The term overweight is used to describe an excess amount of total body weight including all tissues (fat, bone, muscle, etc.) and water. Obesity, in contrast, is an excess amount of body fat. An adult woman or man who has a body-fat percentage exceeding 35 percent (for women) or 25 percent (for men) is considered obese. A person can be overweight without being obese, as many professional football players and bodybuilders are, for such individuals have large amounts of muscle but not much fat. Likewise, a person can be obese without being overweight, such as some elderly individuals or lazy "couch potatoes," who may not weigh a lot but have too much body fat. However, almost all obese people are also overweight.
Because body fat is very difficult to measure accurately, height and weight are used to estimate overweight and obesity. Body mass index (BMI) is a formula that combines both height and weight. It is computed as weight in kilograms divided by height in meters squared, or as weight in pounds times 703 divided by height in inches squared. Normal weight for adults is represented by a BMI of 18.5 to 24.9; overweight by a BMI of 25 to 29.9; and obesity by a BMI of 30 or greater.
John P. Foreyt
Foreyt, John P., and St. Jeor, Sachiko T. (1997). "Definitions of Obesity and Healthy Weight." In Obesity Assessment: Tools, Methods, Interpretation, edited by Sachiko St. Jeor. New York: Chapman & Hall.
Poston, Walker S. C., and Foreyt, John P. (2002). "Body Mass Index: Uses and Limitations." Strength and Conditioning Journal 24(4):15–17.
U.S. Department of Health and Human Services (2001). The Surgeon General's Call to Action to Prevent and Decrease Overweight and Obesity. Rockville, MD: U.S. Department of Health and Human Services, Public Health Service, Office of the Surgeon General.
National Heart, Lung, and Blood Institute. "Clinical Guidelines on Overweight and Obesity." Available from <http://www.nhlbi.nih.gov>