BMI Calculator

How BMI is calculated

Body mass index is your weight divided by the square of your height: BMI = kg ÷ m². In imperial units the formula is BMI = 703 × lb ÷ in². For example, someone 175 cm tall weighing 70 kg has a BMI of 70 ÷ 1.75² ≈ 22.9, which is in the healthy range.

What BMI can and can't tell you

BMI is a quick screening number for adults. It doesn't measure body fat directly, so very muscular people can score as "overweight", and it can underestimate fat in older adults. Waist-to-height ratio adds useful context: keeping your waist under half your height (a ratio below 0.5) is a widely used guide.

Does BMI differ for men and women?

For adults, no: the formula and the WHO categories are the same for everyone. Sex matters elsewhere. Women carry more body fat than men at the same BMI, so the body fat estimate uses your sex and age (Deurenberg formula: 1.2 × BMI + 0.23 × age − 10.8 × sex − 5.4, with sex = 1 for men and 0 for women). WHO waist limits also differ: risk rises above 94 cm (37 in) for men and 80 cm (31.5 in) for women, and is substantially increased above 102 cm (40 in) and 88 cm (34.5 in).

Children and teenagers

Under 20, BMI is compared with children of the same age and sex on the CDC growth charts, because body fat changes as children grow and differs between boys and girls. Enter age and sex to see the BMI-for-age percentile. A whole number of years is read as halfway through that year, so enter 7.25 for a child of 7 years and 3 months. BMI isn't used under age 2.

Pregnancy and other cut-offs

BMI isn't meaningful during pregnancy. Some health bodies use lower thresholds for people of Asian descent (for example overweight from 23 and obesity from 27.5).

This tool is for general information only and isn't medical advice. Your numbers stay in your browser.