When it comes to a pound of muscle versus a pound of body fat, gym myths abound. Muscle is more metabolically active than fat, meaning it burns more calories when you are at rest, but the level of this activity often is overstated.
Another common statement, that one pound of muscle weighs more than one pound of fat, brings smiles to exercise physiologists who point out that one pound is one pound, regardless of what it's made up of. Still, pound for pound, muscle does take up less space than fat.
A pound of muscle doesn't weigh less than a pound of fat, but it does take up less space.
It's a Matter of Space
Muscle is more dense than fat, which means it takes up less space than fat. While you may have heard that muscle takes up one-third the space of fat, the truth is a bit less dramatic: Muscle takes up approximately four-fifths as much space. Two people may be the same height and weight, but the person with a higher body fat percentage will wear a larger clothing size.
Weight by Size
Since it is denser, muscle does weigh more than fat if you compare same-size portions. On average, the density of fat is 0.9 g/mL. The density of muscle is 1.1 g/mL. Using the averages, 1 liter of muscle weighs 1.06 kg, or 2.3 lbs., while 1 liter of fat weighs .9 kg, or 1.98 lbs.
Metabolism and Burning Calories
Common gym lore says that 1 pound resting muscle will burn 30 to 50 calories daily compared to fat, which burns no calories. The truth is that muscle tissue will burn 6 calories daily per pound, says the National Council on Strength and Fitness. Fat burns 2 to 3 calories daily per pound.
If you utilize a strength-training regimen, you can expect to gain 3 to 5 pounds of muscle mass in three to four months, boosting your resting metabolism by 18 to 30 calories per day. The best way to benefit from the calorie-burning potential of your muscles is to actually use them.
Your basal metabolic rate, or the number of calories your body uses when you are at rest, typically accounts for 60 to 75 percent of the calories you burn in a day.
More Muscle and Less Fat
Though swapping fat for muscle might not raise your basal metabolic rate as high as gym myths indicate, it's still a good idea to stay lean. High body fat percentages are associated with elevated risk for obesity-related diseases such as Type 2 diabetes, heart disease, high blood pressure, breathing problems, gallstones and certain cancers.