How is obesity measured?

John La Puma
Endocrinology Diabetes & Metabolism
In adults, with body mass index (BMI): your weight in pounds x 703 divided by your height in inches. Over 30 is class I, over 35 is class II, over 40 is class III. But body fat, muscle and waist size probably are more important.
William L. Wilson, MD
Family Medicine

Currently the experts recommend using body mass index (BMI) to diagnose overweight and obesity. BMI is actually a height/weight formula that reflects your size more than anything else. Because obesity is defined as excessive body fat, to me it would make more sense to directly measure body composition to diagnose obesity. Tanita makes scales that use bioelectrical impedance and Futrex makes equipment based on near infrared impedance to directly measure the relative amount of fat in your body. If you lose 10 pounds, you want to lose 10 pounds of fat, not 10 pounds of muscle. In my medical practice I have taken over 18,000 body composition readings on my patients and it is clear that "super dieters" who reduce their calories too much lose mainly muscle, not fat. 

Because a pound of fat takes up more space than a pound of muscle, pay attention to body shrinkage. If your body is shrinking and your clothes are getting looser even if you aren't losing much weight, you are probably losing more fat than muscle--exactly what you want. Because thin or normal size people can also have too much fat on their body (the flabby thin person), it makes sense to measure your body composition rather than relying on BMI when it comes to diagnosing obesity.

Dr. Mehmet Oz, MD
Cardiology (Cardiovascular Disease)
Scientists and doctors use different tools for measuring obesity. But the most widely accepted measurement is called the body mass index (BMI). The BMI is calculated by plugging your height and weight into a mathematical formula. This formula produces a number that reflects whether your weight is healthy. Here are the BMI ranges and how they are associated with various weight levels, according to the Centers for Disease Control and Prevention (CDC):
  • below 18.5: underweight
  • 18.5 to 24.9: normal
  • 25.0 to 29.9: overweight
  • 30.0 and higher: obese
The BMI isn't perfect. Some highly trained athletes have high BMIs, since muscle weighs a lot. However, knowing your BMI can play an important role in weight management. You can find a BMI calculator on the CDC's website.
Assessing your weight to determine if you are obese involves two key measurements -- body mass index (BMI) and waist circumference.

A woman or man with a BMI between 25 and 29.9 is considered overweight; 30 or more is considered obese; and 40 or greater is considered extreme obesity. Women with a waist circumference over 35 inches and men whose waist measure over 40 inches are at greater risk.

Understanding your risk factors for conditions associated with obesity, according to clinical practice guidelines issued by the National Heart, Lung, and Blood Institute, is important. First, your healthcare professional should determine your BMI, which describes your body weight relative to your height. It is strongly correlated with total body fat content in adults. Your BMI is your weight in pounds divided by your height in inches squared, then multiplied by 703.

Charts that use the same mathematical principles as the BMI can provide figures for your ideal weight based on your height. For example, a height-weight chart that shows lower and upper limits of overweight, obese and extreme obesity can be found at multiple web sites by searching the keyword BMI.

Women in the highest obesity category have a significantly higher risk of hypertension and/or high blood cholesterol than women of normal weight.

And what about that waist circumference?

Unlike fat around the thighs, which is more common in women and is more likely to serve as an energy reservoir, abdominal fat delivers fatty acids directly into the bloodstream for immediate short-term energy, increasing triglyceride and, eventually, cholesterol levels. Healthcare professionals aren't certain why this proves detrimental to your health, but higher proportions of abdominal fat are associated with higher risks of insulin resistance, diabetes, high blood pressure and cardiovascular disease (heart disease and stroke). Women with a waist circumference over 35 inches (and men over 40 inches) are at greater risk.
The two most common methods healthcare providers use to assess your body weight and related health risks are body mass index (BMI) and waist circumference. Neither of these methods directly measures body fat -- but both methods are simple to use, and evidence shows that they are good predictors of health risks associated with overweight and obesity. Body fat percentage measured by skinfold calipers or underwater weighing may also be useful if done by a trained professional.
Marjorie Nolan Cohn
Nutrition & Dietetics

For adults, overweight and obesity ranges are determined by using weight and height to calculate a number called the "body mass index" (BMI). BMI is used because, for most people, it correlates with their amount of body fat.

It is important to remember that although BMI correlates with the amount of body fat, BMI does not directly measure body fat. As a result, some people, such as athletes, may have a BMI that identifies them as overweight even though they do not have excess body fat. For more information about BMI, visit Body Mass Index.

Other methods of estimating body fat and body fat distribution include measurements of skinfold thickness and waist circumference, calculation of waist-to-hip circumference ratios, and techniques such as ultrasound, computed tomography, and magnetic resonance imaging (MRI).
Generally speaking, obesity is measured by one’s BMI.
BMI, or Body Mass Index, is a measure of body fat based on height and weight that applies to adult men and women.
Multiply your weight in pounds by 703. Divide that number by your height in inches. Finally, divide that answer by your height in inches one more time.
A BMI of less than 18.5 is considered “underweight.” A BMI between 18.5 and 24.9 is considered “normal weight.” A BMI of 25 to 29.9 is considered “overweight.” A BMI of 30 or greater is considered “obese.”
Morbid obesity is defined as having a body mass index (BMI) of more than 40. For men that equates to approximately 100 pounds overweight for women, 80 pounds overweight.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.