Lean Body Mass and Muscle Mass What’s the Difference?

Photo by Scott Webb on Unsplash 
Editor’s Note: This post was updated on June 8, 2021 by InBody, USA, for accuracy and comprehensiveness. It was originally published on September 24, 2015

Consider the following three statements:
  • “I’m not working out to get huge; I just want to build strength and put on five pounds of lean muscle.”
  • “My goal is to work out more and put on a healthy five pounds of muscle mass before next season.”
  • “I’m going to add more protein to my diet and hopefully gain 5 pounds of lean body mass by the end of the month.”
In each one, someone wants to gain five pounds of something but is using three different terms. Are these three ways of saying the same thing? Or are they different?
Let’s get one thing out of the way: “lean muscle” is a bit of a misnomer. Although there are indeed different types of muscle from a biological point of view, there is no such thing as “lean muscle”. The word “lean” usually suggests the absence of body fat. But here’s the truth: all muscle is “lean muscle”.
What about Lean Body Mass and Muscle Mass? Both exist. However, they are two very different parts of your body composition, and to understand your weight, health, and fitness goals properly, you’ll need to understand the differences between them. Let’s take a look below.

Lean Body Mass vs. Muscle Mass

While you may hear that it is essential to build lean muscle mass as you age, it is more important to focus on building lean body mass. Lean muscle mass is not an accurately phrased body composition factor, as muscles are lean by nature, but it is a widely used expression worthy of consideration. Lean body mass is more reflective of body composition and is, therefore, more worthy of examining.

What is Lean Body Mass?

Lean Body Mass (also sometimes known as simply “lean mass,” likely the source of the word “lean muscle”) is the total weight of your body minus all the weight due to your fat mass.

Lean Body Mass (LBM) = Total Weight – Fat Mass

LBM includes the weight of:
  • Organs
  • Skin
  • Bones
  • Body Water
  • Muscle Mass
Unlike lean muscle, Lean Body Mass correctly uses the word “lean” as it describes the entire weight of your body minus fat. This is why it is also known as “Fat-Free Mass.”
Because your Lean Body Mass comprises so many parts, any change in the weight of these areas is recorded as changes in LBM; keep in mind, the weight of your organs will not change much. Bone density will decrease over time, but it will not significantly affect the weight of your LBM.
Two major areas to focus on with Lean Body Mass are
  • Body water
  • Muscle mass.

What is Lean Muscle?

Lean muscle is a broad, frequently used term for discussing body composition.
In truth, most muscles are lean; therefore, the phrase lean muscle becomes a misnomer. Sometimes, the term lean muscle refers to the shape of the muscle build compared to a “bulky” muscle build; however, both types of muscle are lean and, in essence, fat-free. In effect, the term lean muscle is redundant and not accurate when discussing body composition analysis.

What is the difference between muscle mass and lean muscle?

Muscle mass, strictly defined, is the weight of the muscles of the body. When people say they have gained muscle mass, they usually mean that their muscles look and feel bigger. That said, the exact weight of the muscles is impossible to analyze, so muscle mass is generally a more subjective expression than a clinical definition.
Similarly, lean muscle is a phrase often used for lean muscle mass when someone is referring to the weight of the muscles, disregarding the amounts of fat that may be present within a muscle. Other times, lean muscle is a descriptive term, as opposed to a bulky muscle. In either case, there are more accurate descriptive factors to address.
lean body mass graphic

The Problem with “Lean Gains”

Because an increase in Skeletal Muscle Mass is an increase of Lean Body Mass, people will lump them together as “gaining lean mass” or “lean gains.”
However, it does not work the other way: an increase in Lean Body Mass is not always an increase in muscle. That is because body water makes up a significant portion of your Lean Body Mass. To illustrate this point, here is a body composition analysis of a 174.1-pound male.
Body composition analysis

Body Composition Analysis taken using the InBody

98.1 (Total Body Water) + 35.5 (Dry Lean Mass) = 133.6 Lean Body Mass
Water made up more than 55% of total body weight, which is normal for healthy adult males.
Notice how from a body composition standpoint, Lean Body Mass consists of three components, two of which are water. Everything else groups together in what’s called your “Dry Lean Mass,” which includes your bone minerals, protein content, etc.
Muscle gains contribute to LBM gains, but so does water, which can fluctuate throughout the day depending on hydration status, diet, and physical activity.
It is also important to note that muscle tissue itself contains water – a lot of it. According to the USGS, muscle tissue contains up to 79% water content. Research has also shown that resistance training promotes the increase of intracellular water in both men and women.
All of this points to two main problems when talking about “lean gains”:
  1. Big Lean Mass gains, when it occurs quickly, are mostly increases in body water.
  2. It’s difficult to say with any certainty how much any gain in Lean Body Mass is due to Skeletal Muscle Mass without using the right tools.

