Bioelectrical Impedance is a very quick and easy way to test body fat. Using a BIA scale (Bioelectrical Impedance Scale, often called a Body Fat Monitor) the individual stands or holds a scale and a safe, low electrical current passes through the body and measures body composition and the % of body fat they have.
The Bioelectrical Impedance works as the faster the signal travels the more muscle you have. This is owing to Fat containing little to no water and muscle containing 70% water. The water in our body conducts electricity so from the speed the current passes through the BIA scale can calculate our body fat and temperature.
This method of testing can be very accurate (4% margin of error) but is affected by hydration, food intake and skin temperature.