People can calculate their body mass index by dividing their weight by the square of their height in meters, explains the Centers for Disease Control and Prevention. Alternatively, one can divide weight in pounds by the square of height in inches and multiply by 703.
BMI is a measure of how a person's weight compares to his height. A person who has a BMI of between 18.5 and 24.9 has a healthy weight, reports the CDC. A BMI of less than 18.5 indicates that a person is underweight, while a person with a BMI between 25.0 and 29.9 is overweight. A BMI of more than 30.0 indicates that a person is obese. Having a BMI outside the healthy range can increase a person's risk of health problems.
The same formula is used to calculate BMI in children and adults. However, the definitions of normal weight, underweight and overweight are different for children up to the age of 19, according to the CDC. Doctors compare children to others of their own age and gender to determine whether they are overweight, underweight or normal weight. This system defines children as being obese if they weigh more than 95 percent of their peers.