Overweight and obesity don't mean exactly the same thing—obesity is a more severe form of being overweight. Because your ideal weight is related to how tall you are, doctors use a scale called the body mass index (BMI) to determine what a healthy weight for you might be. Your BMI is calculated by taking your weight in kilograms and dividing it by your height in meters squared. You can calculate your own BMI here (it will do the conversion to kilograms and meters for you). A BMI between 25 and 29.9 is considered overweight, while obesity is defined as having a BMI of 30 or more. A woman with a BMI of 30 is about 30 pounds overweight. Obesity is then further broken down into 3 categories: Class I, Class II, and Class III. The table below shows the BMI cutoff points for underweight, normal weight, overweight, and the 3 classes of obesity.
|Cutoffs for Overweight & Obesity by BMI1|
|Underweight||Less than 18.5|
|Class I obesity||30-34.9|
|Class II obesity||35-39.9|
|Class III (extreme) obesity||More than 40|
The main causes of being overweight or obese are eating too many calories and not being physically active enough. If you eat more calories than your body burns up, the extra calories are stored as fat. Other factors that may affect your weight include your genes (obesity tends to run in families),2, 3 your metabolism (how your body processes food),4 and your age (your metabolism slows down as you get older). Sometimes an illness or medication can contribute to weight gain. However, total caloric intake and physical activity remain the keys to weight control.
How common is overweight and obesity?
The National Center for Health Statistics estimates that 67% of all Americans over 20 years old are overweight or obese, including nearly 62% of American women.5 The prevalence of obesity tends to vary between racial and ethnic groups. It is also a growing problem among children.6
|Rates of Obesity Among US Women Age 20 and Older7-12|
|% of Women Who Are Overweight
(BMI 25 or higher)
|% of Women Who Are Obese
(BMI 30 or Higher)
|*Numbers for women alone not available: total is for both men and women 18 years of age and older|
Is BMI a good measure of a healthy weight for me?
For some people, BMI might not accurately gauge their health risk. For example, because muscle weighs more than fat, a very muscular person may have a BMI over 25, even though they are not really overweight or at increased risk for heart disease. For South Asians, Arabs, and mixed-race Africans, BMI can also be inaccurate because people with this ancestry tend to have a higher percentage of body fat than white people.13-16 For Asian Americans, BMI may underestimate their health risk.17, 18 Increasingly, studies show that waist measurement and/or waist-to-hip ratio may be better than BMI for assessing the risk your weight poses to your overall health.1