A whole body MRI study of 468 adults aged 18 to 88 found average skeletal muscle mass was 38.4% of body weight in men and 30.6% in women. These are averages from that particular sample, not ideal values or cutoffs for every adult. The study found that relative muscle mass tended to decrease with age, while a noticeable decrease in absolute muscle mass appeared later, particularly after the fifth decade.
This is why an online chart that gives a precise “good” muscle percentage for every decade should be treated cautiously unless it explains how the number was measured and where its data came from. A percentage can be useful for following a trend on the same device, but it is not a standalone diagnosis or a complete measure of muscle health.
What does muscle mass percentage measure?
Muscle mass percentage usually means the estimated mass of muscle divided by total body weight, multiplied by 100. But devices and studies do not always use the same definition of muscle. Some consumer scales estimate skeletal muscle, while others report lean mass or fat free mass. Lean mass includes more than muscle, such as water, organs, and connective tissue. Those numbers should not be treated as interchangeable.
The percentage can also change even if your muscle mass stays the same. For example, if someone loses body fat while maintaining their muscle, muscle will make up a larger share of their total weight. That does not mean they gained muscle. In the same way, an increase in body weight from fat or fluid can lower the percentage without a loss of muscle tissue.
The equipment matters too. MRI can measure skeletal muscle directly in research, while DXA measures lean tissue in different body regions. Many smart scales use bioelectrical impedance analysis, or BIA, which estimates body composition from how a small electrical current moves through the body. BIA estimates can be useful for tracking changes, but the result depends on the device’s equations and measurement conditions.
What the research says about age and gender
The available evidence supports a broad pattern: men have more skeletal muscle, on average, than women, and muscle measures tend to decline with age. It does not support one simple set of age-specific muscle percentage targets that applies to everyone.
In the MRI study, men had more skeletal muscle than women both in total kilograms and as a share of body weight. The authors also observed that relative muscle mass began to decrease from the third decade. However, absolute muscle mass did not show a noticeable decrease until later in life. Body size also explained some, but not all, of the differences between people.
Other research has created age and sex reference values for DXA measurements of lean mass. These data help show how measurements differ across age groups, but they are specific to the measurement method and population studied. Lean mass is not identical to skeletal muscle mass, and reference values from one DXA manufacturer may not transfer directly to another. So, these studies can provide context, but they do not establish a universal muscle mass percentage target.
A more useful clinical measure than muscle percentage
For older adults, clinicians may assess appendicular lean mass, which is lean tissue in the arms and legs. It is often divided by height squared to produce the appendicular lean mass index, or ALMI, measured in kilograms per square meter. This allows a clinician to consider lean mass in relation to height rather than relying on body weight alone.
The European Working Group on Sarcopenia in Older People uses an ALMI below 7.0 kg/m² for men and below 5.5 kg/m² for women as cutoffs for low muscle quantity when assessed with DXA. These are clinical cutoffs used as part of a broader assessment. They are not targets for fitness, and being above them does not by itself prove that someone has healthy muscle function.
The same consensus emphasizes muscle strength as the starting point for identifying possible sarcopenia. Low muscle strength can signal a problem even before low muscle quantity is confirmed. A clinician may consider grip strength, a repeated chair stand, physical performance, and medical history alongside a body composition measurement. Muscle percentage alone cannot establish a diagnosis.
Why smart scale results can be hard to compare
A BIA scale does not directly see muscle. It uses electrical measurements and an equation to estimate body composition. Since body water affects electrical conductivity, hydration and other short term changes can affect the reading. Recent exercise, food and drink, and the time of day can also make comparisons less consistent.
The equation used by one brand may differ from another, so two devices can report different muscle estimates for the same person. A BIA estimate can still be useful if you use the same scale under similar conditions and focus on longer term trends. For example, taking measurements at the same time of day and under similar hydration conditions may make repeated readings easier to compare. A single reading should not be treated as an exact measure of your muscle tissue.
DXA and MRI measurements have their own limits. They are more controlled than a home scale, but they measure different aspects of body composition and use different methods. If you are comparing results over time, using the same method and facility can make the trend more informative.
How to judge whether your muscle health is on track
If your goal is fitness, look beyond a percentage. Track strength and performance in ways that match your training, such as the loads you can lift with good technique, how many repetitions you can complete, or how easily you can perform everyday tasks. These measures do not tell you your exact muscle mass, but they provide practical information about what your body can do.
For older adults, or anyone concerned about unexplained weakness, difficulty standing from a chair, falls, or declining mobility, it is sensible to discuss the concern with a healthcare professional. A clinician can decide whether strength testing or a body composition assessment is appropriate. The EWGSOP2 approach considers strength first, then muscle quantity and physical performance.
If you are using a smart scale to monitor progress, treat its muscle percentage as an estimate and compare readings from the same device over time. Do not assume that a number from a different device uses the same definition. Also remember that a change in percentage may reflect a change in body weight rather than a change in muscle tissue.
The takeaway
A useful answer to “What is a good muscle mass percentage by age and gender?” is that science has not established one universal percentage chart. A large MRI study reported average skeletal muscle values of 38.4% of body weight in men and 30.6% in women, but these are sample averages, not health targets. Age, height, body weight, measurement method, and individual variation all affect how a result should be interpreted.
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For a more meaningful picture, pair body composition estimates with strength and physical function. If a clinician is assessing low muscle quantity, they may use validated measures such as DXA-derived appendicular lean mass index, interpreted alongside muscle strength and performance.
References
- Cruz-Jentoft, A.J. et al. (2019) ‘Sarcopenia: revised European consensus on definition and diagnosis’, Age and Ageing, 48(1), pp. 16–31.
- Imboden, M.T., Swartz, A.M., Finch, H.W., Harber, M.P. and Kaminsky, L.A. (2017) ‘Reference standards for lean mass measures using GE dual energy x-ray absorptiometry in Caucasian adults’, PLOS ONE, 12(4), e0176161.
- Janssen, I., Heymsfield, S.B., Wang, Z. and Ross, R. (2000) ‘Skeletal muscle mass and distribution in 468 men and women aged 18–88 yr’, Journal of Applied Physiology, 89(1), pp. 81–88.
- Janssen, I., Heymsfield, S.B., Baumgartner, R.N. and Ross, R. (2000) ‘Estimation of skeletal muscle mass by bioelectrical impedance analysis’, Journal of Applied Physiology, 89(2), pp. 465–471.
Key Takeaways
| Topic | Key takeaway |
|---|---|
| Muscle percentage targets | There is no universally accepted healthy muscle mass percentage by age and gender. |
| MRI study averages | One study found average skeletal muscle mass was 38.4% of body weight in men and 30.6% in women. These are sample averages, not targets. |
| Age | Research shows muscle measures tend to decline with age, but the timing and amount vary between people. |
| Consumer scales | BIA readings are estimates. Use the same device under similar conditions to follow trends. |
| Clinical assessment | Muscle strength and physical function matter alongside muscle quantity. |
| DXA measure | EWGSOP2 uses ALMI cutoffs below 7.0 kg/m² for men and 5.5 kg/m² for women as one part of clinical assessment. |
