Body fat scales

I would like to purchase a set of bathroom scales.
 
I have seen at places like K-Mart scales that are meant to calculate your body fat and I fail to understand how these things could be at all accurate.  I haven't tried them.
 
Has anybody had any experience with these things?  Do they work? (I very much doubt it)  If so, how do they work, and just how accurate are they?

These devices use a tiny electric current (completely imperceptible) to measure your bodyfat.  The theory is that lean muscle conducts electricity better than adipose tissue (fat) and water.  By measuring the resistance your body introduces to this tiny electric current, the device spits out a bodyfat percentage.

The science is somewhat sound, but the results are subject to about four hundred million variables, like whether you were perspiring when you stepped on the thing (or grabbed it), or whether your feet were wet, or whether you are over-hydrated, or whether you eat like a bodybuilder, or whether you are a girl or a boy, completely sedentary, or athletic, or... you get the picture.  Most of the off-the shelf impedance scales are pre-programmed with most of these assumptions, and I can tell you from personal experience that once your BF drops below 10%, and your activity level approaches that of a trained resistance exercise athlete, the reliability of the readings drops to LESS THAN ZERO.

Save your money and get a skin fold caliper instead.

My home away from DBB

I can't thank you enough for your post veggeep.  A very detailed and accurate reply.
 
That said, being a silly fool with too much $$$ and a need for a set of scales I have bought a set of scales that display BF% anyway (cost me $200).  True to what you said, I reckon they are out by 10%.  I gave them a bash when I first bought them and they placed my BF% at 28% - can't be right I'd place my BF% at 20% or less (probably around 16%).  Then again I had done everything that the manual said not to - it was late in the afternoon, I'd drank a lot of water throughout the day (as I normally do), I had had a workout (cardio - I went for my daily walk) and I'd more than likely had something to eat as well (can't really recall atm).
 
I gave it a bit of a test by weighing myself and then going to the toilet straight after and it responded accordingly.
 
I'll see in the next couple of weeks how accurate it is by following the instructions.  I'll record it on my website and give a review.
 
Thanks again.  I really appreciate your advice.

Worthless. The electrical impedance is based on the current traveling up one leg, through the groin, and down the other leg, while ignoring the fat from the waist up!!! It 'guesses' as to what the rest of the body is like based on the fat of the lower body. The more muscular you are, the less accurate it will be. The fatter you are, the less accurate it will be. The current MUST travel through the entire length of the body, from the right hand, all the way up the arm, down the torso, down the right leg, and then to the foot. This is how BioAnalogics.com works, backed by research that compares it to hydrostatic weighing in consistency and accuracy, with various ages, sexes, cultures, athletic, fat, lean, thin, etc.

Brian D. Johnston
Zone-Training.net
ExerciseCertification.com