We've all heard that human muscle weighs more than fat cells given the
same volume.  OK, I believe that, but how much more does it weigh?  In
other words, given a liter of fat and a liter of muscle, how much
would each weigh?  Of if you prefer, given a pound of fat and a pound
of muscle, how much more volume would the fat have?

Please cite only credible information here.  I saw one site that said
muscle weighs 3 times more than fat but they don't cite any references
so I wouldn't consider that credible.  I also saw one site that said
fat weighs 7 lbs per gallon; but again, no references about where the
info came from.

Hello Holstein13, 

Thanks for your question. I hope you will find this answer to be
provided with solid scientific references.

Summary: Muscle density is 1.06 g/ml and fat density is (about) 0.9
g/ml. Thus, one liter of muscle would weight 1.06 kg and one liter of
fat would weight 0.9 kg. In other words, muscle is about 18% denses
than fat. This should not be confused with the "energy density" of
muscle and fat, which may be where you got the 3x figure that you
mention in your question.

I have made a special effort to find credible sources for the answer.
Below I provide sources for the two numbers separately.

1. The density of mammalian skeletal muscle is 1.06 g/ml. 
"... 1.06 g/cm-3 which is the density of mammalian skeletal muscle"

Source for quote: The Journals of Gerontology Series A: Biological
Sciences and Medical Sciences 56:B191-B197 (2001)
Specific Force Deficit in Skeletal Muscles of Old Rats Is Partially
Explained by the Existence of Denervated Muscle Fibers
Melanie G. Urbancheka, Elisa B. Pickenb, Loree K. Kalliainenc and
William M. Kuzon, Jr.a,d
http://biomed.gerontologyjournals.org/cgi/content/full/56/5/B191#R23 

Reference given by the authors: Mendez J, Keys A, 1960. Density and
composition of mammalian muscle. Metabolism 9:184-188.

2. The density of adipose tissue (fat) is about 0.9 g/ml
"...by multiplying the density of adipose tissue (0.9196 g/ml)"
Source: Association of adiponectin and resistin with adipose tissue
compartments, insulin resistance and dyslipidaemia
M. S. Farvid1, T. W. K. Ng2, D. C. Chan2, P. H. R. Barrett2 and G. F. Watts2*
http://www.blackwell-synergy.com/doi/abs/10.1111/j.1463-1326.2004.00410.x

"...on the assumption that the density of adipose tissue is 0.9g/cm3" 
Source of quote: Pediatric Research 55:437-441 (2004)
Distribution of Adipose Tissue in the Newborn 
TRACEY A.M. HARRINGTON, ELIZABETH LOUISE THOMAS, GARY FROST, NEENA
MODI and JIMMY D. BELL
Source given by authors: 
Ross R, Léger L, Guardo R, De Guise J, Pike BG 1991 Adipose tissue
volume measured by magnetic resonance imaging and computerized
tomography in rats. J Appl Physiol 70: 2164?2172

I hope you find this to be useful and a complete answer to your
question. Let me know if there is anything I can clarify.

Regards, 
RMH

Search Strategy: 
I searched Google using the phrase "density of mammalian skeletal
muscle" and "density of adipose tissue". I used a computer in a
medical center that has free, online access to many scientific
journals.

"A corollary to this is that you may gain weight due to lifting
weights, even while loosing fat weight. This is one of the greatest
exercise myths. The myth is based in fact: muscle is more dense and
weighs more than fat. The same weight of muscle take up nearly half
the volume as the same weight of fat.

The problem with this is that not very many people will be able to add
the volume of muscle mass relative to fat lost to gain weight.
Remember, the average man will add only 4 Kg of muscle mass in a
rigorous training program."

http://www.athleticadvisor.com/Weight_Room/fitness_myths.htm

It is interesting that you mention this myth because I never mentioned
it in my post.  However, you are right on target.  I hear so called
"exercise experts" telling their subjects to ignore the scale.  It
seams to me that dieting is a far more effective method to reduce fat
than exercise alone.

You wrote "The same weight of muscle take up nearly half the volume as
the same weight of fat."  Where did you get that info from?  This
seams like a very easy question to answer.  All you need to do is buy
a pound of pork and extract the same volume of fat and lean meat from
the product.  Then you can weigh each of them.  Unfortunately, I don't
have the tools to do that.

Everything in my previous post was a quote from this site:
http://www.athleticadvisor.com/Weight_Room/fitness_myths.htm

The diet/excersize plan you should take depends greatly on your goals.
 In the long term, working out is much more effective for losing fat
than dieting alone.  There is of course the short term effect of
burning fat as you excersize, but the better benefits occur when the
muscle you create by exercising greatly increases your normal
metabolic rate (how many calories your body burns to sustain itself).

hey, i was just wondering about this BMI (body mass index).
i weigh about 70kg and im 1,70 metres tall. usually to find your BMI
you need to devide your weight by your hight squared.
my BMI was 24.22. that shoudl mean that im nearly overweight. but the
odd thing is im not. i look perfectly normal. i used to train martial
arts before even. im pretty fit actually. why is that?