Body mass index calculators are available on the websites of the Centers for Disease Control and Prevention; the National Heart, Lung, and Blood Institute; and Mayo Clinic. The data needed to make a BMI calculation and measure a person's body fat include birth date, weight, sex and height.
A BMI of less than 18.5 means that a person is underweight, while an index of 30 or more indicates that the person is obese. The normal range for BMI is between 18.5 and 24.9. The range between 25 to 29.9 is considered being overweight. The BMI calculators for adults may only ask for weight height and sex, while one for children and teens can include this data and age or date of birth.