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 relation 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). This can be done by dividing your weight in kilograms by your height in meters squared. Your BMI can easily be calculated using a height and weight table. The BMI calculation does not take into account whether your weight is composed mostly of fat or muscle. Some muscular people may have a high BMI without being overweight or obese.

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

There are other tests that can estimate your percentage of body fat. The 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 circumference, your BMI can usually provide a valid estimate of whether you are overweight or obese.

Methods to diagnose obesity:

  • BMI in adults:
    • Overweight is defined as a body mass index (BMI) greater than or equal to 25.
    • Obesity is defined as having a BMI greater than or equal to 30.
    • Severe or morbid obesity is defined as having a BMI greater than or equal to 40.
  • BMI in children is based on growth charts:
    • Overweight is BMI greater than or equal to 85th percentile for gender and age.
    • Obesity is BMI greater than or equal to 95th percentile for gender and age.
    • Severe obesity is BMI greater than or equal to 99th percentile for gender and age.
  • Other methods to diagnose obesity include:
    • Waist circumference, sagittal 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 0.8 in women is considered obese. Waist circumferences that exceed 40 inches (102 centimeters) men or 35 inchess (88 centimeters) 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 inside the abdomen.
    • Water displacement tests—Fat floats, but 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 can be done to rule out other medical conditions that may cause excess body weight, such as thyroid or adrenal disorders