You're referring to bio-electrical impedence. This is the same methodology used in the Tanita scales. It is highly dependent on your hydration level. So, take your weight in the morning when you're dehydrated, and your weight is lower but your body fat percentage will be higher. Take it in the evening when you're better hydrated, your weight will be higher but your body fat lower.

Bio-electrical impedence is best used to measure trends over time. For example, if I step on my Tanita scale the same time everyday and it says I have 19% body fat, then it begins falling to 18% and then 17%, life is good. If, however, it begins trending upward, I might be concerned. Now my BF% might not really be 19%, but I can see the trend -- the change over time.

The most accurate ways to measure BF% (not BMI which is simply a calculation) is with a skilled technician using fat calipers. You should try to find someone who does this many times a day. Most personal trainers will be able to do it, however the secret is to find someone who is skilled. Then, in three months or so, return to that same person to do it again.

There are many other methods (including a couple of displacement methods --- water and air) that can be more accurate, but they both depend on the clients ability to perform the test correctly.

Lorri