In both overweight and obese people, the defining characteristic is excess body fat. The difference is a matter of degree. Overweight people usually have a body mass index (BMI), or ratio of weight to height, of 25 to 29.9. Obesity is usually defined as a BMI of 30 or more. Another measure of obesity is having more than 30 percent body fat. While overweight people have a higher incidence of certain diseases and conditions than normal weight individuals, as people gain weight, they become even more prone to these often life-threatening problems.