How To Calculate Body Fat

Here are the most popular methods used to calculate body fat. Keep in mind that none of these methods is 100% accurate.

Circumference Test

The circumference test involves using a tape measure to determine the circumference of different areas of the body. The circumferences, along with other data such as height and/or weight, are plugged into a formula that estimates body fat. The circumference test is a simple way to calculate body fat but it's much less accurate than other methods.

Skinfold Caliper Test

The skinfold caliper test is based on the fact that you store most of your body fat directly beneath your skin. These types of fat deposits are called subcutaneous fat. The remainder of your body's fat is located around internal organs (visceral fat) and inside muscle tissue (intramuscular fat).

The skinfold caliper test is performed with a simple, vice-like instrument called a skinfold caliper. The jaws of the caliper pinch a fold of skin and fat and measure the thickness of the fat fold in millimeters. The thickness is plugged into a formula to calculate body fat.

The skinfold caliper test is practical and easy to perform and can be very accurate. But the accuracy depends on the skill level of the tester. The greatest errors are human errors from not pinching at the right spot or using improper technique when pinching.

Bioelectrical Impedance Analysis (BIA)

BIA calculates body fat by testing the electrical conductivity of your body's tissues. BIA sends a weak (and harmless) electrical current through your body. The slower the electrical current travels the more fat you have. Fat impedes the electrical current because fat has a low water content and doesn't conduct electricity well. The electrical current travels quickly through lean mass because lean mass has a high water content and conducts electricity well.

BIA is a fairly reliable way to calculate body fat but it can have a large margin of error, especially if you're extremely fat or extremely lean. Also, because BIA is based on your body's water status, the results can fluctuate based on your state of hydration. If you're dehydrated from alcohol, caffeine, exercise or heavy sweating, the results can be flawed. Because your water balance also varies depending on the time of day, test results in the morning can fluctuate greatly from results at night.

Underwater Weighing

Underwater weighing, also known as hydrostatic testing, can be a very accurate way to calculate body fat. You sit on a scale in a tank of warm water about the size of a jacuzzi. You exhale as much air as possible and immerse yourself in the water for about 10 seconds while your underwater weight registers on a digital scale. The result is then plugged into an equation to determine your body fat.

Underwater weighing is based on the premise that muscle sinks and fat floats. The fatter you are the more buoyant you will be, and the less you will weigh underwater. The leaner you are the more easily you will sink, and the more you will weigh underwater.

Several factors can affect the accuracy of underwater weighing. For example, African-Americans have denser bones than other races, and as a result may appear to have lower body fat percentages when measured hydrostatically. Also, men usually have denser bones than women, and younger people have denser bones than older people. Unless race, age and sex are all carefully taken into consideration, the estimate of body fat could be significantly in error.

Another factor that can influence the results of underwater weighing is your "residual volume." Residual volume is the amount of air left in your lungs after a complete exhalation. Before being lowered into the tank, you have to blow all the air out of your lungs. If you can’t blow every bit of air out of your lungs before being submerged, you'll appear to have a higher body fat percentage than you really do.

Dual Energy X-ray Absorptiometry (DEXA)

DEXA is the most accurate method to calculate body fat, but it's also the most expensive method. You lie on a special bed while low doses of two different X-ray energies scan your body from head to toe. In addition to measuring how much body fat you have, DEXA also determines where the fat is located on your body, which is a more relevant health indicator.

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