A small and completely harmless electrical current is passed through your body. The electrical current passes more quickly through fat free tissue like muscle than it does through fat or bone tissue. The amount of resistance to the electrical current relates to how much fat-free mass a person has and their body density.
When buying Body Fat Monitory Scales, you should consider the following points:
Use them to measure your progress only. Don't compare your body fat percentage to tables or to your friends score. It is probably inaccurate no matter what the manufacturers say.
Choose body fat scales that have the right profile for you. If your children are going to use them make sure they can be calibrated for children. If you're an athlete (+10 hrs of strenuous exercise a week) same thing applies. This is important. It makes a big difference.
Measure out some water one hour before you test yourself. Make sure you always drink the same amount of water one hour before you test yourself.
Measure yourself at the same time of day for each test.
Ensure that you keep a detailed record of your statistics.
Your skin temperature affects the electrical current used by the scales. It's difficult but try to test yourself in a similar room temperature each time.
Don't test yourself after exercising. When you exercise you sweat and when you sweat you lose water. This affects your hydration levels and hence the results.
Thoroughly clean the foot pads, preferably with alcohol and then dry them off each time you test.
Buy the most expensive body fat scales you can afford. Accuracy will almost certainly increase with price.
If ever you get the chance, have your body composition measured by a professional using skin fold callipers. It will give you a frame of reference as to how accurate your scales are.