Body mass index (BMI) is a value derived from the mass (weight) and height of a person. It is a convenient rule of thumb used to broadly categorize a person as underweight, normal weight, overweight, or obese. Commonly accepted BMI ranges are underweight: under 18.5 kg/m2, normal weight: 18.5 to 25, overweight: 25 to 30, obese: over 30.