Body mass index is a quick screening number that compares your weight to your height. It is worked out as your weight in kilograms divided by the square of your height in meters, and the pounds and inches version uses the same idea with a conversion factor. The common adult ranges are below 18.5 for underweight, 18.5 to 24.9 for a healthy weight, 25 to 29.9 for overweight, and 30 or higher for obesity. BMI does not measure body fat directly, so it can read high for very muscular builds and it does not account for age, sex, or where weight sits on the body. It is meant for adults, so it works differently for children, teens, pregnancy, and athletes. For a complete picture of your health, talk with a doctor or another qualified health professional.