The formula
BMI is weight divided by height squared: kilograms ÷ metres². In pounds and inches it is 703 × pounds ÷ inches². A person 1.75 m tall weighing 72.5 kg has a BMI of 72.5 ÷ 1.75² ≈ 23.7.
The categories
For adults, the World Health Organization and the US Centers for Disease Control and Prevention use the same bands: under 18.5 is underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight, and 30 or more obesity, split into three classes. The healthy range shown is the weight that gives a BMI of 18.5 to 24.9 at your height.
What BMI cannot tell you
BMI is a quick screening number, not a measure of health. It cannot tell muscle from fat, so it reads high for many athletes; it says nothing about where fat is carried; and the same number can mean different things at different ages, for men and women, and across ethnic groups. Some health bodies use lower cut-offs for people of Asian descent. A doctor looks at much more than BMI.
Questions
Is BMI different for men and women?
Not for adults: the World Health Organization and the CDC use one set of bands for everyone aged 20 and over, so the same height and weight give the same BMI and category. What differs is what the number hides. At the same BMI, women on average carry more body fat than men, and men more muscle. For children and teenagers, BMI is compared with others of the same age and sex, so there it does differ.
Can I use it for a child or teenager?
No. Under 20, BMI is read against growth charts for age and sex, as a percentile, which this calculator does not do. Ask a doctor or use a children's BMI tool from a health service.
Does it work during pregnancy?
No. Weight gain in pregnancy is expected and BMI does not apply. Ask your midwife or doctor.
Is my weight stored?
No. It is worked out in this page and goes nowhere.