Body Fat scales

These body fat scales most stores sell, how do they function, and do they work? I have seen some claim that they pass an electrical current through you and measure which is fat and which is muscle, but wouldn’t you have to stick electrodes on you or something? Surely it can’t tell from just passing a miniscule current in the soles of your feet?

Ok, I admit, sounds like junk science to me off hand. Sort of like the Ionic Breeze or wearing a copper bracelet. I am willing to admit I am wrong, but I am curious as to what the Straightdope has to say about these things.

Work, not work? How? Scam or great product?

I have one, and it seems to work. This is because it shows a higher body fat % at times when I’m less hydrated (e.g. in the morning). At timeswhen I’m more hydrated, the number is lower. If it were a scam, I wouldn’t be getting such consistent results.

But don’t use one if you have a pacemaker.

Maybe this will help explain it. I have a body fat monitor scale too and it seems to work. I don’t think it’s junk science, and it’s an easier way to measure body fat than the dunk yourself in water or the X-ray methods.

IIRC per CR reviews they suck as to the absolute accuracy in terms of telling you what your actual (accurate) body fat percentage is, but they will correctly indicate changes from their (inaccurate) baseline so at least it can tell you if you are improving or not.

When I joined the local health club, I was given a reading on one of those things during the tour, and again the next day when I signed the contract. They were on two different “identical” scales, and the body fat percentages differed by about 10%. YMMV. :dubious:

Oh, and although the thread title seems to say otherwise, body fat does not cause you to have scales on you. :wink:

So I wonder, this Bioelectrical Impedance method, how does it work exactly? I mean, a very low electrical “signal” (as the site above mentions), wouldn’t travel very far into the skin, or very far along the muscles, certainly not above the calves. So how does this machine claim to tell your whole bodyfat percentage, when the most you are getting is your legs measured?

Or is there some new fangled electricity that travels your whole body with very little amperage, does no damage, and isn’t felt beyond a slight tingle?

On my scales you stand with each foot on a different electrode. I assume they measure current going up one leg and down the other. It also asks for your height so perhaps it uses some average height/leg length ratio in its calculation.

Oh, and all calculations will be made on the assumption of an average fat distribution so value measure for your legs can be used to approximate for your whole body. No doubt they have been calibrated with the results of more precise but cumbersome measurements for a range of subjects.

Yes. So if someone is more “apple” (fat around the stomach) or “pear” (fat around the hips), that will also affect the readings.

Most scales that you buy at home or use at the gym measure the current going from one foot to the other. You can also get body fat analyzers that you grab with both hands, and the current travels across your upper body. These are “two lead” systems - one electrode on each foot, or one on each hand.

A “four lead” system is more accurate. You attach an electrode to one hand, and the opposite foot, as well as two other electrodes. Thus the current travels all the way acros the body, not just the upper half or lower half. The extra two electrodes help “calibrate” the amount of impedance you have, which makes the reading more accurate.

This site gives a good overview of bioelectrical impedance measurement, and it states:

Choice magazine did a test on the bathroom impedance based body fat scales and compared them to immersion based body fat testing, which is much more accurate.

The best scales ended up with about high 80’s to 90% accuracy from memory, so as long as your body isnt hugely unusual, they dont seem to be too bad for checking overall weight loss if you’re trying to do something like go from 30% to 20% over time, just dont expect it to tell you whether you’re 16 or 17% fat on a particular day.


Seems to me the scale is the easiest way, though not the most accurate. The tank method is, to my understanding, kind of expensive to do and the most accurate. My gym told me they would use calipers and pinch various parts of my body to get the number. I told them it wasn’t that important to me.

Funny aside: The Simpson’s had an episode where the nuclear plant was checking the health of its employees. The technicians are taking readings of the employee’s body fat percentage. The scale reads 110%, zoom to Homer in the tank with a doughnut and one of the technicians shouting, “Hey, no eating in the tank!” I guess you had to see it.

I found mine to be completely useless as a body fat indicator. As a scale, it’s great, squares with the professional balance scale at school pretty well. The problem is that it tells me I’m at 26% body fat when my first reading on the thing was 24% body fat. I’ve lost 9 kg since that first reading, a reduction of more than 10% of my gross body weight, and I have gained visible and tangible amounts of muscle, which means that I’ve probably lost even more than 10% body fat. It wasn’t an el-cheapo scale either, which makes me distrust the whole technology.