It's very common -- more than 1 in 3 U.S. adults are obese. If you're one of them, you can work to lose weight. Although it's not easy, dropping some of those extra pounds -- maybe fewer than you think -- starts to turn things around for you. What Your BMI Says. For adults, experts usually define obesity based on body mass index, or BMI. This formula relates your weight to your height. For instance, if two people weigh the same amount but one is taller than the other, the taller person will have a lower BMI. To find your body mass index, plug your height and weight into a BMI calculator. If your BMI is: Below 18.5: underweight; 18.5-24.9: normal; 25-29.9: overweight; 30 or higher: obese; If you're obese, your doctor might talk about the categories of obesity: Obesity level l: BMI of 30-34.9; Obesity level ll: BMI of 35-39.9