varies between populations.