Body mass index, or BMI, is the most common measure of obesity. It is calculated by dividing weight in kilograms by height in meters squared. A result of 30 or higher is obese, according to the Centers for Disease Control and Prevention. Obesity is a serious condition that increases a person’s risk of developing a variety of health problems, including type 2 diabetes, heart disease, and stroke.

Medical experts rely on BMI because it is simple, cheap, and generally accurate. At times, however, the index misclassifies patients because it does not distinguish fat from muscle.

Holly F. Lofton, MD, an internist board-certified in obesity medicine, tells The Wall Street Journal that “You can have two people with the same BMI, but one has 50 percent body fat and the other has 30 percent body fat.”

Dr. Lofton, who is also the director of NYU Langone’s Weight Management Program, says health insurance will only provide treatment for patients with certain BMI levels. However, an equally low-cost gauge for obesity, the medical waist measurement, is less likely to misclassify a muscular person as overweight. If the waist measured right above the hip bones is greater than 40 inches for males or 35 inches for females, it more directly correlates with obesity.

“BMI and waist circumference are not diagnostic tools for disease risks,” says Dr. Lofton. “If you go online and find your BMI is high, the next step is to go to a medical professional and see what that means for you and your health risk.”

Read more from The Wall Street Journal (subscription required).