How to Measure Your Lean Body Mass and Muscle Mass

How to Measure Your Lean Body Mass and Muscle Mass

Since there’s a significant difference between Lean Body Mass and Skeletal Muscle Mass, how is it possible to know how much of each you have?
Let’s start with what not to do: do not try to use a scale to calculate changes in Skeletal Muscle Mass.
It’s a popular method to estimate muscle gain using a combination of the number on the scale and advice from fitness magazines.
The problem with using a scale to estimate progress is there are so many factors that can influence an increase in body weight, a few of which include:
There is only one way to calculate what is happening to your Lean Body Mass: getting your body composition analyzed. Without testing your body composition, there will be no way to know what any gain or loss in your body weight is due to.
Most methods of body composition analysis will, at the minimum, divide your body into Lean Body Mass (also known as Fat-Free Mass) and Fat Mass. These methods include:
  • Skinfold calipers
  • Hydrostatic Weighing
  • Air displacement plethysmography
Each of these has its pros and cons, and accuracy may vary depending on a number of factors unique to each testing method.
For more in-depth body composition analysis, dual-energy X-ray absorptiometry (DEXA) and Direct Segmental Multi-Frequency bioelectrical impedance analysis (DSM-BIA) are better options. Not only will these methods tell you how much fat you have, but they will differentiate your skeletal muscle mass from your lean body mass.

Will a Lean Body Mass Calculator Help?

A lean body mass calculator is a computation based on various inputs of height, weight, gender, and age. In general, it is the difference between total body weight and body fat weight. Most of the time, the calculation is more helpful for physicians to determine an appropriate amount of drug prescription or anesthetic than reflective of overall body composition.

Does Paying Attention to Just Weight Loss Work?

n short, no, weight loss is an inaccurate reflection of any projections of lean body mass, muscle mass, or lean mass. Weight loss, or gain for that matter, does not parallel overall health and body composition findings. 

Is My Body Fat Percentage Important?

Body fat percentage is part of the computation to discover your lean body mass, so yes, it is a vital factor to consider. Body fat percentage has a different healthy range for men and women and can provide insights into the overall health of a person. No one factor, however, is entirely predictive.

So, Lean Body Mass, Muscle Mass, Lean Mass, Which Is it?

Back to our original three statements: which is correct to say? Let’s review:
  • Lean Muscle: You should stop using this term because it is misleading. All muscle is “lean muscle,” and it is a confusing mix of two real terms: Skeletal Muscle Mass and Lean Body Mass.
  • Muscle Mass (or Skeletal Muscle Mass): Yes, it is likely true that if you are performing resistance training/weightlifting workouts and adding enough protein to your diet, a percentage of the change is likely due to muscle mass development. But remember that skeletal muscle mass is part of LBM. Things get tricky when you start putting numbers on your muscle mass gains. Everyone’s body composition is different, and the proportion of your skeletal muscle mass to Lean Body Mass will not be the same as someone else’s. This makes accurate estimations even harder unless you have access to sophisticated tools that differentiate between LBM and SMM.
  • Lean Mass (lean body mass): This is probably the best and safest term to use to describe your gains. When you use this term, you communicate you have gained weight from muscle and water, not body fat.
However, that’s all you can say. Lean muscle mass is redundant, and because of the nature of Lean Body Mass, it is difficult to say how much of the gain is due to water and how much is due to muscle (which is mostly water, to begin with). A 5 pound gain of LBM is not 5 pounds of pure muscle.

Lean Muscle vs. Muscle Mass? Which Is More Important?

Maybe the answer is something else.
When it comes to tracking your muscle gain (or fat loss), it all comes down to what tools you’re using to measure your progress. If you are working with just a weight scale, you will only know that your weight increases or decreases. It would be hard to differentiate the weight gain from water, muscle, or body fat. If you are serious about accurately measuring your muscle gain and assessing your health, certainly get your body composition analyzed. Then—and only then—can you tell people that you gained five pounds of muscle with confidence.

Get your InBody 570 Composition test, FREE to all our members!

Video on Lose The Fat, Keep The Muscle

by Robert Burns

shape-up logo

Like this article?


2101 E Coast Hwy
Corona Del MarCA 92625

Business Hours

MON 6:00 AM – 8:00 PM
TUE 6:00 AM – 8:00 PM
WED 6:00 AM – 8:00 PM
THU 6:00 AM – 8:00 PM
FRI 6:00 AM – 8:00 PM
SAT 8:00 AM – 2:00 PM
SUN 10:00 AM – 2:00 PM