Do scales that calculate body fat really work?

Sort of. The devices, which cost $100 and up, send a mild, unnoticeable electrical current up through one foot to the waist, then down the other leg to measure how much body fat a person has. Since fat is a poor conductor of electricity compared with water (the main component of muscles, liver, brain, etc.), the current flows more slowly through fat tissue. Using complex formulas that include age, sex, height, and activity level, the devices — Tanita and HoMedics are the big players — can thus calculate what percent of a person’s weight is attributable to fat.