Body fat scales vary from bioelectrical imepdence analysis (BIA) and calipers, two types you can purchase for home use, to those tests that can only be conducted in clinical settings. The technology, accuracy, methods and level of skills needed to perform body fat readings using each scale also vary widely.
Types
BIA scales often look like regular bath scales or can come in hand-held models. Calipers are small plastic or metal tools with an opening and marked measurements used to pinch the outer layer of fat. Hydrostatic underwater weighing, considered the "gold standard" of body fat scales, requires weight measurements outside and inside a tank of water large enough to completely submerge you.
Methods
BIA technology is based on the idea that fat does not conduct electricity as well as lean tissue. A painless electric current is sent through the body to measure lean tissue and fat.
Calipers are used to pinch fat on certain areas of the body--under the arms, at the shoulder blades--to measure the outer layer of fat.
Hydrostatic underwater weighing is based on the fact that fat makes you float and lean tissue makes you sink. Your weight on land is compared with underwater weight to account for water displacement.
Accuracy
Hydrostatic underwater weighing is considered the most accurate body fat scale. According to research from the European Society for Clinical Nutrition and Metabolism, BIA technology can be an accurate scale for measuring body fat in people with stable hydration and electrolyte balances; and if algorithms take into consideration a person's age, sex and race. Accuracy in BIA is compromised in people who are at extreme ends of the BMI spectrum, or for fluctuations in hydration. The accuracy of caliper readings depends on the skills of the person conducting the test.
Availability
BIA scales and calipers can be purchased for home use and have the benefit of being portable. Costs vary, but can start as low as $30 for a handheld BIA scale and
$5 for calipers. More advanced BIA technology, with stronger electrodes, is used in clinical settings. Hydrostatic underwater weighing is only conducted in clinical and university research settings.
Alternatives
Recent body fat scales have emerged to replicate the accuracy of hydrostatic underwater weighing without the complications. The "Bod Pod," which measures air displacement, was found to be as accurate as BIA and hydrostatic underwater weighing, according to research from the American Journal of Clinical Nutrition. DEXA, or dual energy X-ray absorptiometry, is a body scan that measures fat, muscle and bone density and has been used to measure body fat in large population studies, such as those conducted by the Centers for Disease Control and Prevention.
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