A BMI calculator is a tool used to estimate a person's body mass index (BMI), which is a measure of body fat based on a person's weight and height. BMI calculators are typically used to assess a person's risk of health problems related to being underweight, overweight, or obese. The calculator works by dividing a person's weight in kilograms by their height in meters squared. The resulting number is compared to BMI categories to determine if the person is underweight, normal weight, overweight, or obese.