As our knowledge of obesity has grown, the way physicians define obesity has changed.

The original definition of morbid obesity was any individual who was greater than or equal to twice their ideal body weight, or greater than or equal to 100 pounds above their ideal body weight. The figures for ideal body weight were determined by the 1983 Height and Weight Standards of the Metropolitan Life Insurance Company.

Morbid obesity was also defined by the amount of total body fat, although this value is not easily obtained. Normally 20 to 25 percent of body weight is fat. If 40 percent or more of the body weight is fat, morbid obesity is diagnosed.

The modern definition of obesity is based on body mass index (BMI), a calculation that compares your weight (measured in kilograms) with your height (measured in meters, then squared). It was developed through a collaboration between the National Institutes of Health’s National Heart, Lung, and Blood Institute and the North American Association for the Study of Obesity.

Using this calculation (BMI=kg/m2), you can determine how physicians would classify your body’s current size.

Calculate your BMI

• BMI of less than 24.9 is considered normal.
• BMI between 25 and 29.9 is considered overweight.
• BMI of 30 or greater is considered obese.

Obese patients are placed in three classes, based on their BMI:

• Class 1 = 30 to 34.9
• Class 2 = 35 to 39.9
• Class 3 = 40 to 49.9

Class 3 BMI is required to be diagnosed morbidly obese. Individuals with a BMI that is greater than 49.9 are considered super obese.

Waist circumference is also an important consideration in determination of the health risks related to obesity. Men with a waist circumference greater than 40 inches and women with a waist circumference greater than 35 inches are at increased risk of complications.

To reflect this increased risk, the classifications defined by BMI can be increased one level.

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