Body mass index (BMI) is a quick screening measure that relates your weight to your height. It is widely used to flag whether an adult may be underweight or carrying excess weight.
BMI does not measure body fat directly and does not account for muscle mass, age, sex or ethnicity, so treat it as a starting point and talk to a health professional about your individual situation.
Formula
BMI = weight (kg) ÷ height (m)²
In imperial units the equivalent is BMI = 703 × weight (lb) ÷ height (in)².
BMI categories for adults
Below 18.5: underweight. 18.5 to 24.9: healthy weight. 25 to 29.9: overweight. 30 and above: obesity.
Worked example
A person who weighs 70 kg and is 175 cm tall has a BMI of 70 ÷ 1.75² = 22.9, which is in the healthy range.
Frequently asked questions
Is BMI accurate?
BMI is a useful population-level screening tool but can misclassify individuals — muscular people may score as overweight, for example. Waist measurement and other checks give a fuller picture.
Can children use this BMI calculator?
No. Children and teenagers are assessed with age- and sex-specific BMI percentiles, not the adult categories used here.
Last updated · Calculations checked by automated tests· Editorial policy