What is BMI?
Body mass index (BMI) compares your weight to your height. It is calculated as BMI = weight (kg) / height (m)2, or 703 × weight (lb) / height (in)2 in US units. The result places adults into broad categories used by the World Health Organization and the US CDC.
BMI categories for adults
- Below 18.5: underweight
- 18.5 to 24.9: healthy weight
- 25 to 29.9: overweight
- 30 or higher: obesity
Limits of BMI
BMI is quick and free, which is why it is used in studies and screenings, but it only uses height and weight. It cannot tell muscle from fat, so an athlete with a lot of muscle can have a high BMI while carrying little body fat. It also ignores where fat is stored, and the best cut-off points can differ between populations. Waist circumference, blood pressure, blood sugar and cholesterol give a fuller picture of health.
What to do with your result
If your BMI is outside the healthy range, treat it as a prompt for a conversation with your doctor rather than as a verdict. Sustainable changes in food, movement and sleep matter more than any single number.
Frequently asked questions
What is a healthy BMI?
For adults, a BMI from 18.5 to 24.9 is classed as a healthy weight.
Is BMI accurate for athletes?
Often not. Muscle weighs more than fat, so very muscular people can have a high BMI without excess body fat.
Does BMI differ for men and women?
The adult formula and categories are the same for both. Women tend to carry more body fat than men at the same BMI.
Can I use this BMI calculator for children?
No. For people under 18, BMI is compared with age- and sex-specific percentiles, which this adult calculator does not use.