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Body mass index (BMI) is the standard most often used to determine if a person is overweight or obese. BMI is a number that shows body weight adjusted for height. It is a metric measure used to estimate the amount of body fat the person carries. It is arrived at by dividing weight by 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) and multiply by 705.

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.

In general, overweight is defined as a BMI greater than or equal to 25, and obesity is a BMI greater than or equal to 30. BMI ranges include the following classifications, according to the National Institutes of Health (NIH):

The BMI standard of measurement was developed by the NIH in 1998 in an effort to define classes of excess body weight and obesity. Most 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 (e.g., body builders, weightlifters, athletes) 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 (e.g., the elderly).

Waist measurement may be used in combination with BMI to calculate a person's risk for diabetes. The following chart provides healthy versus obese ranges for men and women:

A third tool for estimating body fat 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 1.5 times larger than the hip measurement, indicating abdominal or visceral fat (android obesity), the risk of various diseases (e.g., diabetes, cardiovascular disease) is increased.

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.
  
  
• Bodpod. A test that uses a computerized, oval chamber to measure a person’s volume and mass. The person’s whole-body density can then be calculated, including fat and muscle percentages.
  
  
• DEXA (dual energy x-ray apsortiometry) x-ray. Often used to measure bone density, the DEXA x-rays also show the percentage of body fat, as well as where and how much fat a person has.
  
  
• Skinfold measurements. Skin calipers measure the thickness of skin and subcutaneous fat just beneath the skin on the back of the arm, waist or thigh.
  
  
• Underwater weighing (hydrostatic weighing). The patient is weighed underwater, showing how much lean body mass and body fat a person has.

A diagnosis of obesity comes from calculating BMI. A BMI equal to or greater than 30 is considered obese, which is defined as having an excess amount of body fat.