Smart scales calculate your estimated body fat percentage using bioelectrical impedance. Sensors underneath the feet send an undetectable electric current from one foot through the body to the other foot. Vladimir Karpukhin, MSBCR (Medical Bariatric Surgery Clinical Reviewer) at Cedars-Sinai Marina del Rey Hospital, notes that electricity is conducted in the body by the water present in the muscle tissue. “In contrast to water, fat is a poor conductor of electricity. The more electrical current resistance encountered, the higher the percentage of fat in the body," he said.