What BMI tells you
Body mass index is weight in kilograms divided by height in meters squared. The World Health Organization uses it to classify adults: below 18.5 underweight, 18.5 to 24.9 normal weight, 25 to 29.9 overweight and 30 or more obesity, in three classes. The healthy weight range shown is the weight that gives a BMI between 18.5 and 24.9 at your height.
BMI is a quick screening number, not a diagnosis. It does not distinguish muscle from fat, so athletes may show a high value, and older people may have a normal BMI with little muscle. Waist size and a doctor's assessment give a fuller picture.
Children and special cases
For children and teenagers BMI is compared with growth charts by age and sex, so these adult categories do not apply. Some health organizations use lower thresholds for people of Asian descent. During pregnancy BMI is not meaningful.