It works by sending a very low electrical current through your body, body fat transmits this diffrently than muscle a computer measure the time the current takes and your weigth does a little math and spits out an answer that is probably incorrect. However, if you use the same time and with the same conditions every day you can certaintly tell wether you have gained or lost body fat.

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Jim

**Note above poster works for a retailer selling bikes and related gear*

I always hear how these scales are way off and totally inaccurate (usually from folks who say the reading "has to be too high" *wink*). I use mine occassionally upon waking, after 1st urination, and it is usually correct within 2-3%. At the very least, as Jim said, it will tell you whether your BF% is going up or down.

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-- Every morning brings opportunity;
Each evening offers judgement. --

They measure impedence. Fat has higher impedence (resistance) than muscle (a lot higher actually). They can be very accurate, but they are very sensitive to hydration status. It will read high when you are dehydrated, and low if you are super-hydrated. I have a Tanita that reads within 0.5% of my true BF (measured via water immersion). I have read that most people are euvolemic (neutral hydration) just before bed. That being said, I weigh myself each morning after using the loo. It reads 1-1.5% higher then, but it's consistent.

Scott

I bought one a few months ago as I decided it was about time I lost some weight and got fit again. As Scott says, it's very much based on your hydration status, so make sure you weigh-in under the same conditions every time.

I weigh-in when I get home after work, and I make sure I'm properly hydrated in work. It's usually about 1.5% higher in the morning when I'm always dehydrated.

I tried one of the hand held models at a local health food store a few weeks back and am glad to know that being dehydrated messes things up. I was also told having cold hands was causing it some problems (it was about -25 celsius outside and it refsued to take any reading at all until I rubbed my hands together for a minute or so). I had just finished doing a workout with an hour of swimming, an hour in a spin class and then an hour run and stopped in at the store where they were demonstrating the device on my way home. I know that I've put on some extra weight over the winter but just about fell over when I saw how "fat" I supposedly was :)