Body fat

I just joined a gym. They tested my height, weight, blood pressue and
some other things. They put the figures into a computer which gave a print
out including my body fat percentage.

After 2 weeks I've noticed a slight loss in fat from my appearance but my
scales tell me I've lost a pound max. But I've also built a bit of muscle
as well.

Question is, this body fat percentage only seems to go off by height and
weight. How do they know what is fat and what is muscle?

Tommy wrote:
> Question is, this body fat percentage only seems to go off by height and
> weight. How do they know what is fat and what is muscle?

There are many methods that try to indicate how much fat you have.
Actually, what you describe -- BMI -- is not a measure of fat at all,
as you correctly realize. It is intended to be a triage tool to
indicate that perhaps we should look further. But unfortunately, some
people use it as the "end result".

If you google for "percent body fat" or "body fat composition",
eventually you will discover some web pages that provide a good
overview of all the methods currently favored to measure or give an
indication of "fatness". For example,
http://www.new-fitness.com/body_fat_analyzing.html .

But be advised: there is no perfect method, although the hydrostatic
method (water immersion) and DEXA (high-energy scan) are considered to
be the "gold standards". But all methods incorporate statistical
inferences to some degree -- some more than others. So there is always
a degree of "error" in the results -- again, some more than others.