Obesity is an abnormally high proportion of body fat. The doctor can often determine if you are obese by looking at your body and assessing the percentage of body fat. Methods of assessing body fat are discussed below.

Measuring your weight in proportion to your height is the traditional way of determining whether you are overweight, obese, or at an appropriate weight. Your doctor can often determine if you are overweight or obese by calculating your body mass index (BMI), which is derived by dividing your weight in kilograms by your height in meters squared. Your BMI can easily be calculated using a height and weight table.

In addition, there is also risk associated with abdominal fat accumulation, even if your total weight is not particularly high. So measuring the circumference of your waist is also an important measure of whether you need to lose weight.

The BMI calculation does not take into account whether your weight is composed mostly of fat or muscle. Some very muscular people may have a high BMI without being overweight or obese. Your doctor will likely be able to estimate whether you have too much body fat by measuring your waist-to-hip ratio, but there are also tests (see below) that can estimate your percentage of body fat. Accuracy of these tests varies and some are so expensive that you are not likely to have them at the doctor’s office. When combined with your visual appearance and waist-to-hip ratio, your BMI can usually provide a valid estimate of whether you are overweight or obese.

Tests to diagnose obesity include:

  • Body Mass Index (BMI)—Calculated by dividing your weight in kilograms by your height in meters squared. A BMI of 25 to 29.9 is considered overweight and 30 or higher is considered obese for.
  • Waist circumference, saggital diameter, and waist-to-hip ratio—Simple measurements that estimate the amount of fat deposited in the skin and inside the abdominal cavity. Waist-to-hip ratio greater than 1 in men or greater than 0.85 in women is considered obese. Waist circumferences that exceed 102 centimeters (40 inches) men or exceed 88 centimeters (35 inches) in women are associated with an increased risk of heart disease.
  • Skinfold caliper—Most fat is deposited beneath the skin. This test measures fat just beneath the skin, but cannot measure fat accumulated inside the abdomen.
  • Water displacement tests—Fat floats; the rest of your body tissues sink. Determining how well you float provides an estimated ratio of fat to body mass.
  • Electrical measurements—A couple of tests calculate your percentage of body fat by measuring the difference between the electrical characteristics of fat and other tissues in your body.
  • Blood tests—To rule out other medical conditions that may cause excess body weight, such as a thyroid disorder, your doctor may order some blood tests.