Body mass index (BMI) is a popular method of defining a healthy weight vs. being underweight, overweight or obese. It should be used as a guide, along with waist size, to help estimate the amount of body fat. To calculate your body mass index, insert your weight in pounds and select your height. Click on the Compute button to see your body mass index. A BMI of 25 to 29.9 is considered overweight and one 30 or above is considered obese.