Guides
What is BMI and how is it calculated?
Body mass index is one number that compares your weight with your height.
BMI is weight in kilograms divided by height in meters squared. It was developed from work by the Belgian statistician Adolphe Quetelet in the 1800s and named "body mass index" by Ancel Keys in 1972. It is used to screen groups of people and to give a rough indication for individuals.
The formula
- Metric: BMI = weight (kg) / height (m) squared.
- Imperial: BMI = 703 x weight (lb) / height (in) squared.
Example: a person who is 1.75 m tall and weighs 70 kg has a BMI of 70 / 3.0625 = 22.9.
Where it comes from
Adolphe Quetelet noticed in the 19th century that, among adults, weight rises roughly with the square of height. The index was called the Quetelet index until the physiologist Ancel Keys proposed the name body mass index in a 1972 paper.
What it is used for
- Screening: a quick, cheap way to flag people who may benefit from a closer look.
- Public health: comparing groups and tracking trends over time.
- A starting point in a doctor's visit, alongside other measurements.
Because it uses only height and weight, BMI cannot see body composition. Read how accurate BMI is and the category chart.
Frequently asked questions
Does BMI depend on age or sex?
The formula does not. For adults, the WHO categories are the same for men and women. Interpretation can differ with age, and children use percentile charts instead.