Obesity and overweight are often used interchangeably. However, there is a distinct difference between the two terms. The traditional standard used to determine if a person is overweight or obese has been the body mass index (BMI).

BMI is a number that shows body weight adjusted for height. Using standard measurement, divide weight (in pounds) by height (in inches, squared) and multiply by 705. Using the metric system, divide weight (in kilograms) by height (in meters, squared).

For example, a man who weighs 250 pounds and is 6 feet tall would have a BMI of 33.9. This is 250 divided by 5,184 (which is 72 inches squared) and multiplied by 705.

BMI ranges include the following classifications, according to the National Institutes of Health (NIH):

The BMI was developed by the NIH in 1998 in an effort to define classes of overweight and obesity. Many physicians believe that BMI offers a better way to calculate body fat and the associated health risks than the previous standard of using height and weight tables.

However, BMI is only an estimate of body fat and does not take a number of factors into account, including the amount of muscle mass. Extremely muscular individuals may have a high BMI but are not obese and do not have increased risks for disease. Conversely, BMI may not provide an accurate measurement of people who have high concentrations of abdominal fat or those with low muscle mass.

Waist measurement may be used in combination with BMI to calculate a person’s risk for diabetes. People with central obesity (abdominal fat, sometimes referred to as an “apple shape”) are at an increased risk of diabetes. The following chart provides healthy versus obese ranges for men and women:

Another tool is the waist-to-hip ratio. This ratio is determined by measuring the circumference of the waist and dividing it by the circumference of the hips. If the stomach measurement is larger than the hip measurement, indicating abdominal or visceral fat, the risk is increased for development of diabetes, as well as many cardiovascular diseases. Some recent research suggests the waist-to-hip ratio may be a more valuable tool than the BMI in identifying cardiovascular and diabetic risks, especially in elderly people. 

These are general tools most health practitioners can use to gauge risk. Other tools provide more precise information but may be expensive or impractical for many patients:

• Bioelectrical impedance analysis (BIA). A noninvasive electrical current is sent through the body and conductivity is measured. The higher the conduction, the more muscle and lean tissue.
  
  
• Body composition tracking system. A test that uses a computerized, pod-like chamber to measure a person’s volume and mass. The person’s whole-body density can then be calculated, including fat and muscle percentages.
  
  
• Bone density scan. Often used to measure bone density and screen for osteoporosis, dual energy x-ray absorptiometry (DEXA) also shows the percentage of body fat, as well as where and how much fat a person has.
  
  
• Skinfold thickness. Skin calipers measure the thickness of skin and subcutaneous fat just beneath the skin on the back of the arm, waist or thigh.
  
  
• Underwater (hydrostatic) weighing. The patient is weighed underwater, showing how much lean body mass and body fat there is.