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. Let’s clear up a common misconception: all muscle is lean. The term “lean muscle” isn’t scientifically accurate. But what is lean body mass and what is muscle mass?
These are real terms, and they refer to different aspects of your body composition. Understanding the difference is key if you want to accurately calculate lean body mass, track your skeletal muscle mass, and reach your fitness goals. Let’s take a look below.
Lean Body Mass vs. Muscle Mass
Many people use lean body mass, muscle mass, and even “lean muscle” interchangeably, but they aren’t the same thing:
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 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 can be 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 won’t significantly affect the weight of your LBM.
The two biggest factors that influence lean body mass are body water and muscle mass.
What is Muscle Mass?
When people talk about gaining muscle through strength training and protein intake, they’re actually referring to increasing their skeletal muscle mass — the only muscle type you can actively grow through exercise and nutrition.
When people talk about gaining muscle by eating more protein or muscle-building workouts, what they’re really talking about is gaining or building their Skeletal Muscle Mass — the only muscle type you can actively grow through exercise and nutrition.
However, muscle mass as a whole includes:
Skeletal Muscle Mass (the type you can build)
Cardiac Muscle (heart muscle, which functions involuntarily)
Smooth Muscle (found in organs, also involuntary)
This distinction is important because skeletal muscle vs. muscle mass isn’t the same thing. Many assume they’re interchangeable, but skeletal muscle mass refers specifically to the muscles you can develop through training.
Another key consideration is body water, which is a major component of lean body mass. This can lead to confusion when people use “muscle gain” and “lean gains” interchangeably. A sudden increase in lean body mass doesn’t always mean you’ve gained muscle — it could simply be fluctuations in body water.
The Problem with “Lean Gains”
Because an increase of Skeletal Muscle Mass is an increase of Lean Body Mass, people will lump them together as “gaining lean mass” or “lean gains.”
However, it doesn’t work the other way: an increase of Lean Body Mass is not always an increase in muscle. That’s because body water makes up a significant portion of your Lean Body Mass. To illustrate this point, here’s a body composition analysis of a 174.1-pound male:
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 is made up of three components, two of which are water. Everything else is grouped together in what’s called your “Dry Lean Mass,” which includes your bone minerals, protein content, etc.
Muscle gains definitely contribute to LBM gains, but so does water, which can fluctuate throughout the day depending on hydration status, diet, and physical activity.
It’s also important to note that muscle itself contains water – a lot of it. According to the USGS, muscle can contain 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”:
Big Lean Mass gains, when it occurs quickly, are largely increases in body water
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
Factors That Influence Lean Body Mass and Muscle Mass
Several factors affect lean body mass and skeletal muscle mass.
Genetics – Your genetics determine muscle-building potential, metabolism, and skeletal muscle mass distribution, affecting how easily you gain or retain lean body mass.
Age – After 30, muscle mass declines by 3–5% per decade without strength training, making resistance exercise and protein intake essential for maintenance.
Nutrition – Adequate protein, calories, and hydration support muscle growth and recovery, while deficiencies can lead to muscle loss and decreased lean body mass.
Exercise – Resistance training increases skeletal muscle mass, while cardio and mobility work help maintain overall lean body mass and body composition.
Hormones – Testosterone, growth hormone, and insulin regulate muscle protein synthesis, influencing muscle gain and retention, especially with aging.
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?
What You Shouldn’t Do
Avoid these common mistakes when trying to calculate lean body mass or track muscle mass:
Don’t rely on a scale alone – A scale cannot distinguish between fat, water, and muscle mass.
Don’t assume weight gain equals muscle gain – Changes in body weight can come from:
Undigested food and drinks
Water retention from glycogen or sodium
Fat gain from a caloric surplus
Don’t estimate muscle mass using generic formulas – Without proper measurement, you won’t know if lean body mass changes are from skeletal muscle mass or water fluctuations.
Don’t overlook hydration levels – Water makes up a large portion of lean body mass, and dehydration can make muscle appear smaller than it actually is.
What You Should Do
For accurate body composition tracking, follow these best practices:
Get a body composition analysis – The most reliable way to calculate lean body mass and skeletal muscle mass is through body composition testing.
Choose the right measurement method:
Skinfold calipers – Measures body fat at specific locations.
Hydrostatic weighing – Determines body density using underwater weighing.
Air displacement plethysmography – Uses air pressure to estimate body composition.
DEXA scan – Differentiates between fat mass, lean body mass, and bone density.
DSM-BIA (Direct Segmental Multi-Frequency Bioelectrical Impedance Analysis) – Separates skeletal muscle mass from lean body mass.
Track changes over time – Instead of relying on a single measurement, track trends in muscle growth and body composition over weeks or months.
Factor in hydration levels – Muscle mass contains up to 79% water, so proper hydration is key for accurate readings.
How to Increase Lean Body Mass Safely
Building lean body mass requires a combination of strength training, proper nutrition, hydration, and recovery. Here’s how to do it effectively and safely:
Strength Training
Prioritize resistance training with compound movements like squats, deadlifts, and bench presses.
Use progressive overload by gradually increasing weights or reps.
Train at least 2–3 times per week to stimulate muscle growth.
Optimize Protein Intake
Consume 0.7–1.0 grams of protein per pound of body weight to support muscle repair and growth.
Include lean meats, eggs, dairy, legumes, and protein supplements in your diet.
Eat Enough Calories
Maintain a slight caloric surplus (eating more calories than you burn) to build muscle without excessive fat gain.
Focus on nutrient-dense foods like lean proteins, healthy fats, and complex carbs.
Stay Hydrated
Muscle mass is 70–80% water so dehydration can reduce muscle performance and recovery.
Drink adequate water throughout the day, especially during workouts.
Get Enough Sleep and Recovery
Sleep 7–9 hours per night to allow muscle repair and growth.
Include rest days to prevent overtraining and support lean body mass development.
So, Lean Body Mass, Muscle Mass, or Lean Mass. Which Is it?
Back to our three statements: which is correct to say? Let’s review:
Lean Muscle – Avoid Using This Term: 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) – A More Precise Term: If you’re performing resistance training and consuming enough protein, it’s likely that a portion of your weight gain comes from muscle mass development. However, keep in mind:
Skeletal muscle mass is just one part of lean body mass (LBM).
The ratio of skeletal muscle vs. muscle mass varies from person to person.
Accurately measuring muscle mass gains is difficult without specialized tools that can differentiate LBM from skeletal muscle mass.
Lean Mass (Lean Body Mass) – The Safest Term to Use: This is probably the best and safest term to use to describe your gains. When you use this term, you’re telling people that you have gained weight from muscle and water, not body fat. However, this term still has limitations:
Lean body mass includes body water, so you can’t determine exactly how much of your gain is muscle vs. water.
Muscle itself is largely water, making it even harder to separate true muscle mass increases from fluctuations in hydration.
A gain of 5 pounds of lean body mass is not the same as gaining 5 pounds of pure muscle.
Use the Right Tools to Track Muscle Gain or Fat Loss
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 all you’re working with is a weight scale, then all you will ever know for sure is your weight is increasing or decreasing. It would be hard to differentiate the weight gain from water, muscle, or body fat.
If you’re serious about accurately measuring your muscle gain and assessing your health, go get a body composition analysis. Then — and only then — can you tell people that you gained five pounds of muscle with confidence.
