OK I have always been pretty active and never fat.

I just started working out at the beginning of October.  At the time I weighed 190 @ 11.8% body fat.

Now, 2 months later I think I am seeing changes...better muscle tone and so forth.  So today I weighed in and measured my body fat again.

188 @ 11.2% .  I'm not real concerned about my level of body fat, I am checking it as more of a curiosity than anything else.  But here's what I'm confused about.  The way I figured it:

Early October
190 x 11.8% = 22.42 lbs. of fat
190 - 22.42 = 167.58 lbs. of bones, muscle, internal organs and so on

Today, Dec 8
188 * 11.2% = 21.06 lbs. of fat
188 - 21.06 = 166.94 lbs. of bones muscle, internal organs, etc.

So, assuming my internal organs and bones weigh about the same, it looks like I lost 167.58-166.94 = 0.64 lbs. of muscle since I started working out.

This doesn't make sense to me.  Can anyone explain to me what these body fat calculations are good for?  I'd have a hard time believing that I lost muscle mass after I started lifting.

The way they are measuring my fat is by punching in my age, weight and height and then I hold this little computer thing by two handles and I guess it measures resistance through the body or something and after about 10 seconds it spits out a number, say 11.8% fat or whatever.

So what's the real story behind this stuff?

You probably shouldn't worry too much, I doubt the test is that accurate.

Those tests are pretty inaccurate.   I believe they work by measuring the electrical resistance through your body.  You usually see the ones that are handheld or that you step on (typically integrated with a scale).   I did both types at one time a they differed by over 1%.   Being that I was around 10% bf at the time, that's a pretty significant difference.  Not sure if the difference was due to the inherent error in the machines themselves, or in the path the current takes throughout your body.  It would stand to reason that the current pretty much passes through your arms and upper body with the handheld and legs and lower torso with the step-on.  

I believe the only accurate test is the one where you are immersed in a tub of water.   Although that is obviously much less convenient...

Rocko

those are the most inaccurate bodyfat tests.  get a 7 or 9 point skinfold test.

Get a set of calipers and monitor your progress by skinfold testing.

Pay no attention to the charts that accompany the calipers for determining body fat from the measurements. Just pay attention to the measurements and make sure you take them in the same place and with someone holding the calipers the same way. if the measurements are smaller you are losing fat.

The only accurate way to determine body fat % is underwater weighing.