Does muscle really weigh more than fat?

Generally speaking, I tend to get this question more from women that I do from men. I think this is because men tend to be more concerned with how much muscle they carry on their bodies. That isn’t to say that women aren’t concerned about the same thing, but what I have found is that women tend to use the “mirror” method to diagnose how much muscle they carry, while men tend to use the “numbers” method. What this means is that if you ask a woman how much muscle she feels like she’s carrying, she will refer to her reflection to determine if she thinks she is carrying too much muscle or just enough.

Not many women that I have met – or trained – seem to be of the opinion that they carry too little muscle (but that is a problem for another post). In contrast, many of the men that I talk to or train are more concerned with their “numbers”. What this means is they will tend to know their body weight, body fat percentage, and lean mass weight. Of course, men like to look good too (I am no exception) so we rely on the mirror method a little bit as well.

The reason that I mention this basic difference in how men and women tend to track their muscle and fat is because to really understand where your bodyweight comes from, you have to understand your numbers. It all plays out like this:

First, almost everyone I’ve ever met has heard the phrase “muscle weighs more than fat”. This illustrates an interesting point that we’re going to go over, but it is fundamentally flawed. One pound of muscle weighs the same as one pound of fat; two things that have the same weight, have the same weight no matter what. The major difference between muscle and fat is not how much each weighs, but rather how dense they are. Muscle is more dense than fat, so for similar volumes of fat and muscle, the muscle tissue will be heavier than the fat tissue.

You love science? I love science! Here’s some science.

First, conversion factors and such:

  • Fat Density: 0.9 kg/L
  • Muscle Density: 1.06 kg/L
  • Conversion: 1.0 kg = 2.20 lbs

Now, my current stats looks like this:

  • Weight: 165 lbs
  • Body Fat %: 14.3
    • Fat Mass: (165 lbs) * (0.143) = 23.6 lbs
  • Volume of Fat/Muscle:
    • Fat Mass: 23.6 lbs = 10.73 kg
    • Fat Volume: 10.73 kg / 0.9 = 11.93 Liters of volume

So, I have a pretty good Body Fat %, but you see that it still equates to 6 2-liter bottles of soda in body fat!

…holy crap, I need a minute. I’ve never done that calculation before…

Ok, we’re good. Moving on!

Now, lets assume that I exercise and diet with the target of maintaining the same total body weight, but changing my body composition. I eat a moderate calorie diet, lift weights three times per week, and maintain a clean diet (protein, veggies, some fruit, low carb). The following stats are a pretty reasonable target after, say, 6 months:

Here are my target stats:

  • Weight: 165 lbs
  • Body Fat %: 11.0
    • Fat Mass: (165 lbs) * (0.110) = 18.5 lbs
  • Volume of Fat/Muscle:
    • Fat Mass: 18.5 lbs = 8.41 kg
    • Fat Volume: 8.41 kg / 0.9 = 9.34 Liters of volume

So, with the same weight, adjusting only body fat, the total volume of “person” that I would be carrying around would drastically change. I would lose 2.6 liters of volume while staying the same weight. You can imagine how my visible person would adjust in relation to these body composition changes. I would appear more svelte, my abs would be visible (6-pack!), and I would appear incredibly lean. The additional several pounds of muscle would also affect how defined my muscles appear.

This science lesson all leads us back to answering the initial question:

Why does the scale tell me I’m so heavy, but the mirror tells me I look okay?

It’s all because fat has a greater volume per pound than muscle. For every pound of fat that you replace with a pound of muscle, your overall volume will decrease by about 15%. Yay science!