How Do Scales Measure Body Fat?

Move over body mass index and height and weight charts, it's time to say "hello" to body fat testing as a gauge of health and wellness. Unlike the more common BMI or weight measurements that are unable to differentiate between lean muscle mass and fat mass, body fat testing more accurately gauges whether your body is composed of proper or improper amounts of fat.

Function

Scales that measure body fat use bioelectrical impedance analysis (BIA) in the calculation of fat mass. Stand on a scale that measures body fat, and a small electrical current (don't worry--you can't feel it) will run up one leg, through the body and down the opposite leg. The current will pass more quickly through lean mass, like muscle than fat or bone mass, which impedes the progression of the current. The scale measures the amount of impedance that the electrical current experienced while coursing through your body to determine fat mass.

Accuracy

According to Georgia State University, when used correctly, BIA scales measure body fat to three percent accuracy. This means that if your measurement reads 24 percent, your actual body fat could range from 21 to 27 percent. Because BIA measurements aren't perfect, rather than living and dying by the number on the scale, use the number as a guide.

Tips for Use

BIA testing is effected by the water content in your body, so aim to be at or near the same level of hydration every time you test. Georgia State University and the Sport Fitness Advisor suggested some of the following guidelines for gaining the most accurate results: do not eat or drink within four hours of testing, always test yourself at the same time of the day, avoid exercising at least 12 hours prior to the test, go to the bathroom before the test and avoid ingesting anything that could otherwise effect your hydration levels (e.g., alcohol, coffee and other diuretics). Test first thing in the morning to more easily abide by these guidelines.

Advantages

BIA testing on your bathroom scale is easy, reasonably accurate, inexpensive and can be done from the privacy of your own home. Unlike other methods of body fat testing that are performed by a professional and cost $30 to $100 per test, a body fat scale can range in price from as little as $25 to as much as $1,000, depending on the model.

Disadvantages

If you're interested in a very accurate body fat measurement, body fat scales aren't the best option. According to Georgia State University, BIA scales tend to overestimate very lean individuals and underestimate the fat mass of obese individuals. While they are helpful for tracking changes in body fat, they will not provide a perfectly accurate number.