Body fat scales use the BIA (Bioelectrical Impedance Analysis) technique. This method measures body composition by sending a low, safe electrical current through the body. The current passes freely through the fluids contained in muscle tissue, but encounters resistance when it passes through fat tissue. This resistance of the fat tissue to the current is termed 'bioelectrical impedance', and is measured by body fat scales. When set against a person's height and weight, the scales can then compute their body fat percentage. Since BIA relies on the fluid levels of 'fat free mass', such as muscle tissue, certain factors can alter an individual's body fat reading. For example: - the reading may be lower than normal if you measure your body fat just after a bath or exercising - the reading may be higher than normal if you have just woken up or eaten a meal In general, there is less fluctuation in a person's body weight and hydration levels between the late afternoon and the early evening (two hours after eating lunch and before the evening meal). However, as everyone's daily routine and eating habits are different, you should determine the most suitable time to measure your own body fat. http://www.brianmac.demon.co.uk/fatbia.htm