Search for Health Information You Need

Measuring Obesity and Body Fat: Body Mass Index (BMI)

Body mass index (BMI) is the standard measure most often used to determine if a person is overweight or obese. It replaced the previous standard of height and weight tables. Generally, the BMI index is used to estimate the amount of body fat and to define classes of excess body weight and obesity. You can calculate your own BMI by simply dividing your weight in pounds by your height in inches squared and then multiplying this figure by 705. If you are using the metric system, just divide your weight in kilograms by your height in meters squared.

Normally, you are considered to be overweight if your BMI is equal to 25 or greater, whereas obesity starts from a BMI of 30. According to the National Institutes of Health, BMI ranges include the following classifications:

BMI Range

Classification

% Above Normal

<=18.5

Underweight

n/a

18.5 to 24.9

Normal

n/a

25.0 to 29.9

Overweight

20 to 25 percent

30.0 to 39.9

Obese

25 to 35 percent

40+

Extreme/morbid obesity

35 to 40 percent

However, BMI is only a very simple estimate of body fat which does not take a number of specific factors into account, such as the amount of muscle mass. Extremely muscular individuals (e.g., weightlifters, body builders or wrestlers) may have a high BMI but are not obese and, therefore, do not have increased risks of diseases associated with excess weight. Conversely, BMI may not provide an accurate measurement for people with high concentrations of abdominal fat or those with low muscle mass, like the elderly.

Waist measurement can be used in combination with BMI to calculate an individual’s risk for diabetes. The following chart provides healthy versus obese ranges for both sexes:

Healthy

Obese

Men

< 37 inches (94 cm)

> 40 inches (102 cm)

Women

< 32 inches (81 cm)

> 35 inches (89 cm)

A third tool that is used for estimating body fat is the waist-to-hip ratio. This ratio can be determined by measuring the circumference of the waist which is then divided by the circumference of the hips. The ratio of 1.5 times or greater indicates the presence of abdominal or visceral fat (android obesity), which increases the risk of certain diseases like diabetes or cardiovascular disease.

These are general tools most doctors (as well as patents) can use as a quick reference. But there are also other tools that provide more precise information but these may be too expensive or impractical for some patients:

    • Bioelectrical impedance analysis. A non-invasive method in which electrical current is sent through the body and conductivity is measured to estimate the body composition. The higher the conduction, the more muscle and lean tissue. Bioelectrical impedance analysis is relatively inexpensive but its accuracy is not extremely high.
    • Bod Pod. A more expensive but also more accurate test that uses a computerized, oval chamber to measure an individual’s volume and mass. The person’s whole-body density can then be precisely calculated, including fat and muscle percentages.
    • Dual energy x-ray absorptiometry DXA (formerly DEXA). Often used to measure bone density, the DXA scan also shows the percentage of body fat, as well as where it is deposited. This method is more expensive but accurate.
    • Skinfold measurements. Skin calipers measure the thickness of skin and subcutaneous fat just beneath the skin in various sites such as the back of the arm, waist or thigh. This method is not too accurate for calculating the percentage of body fat.
    • Underwater weighing (hydrostatic weighing). The person is weighed underwater, showing how much lean body mass and body fat they have. This method is not too accurate for estimating the percentage of body fat because the residual volume of air in the lungs can add error.
  • Air displacement plethysmography. This is a scientific method of estimating body composition by measuring the amount of air displaced by the person sitting inside a chamber equipped with special sensors.

Finally, a doctor will determine whether or not the person is clinically obese based on their BMI, waist circumference, medical history and, if necessary, any of the more accurate indicators mentioned above.

See also: Calculating the Basal Metabolic Rate (BMR) and Calculating the Body Adiposity Index (BAI)