BIA (Bioelectrical Impedance Analysis) estimates percent body fat by sending a low, safe electrical signal through the body. The signal passes freely through the fluids found in lean tissue, but encounters resistance when it passes through fat tissue. This resistance of the fat tissue to the current is known as bioelectrical impedance. The bioelectrical impedance monitors have inbuilt formulas that can estimate your % body fat using the impedance value, together with your weight, height, gender and age.