Lean body mass is simply an estimation of how much you weigh without the body fat – the weight of your bones, organs and muscles.
As the other factors are assumed to be relatively static, by monitoring your LBM you can get a fairly accurate estimate of the amount of muscle you are gaining or losing.
It is a critical number to know whether you are trying to lose or gain weight.
It is much more important than your bodyweight. People with the same bodyweights but different LBMs can look radically different (see Muscle Weight Vs. Fat Weight below).
Your goals are not to lose muscle and gain fat, they are to lose fat and gain muscle. Your LBM can give you a quick appraisal of where your training is taking you.
How To Calculate LBM
LBM is fairly easy to calculate once you have weighed yourself and figured your body fat percentage. You just calculate your body fat in pounds and subtract that from your bodyweight.
You can use the equations below or let the Guide’s calculator calculate your body fat percentage and lean body mass.
Body Fat in lbs. =
(Total Bodyweight)(Body Fat Percentage (in decimal form))
So someone who weighs 173 pounds with a body fat percentage of 17.5% would calculate as follows:
= 30.3 lbs. of fat
173 – 30.3
= 142.7 LBM
The accuracy of your LBM number is dependant on the accuracy of your inputs – your weight and your body fat percentage. As body fat percentages can be all over the map based on the methods used, your LBM as an accurate gauge of your fitness level should be taken with a grain of salt.
Use the mirror to make any major judgements about your body, use lean body mass as a progress report for your training program.
The Visible Difference Between Muscle and Fat Weight
You have probably heard the saying, “Muscle weighs more than fat.” This is true. While a pound of muscle weighs the same as a pound of fat, the volume is different. Muscle is denser than fat.
A pound of fat will take up about 18% more space than a pound of muscle. If squeezed into cans, you would need a can 18% bigger to contain the pound of fat.
In addition to volume differences, keep in mind that muscle wraps tightly to the skeletal structure whereas fat hangs loosely. So, all total, you can find two people of the same build and height who weigh the same appearing vastly different based on their lean body mass (LBM).
If your goals are to improve your physical appearance, you should concern yourself more with where your LBM is rather than your bodyweight.
The Importance of Knowing Your LBM?
What if your trip to the scales showed no bodyweight gain in two weeks time? Is it then time to make some big adjustments to your program? Maybe, maybe not.
Your lean body mass number could show that you actually gained muscle (and, necessarily, lost some body fat) which means your program is doing well and you probably wouldn’t want to change a thing.
And, of course, if your bodyweight is moving skyward, you should temper your enthusiasm until you get an accurate estimate of your lean body mass. It may just be body fat you are gaining.
How Much Body Fat Gain Is Acceptable?
On a mass gain program, it isn’t realistic for most people to expect to gain all muscle and no fat. At the most, you want to be gaining at a 1:1 ratio of muscle to fat (1 lb. of muscle for every one pound of fat).
Most beginner and intermediate level trainers should be able to do better, gaining at a 2:1 ratio or better (2 lbs. of muscle for every one lb. of fat). See Eating to Gain Weight: Concerns About Body Fat.
When you switch to a cutting program your focus will be on maintaining your muscle gains while losing the fat.
But… Is this information really necessary for success?
Yes, Yes and YES. Without it you are flying blind when it comes to adjustments. And unless you strike gold right out of the gate, unless you hit on the exact right diet and training program that will quickly take you to desired destination, adjusments will need to be made to maximize your gains.
See some samples of the ways your lean body mass and other body composition numbers will benefit your program by checking out the article, Using Tracking to Keep the Muscle Gains Coming.