Some of the scales use an electric pulse to measure body fat. I have heard those are accurate within a couple percent. The scale you have sounds like it only takes BMI into account, something entirely useless for an active individual. BMI charts assume I am 24-25% body fat, but I am actually right around 10%.
I believe hydrostatic weighing is the only "completely" accurate way. It is probably really expensive and you need to find somewhere that does it, though.
As far as I know all of the scales that give body fat percentage readouts pass an electric current up one leg, and it goes through part of the abdomen and down the other leg. From what I've heard, they are accurate within about 4% of the correct body fat percentage, if used under the correct conditions. Underwater weighing apparently has an error of a couple of percent and a technique using X-rays is even more accurate. Estimates using skinfold measurements with calipers probably fall in between scales and the more sophisticated techiques in accuracy, if
they're done accurately by someone experienced in the technique. Estimates using body circumference measures (waist, neck, height, etc.) such as the Navy technique are thought to be less accurate than correctly done skinfold measurements--generally they just rely on two or three measurements, so they don't fully take into account people's different body proportions.
There are a number of reasons why body fat scales are less accurate than some of the other techniques. Of course there are reliability problems because for them to give consistent readings you have to be at exactly the same place in terms of hydration, amount of food in your stomach, amount of moisture on your feet or other parts of your body when you're standing on the scales (wet feet and dry rest of body is the way you're supposed to do it), etc., and it's hard to be that consistent. As far as their validity, a problem is that the current doesn't pass through the entire body, it just goes up one leg and down the other because that's the quickest route out of the body. The principle behind the technique is that muscle conducts electricity better than fat because it contains more water. But since the current doesn't go through the entire body, people with the same body fat percentage but different distributions of fat are going to have very different readings, given that the calculations are done from the same formula. There are different formulas used for men and women, of course, but obviously individual men and women can have pretty different fat distributions. In my case, I store more body fat in my butt and inner thighs than I think the average man does. I've also, judging from my increases in strength and from looking in the mirror, put on perhaps a couple of pounds of muscle in my upper body over the last few months. So that's why I think the reading it gives for me (24.9% this morning, at 163.6 pounds) is probably a little high.
You can set my scales to an "athletic" setting that uses a formula that's supposed to take into account that people who work out have more heavily muscled upper bodies. But when I do that, it reads out around 11%, and I know that's far too low because if I were at 11% I'd have a totally flat stomach, etc. A one-size-fits-all formula can't differentiate between people with the upper body muscles of a football player or experienced bodybuilder and someone who's merely been working out steadily for a few months.
Of course, what we're doing this for is not to achieve some percentage of body fat, but to improve our level of fitness and strength and the appearance of our bodies! As long as I'm going in the right direction, I'm not going to obsess over what my exact percentage of body fat is (any more than I already have!). But it's interesting to compare the appearance of people with different reported percentages of body fat. Google around and you can find a ton of pages with such pix; here's one: http://forums.johnstonefitness.com/show ... hp?t=31392
. If the body fat percentages people are reporting on this page are anywhere near accurate, then I'd say my body fat right now is somewhere in the low 20s.