Body mass index (BMI) doesn't tell the whole story about your weight and health, says Albert Sun at The New York Times. The formula commonly used to determine obesity (or underweight) is so popular because it's easy to measure height and weight. But in 18 percent of cases, according to data from the Center for Disease Control and Prevention's National Health and Nutrition Examination Survey, BMI gets it wrong. In 11 percent of cases, people labeled overweight have healthy amounts of fat, and 31 percent of those underweight or normal weight have excess fat.