A BMI calculator is a tool used to determine a person's body mass index (or BMI for short). BMI is a measurement that can be used to assess someone's level of body fat based on his or her height and weight. The BMI calculator does not measure body fat directly but correlates very closely to direct measures of body fat. In order to gauge a person's body mass index, the only information you need is his or her height and weight. The BMI calculator is used to determine the body mass index of a person 20 years of age or older. It is not appropriate to use the BMI weight status categories for adults to interpret BMI numbers for children and teens.