Obesity guidelines from the National Heart, Lung and Blood Institute propose that doctors use body mass index (BMI) to assess patients because the index is simple, correlates to fatness and applies to both men and women. 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.