Body mass index (BMI) is a measure based on height and weight. It tries to determine if you're underweight, normal weight, overweight, or obese based on those values. BMI, or body mass index is a tool that broadly tries to categorize a person as underweight, normal weight, overweight, or obese based on simple measurements. Even though BMI correlates with the amount of body fat it does not directly measure the amount of body fat. BMI calculations also can not differentiate between fat and lean body mass. To find out your BMI score use the easy to use tool below. Enter in your height and weight. Your BMI score will be calculated and you'll find out which BMI range you are in. Are you underweight, normal weight, or overweight?