Obesity, as defined by the Centers for Disease Control and Prevention (CDC), is measured by body mass index (BMI), a statistic that generates an estimate of body composition through a calculation involving a person's height and weight.
BMI = weight divided by height squared (in kgs)A BMI of 30 or more indicates obesity. As an example, a person who is 5'9" tall must weigh at least 203 lbs. (just over 92 kilograms) to be obese. A BMI between 25 and 29.9 qualifies as "overweight."
For most, BMI is a reasonable estimate of body composition. But the CDC correctly notes that BMI is not a direct measurement of body fat. A highly muscular person may have a large weight relative to his or her height due to the higher density of muscle.
To illustrate: At the height of his competitive bodybuilding form, Arnold Schwarzenegger was 6'2" tall and weighed 235 lbs. This generates a BMI of 30.2. So, technically, Arnold was obese at this time, but he is obviously a rare example.
A more accurate measure of body fat requires tests such as skinfold caliper measurements or, even better (but more expensive), water weighing or dual energy x-ray absorptiometry (DEXA scanning, a research tool).
Bill's concern about his wife's obesity was actually misplaced, because she was not technically obese: He had considered her weight without considering her above-average height.
