How BMI works
Body mass index divides your weight in kilograms by your height in metres squared. It gives one number that public health services use to sort adults into broad weight bands. Because it needs only height and weight, it is quick to check at home, which is why doctors use it as a first screening step rather than a final answer.
Reading the result honestly
BMI does not know the difference between muscle, bone and fat, and it does not know where fat sits on your body. A weight trainer and a sedentary person of the same height and weight get the same score. Waist measurement, blood pressure and blood tests tell a far more complete story.
Who should not rely on it
- Children and teenagers, who need age and sex percentile charts instead
- Pregnant and breastfeeding women
- Athletes with high muscle mass
- Older adults, whose muscle mass has usually dropped
Frequently asked questions
What is a healthy BMI for adults?
On the standard adult scale, 18.5 to 24.9 is described as a healthy range. Some Asian health guidelines lower the overweight threshold to 23, so check the guidance used in your country.
Does BMI work for children?
No. Children are assessed with age and sex specific percentile charts, because body composition changes a lot during growth.
Is BMI the same in metric and imperial units?
Yes. The imperial figures are converted to metres and kilograms first, so the score is identical either way.