Muscle mass percentage is the portion of your total body weight made up of muscle tissue. For the average adult, skeletal muscle accounts for roughly 38% of body weight in men and about 31% in women, though these numbers shift considerably with age, activity level, and body composition. It’s one of the more useful metrics for understanding your overall fitness and metabolic health, and it’s increasingly available through smart scales, gym assessments, and clinical testing.
What Counts as Muscle Mass
Your body contains three types of muscle: skeletal muscle (the kind attached to bones that you consciously control), cardiac muscle (your heart), and smooth muscle (lining your digestive tract, blood vessels, and other organs). When people talk about muscle mass percentage, they almost always mean skeletal muscle, since it’s the type that responds to exercise and changes meaningfully over your lifetime.
Skeletal muscle is the largest component of what clinicians call “lean mass,” which also includes organs and bone. Some measurement tools report lean mass rather than skeletal muscle specifically, so it’s worth knowing the distinction. A lean mass reading will always be higher than a pure skeletal muscle reading because it bundles in everything that isn’t fat.
Average Percentages by Age and Sex
A large study published in the Journal of Applied Physiology measured skeletal muscle in 468 men and women aged 18 to 88 and found clear patterns. For men, average skeletal muscle mass as a percentage of body weight looked like this across age groups:
- 18 to 29: about 42%
- 30 to 39: about 39%
- 40 to 49: about 37%
- 50 to 59: about 35%
- 60 to 69: about 34%
- 70 and older: about 36% (though with wide individual variation)
Women generally carry a lower muscle mass percentage due to differences in hormones and body composition. The same study found that the rate of decline with age was roughly half as steep in women compared to men. Women’s relative muscle percentage typically ranges from the upper 20s to the mid-30s, depending on age and fitness level. In both sexes, the percentage begins dropping as early as your 30s, though the actual amount of muscle lost doesn’t become dramatic until around age 50.
Men lose roughly 1.9 kg (about 4 pounds) of muscle per decade on average, while women lose about 1.1 kg (roughly 2.4 pounds) per decade. After 50, the annual rate of muscle loss accelerates to 1 to 2% per year, a process called sarcopenia when it becomes severe enough to affect strength and physical function.
Why Muscle Mass Percentage Matters
Muscle is more metabolically active than fat, burning roughly 10 to 15 calories per kilogram per day at rest. Fat tissue, by comparison, burns about one-tenth to one-fifth of that. This difference means that two people at the same body weight can have noticeably different resting metabolic rates depending on how much of that weight is muscle versus fat. A higher muscle mass percentage makes it easier to maintain your weight over time and gives your body a larger reservoir for glucose storage, which supports healthy blood sugar regulation.
Beyond metabolism, your muscle mass percentage is a strong predictor of functional independence as you age. Losing muscle doesn’t just mean looking less toned. It translates directly into reduced strength, slower walking speed, greater fall risk, and a harder time recovering from illness or surgery. Clinicians now recognize low muscle mass as a distinct health concern, not just a cosmetic one.
What Drives Your Muscle Mass Percentage
Several hormones regulate how much muscle your body builds and maintains. Testosterone is the most well-known: studies have shown that testosterone replacement in men with low levels can increase muscle mass by as much as 20%. Growth hormone and its downstream signal, IGF-I, stimulate muscle protein production, and people with growth hormone deficiency consistently show reduced muscle mass and impaired muscle function. Insulin also plays a protective role by slowing the breakdown of muscle protein, which is one reason uncontrolled diabetes can lead to muscle wasting.
On the other side, stress hormones like cortisol actively break down muscle. People with chronically elevated cortisol (as in Cushing’s syndrome) develop visible muscle wasting as a hallmark symptom. Thyroid hormones, glucagon, and epinephrine also promote protein breakdown under certain conditions. The balance between these building and breaking signals shifts with age, generally tipping toward loss as anabolic hormone levels decline.
Genetics, physical activity, and nutrition round out the picture. Of these, protein intake and resistance training are the two factors most within your control.
How Muscle Mass Is Measured
The most accessible tool is bioelectrical impedance analysis, or BIA, which is built into many smart scales and handheld devices at gyms. BIA sends a small electrical current through your body and estimates muscle mass based on how quickly that current travels (muscle, which contains more water, conducts electricity faster than fat). It’s convenient and painless, but accuracy varies considerably.
Compared to DEXA scans (the clinical gold standard that uses low-dose X-rays), BIA can overestimate lean mass by anywhere from 3 to 8 kg in people with a normal to moderately high BMI. The errors get worse at the extremes of body composition. In people with obesity, BIA tends to underestimate body fat because its equations were often developed using normal-weight subjects. Hydration status also throws off readings: being dehydrated or retaining fluid can shift your results meaningfully from one day to the next.
If you’re using a BIA scale at home, the most useful approach is to track trends over time rather than fixating on any single reading. Weigh yourself under consistent conditions (same time of day, similar hydration) and watch the direction of change over weeks and months. For a more precise baseline, a DEXA scan at a clinic or university lab typically costs between $50 and $150 and gives you a detailed breakdown of muscle, fat, and bone by body region.
Maintaining and Building Muscle Percentage
Resistance training is the single most effective way to maintain or increase your muscle mass percentage at any age. Your muscles respond to progressive overload, meaning they grow when consistently challenged with loads that approach their current capacity. This doesn’t require heavy barbells. Body-weight exercises, resistance bands, and machines all work as long as the effort is genuinely challenging in the last few repetitions of each set. Two to three sessions per week targeting all major muscle groups is enough to produce measurable results within eight to twelve weeks.
Protein intake is the nutritional cornerstone. A meta-analysis of studies on muscle preservation found that consuming more than 1.3 grams of protein per kilogram of body weight per day supported muscle growth, while intakes below 1.0 gram per kilogram were associated with muscle loss. For a 70 kg (154 lb) person, that threshold works out to about 91 grams of protein daily. Spreading protein across meals rather than loading it into one sitting appears to optimize the muscle-building response, since your body can only use so much at once for protein synthesis.
Sleep and recovery matter more than most people realize. Growth hormone is released primarily during deep sleep, and chronic sleep deprivation is associated with elevated cortisol, both of which shift the hormonal balance away from muscle building and toward breakdown. Consistently getting seven or more hours of sleep supports the hormonal environment your muscles need to repair and grow after training.