The average woman carries roughly 46 pounds (21 kilograms) of skeletal muscle, which works out to about 31% of her total body weight. That number comes from a widely cited study of 468 men and women measured by MRI, the gold standard for sizing up individual muscles. But “average” hides a lot of variation: your height, ethnicity, age, activity level, and hormonal status all push the number up or down, sometimes by ten pounds or more. And depending on which measurement tool you use and what exactly it counts, the figure you get back can shift dramatically.
What That 46-Pound Figure Actually Measures
The study behind that number scanned adults aged 18 to 88 with whole-body MRI and found that women averaged 21.0 kg of skeletal muscle mass, compared with 33.0 kg for men. In relative terms, skeletal muscle made up about 30.6% of women’s body weight versus 38.4% for men.1PubMed. Skeletal muscle mass and distribution in 468 men and women aged 18-88 yr So the sex gap is real and large in absolute terms, but smaller when expressed as a percentage of total weight.
It matters that this figure refers to skeletal muscle mass specifically, not “lean body mass,” a term that gets thrown around almost interchangeably but means something quite different. Lean body mass includes everything that is not fat: organs, bones, water, connective tissue, and muscle all lumped together. In one analysis, skeletal muscle mass values ranged from about 27 to 37 kg, while lean body mass for the same individuals ran between 51 and 60 kg.2PubMed Central. Assessing skeletal muscle mass and lean body mass: an analysis of the agreement among dual X-ray absorptiometry, anthropometry, and bioelectrical impedance If someone tells you they have 120 pounds of “lean mass,” they are not claiming 120 pounds of muscle. Roughly half of that figure is water, bone, and organs. When you see body-composition results from a gym scale or a DEXA scan, double-check which term is being used before you compare yourself to any benchmark.
Why the Number on Your Report Depends on the Tool
MRI can visualize each muscle individually, which is why researchers trust it for precise measurements. But MRI scans are expensive and time-consuming, so most real-world body composition assessments rely on one of two cheaper tools: DEXA (dual-energy X-ray absorptiometry) or bioelectrical impedance analysis (BIA), the technology inside most consumer smart scales.
DEXA actually measures lean soft tissue, not skeletal muscle per se. It cannot separate water-swollen tissue from contractile muscle fiber. BIA sends a small electrical current through your body and estimates composition based on how quickly the signal travels; results can shift depending on your hydration, when you last ate, and even the time of day. Both methods tend to overestimate muscle in people who carry more body fat and underestimate it in leaner individuals.
A newer approach, the D3-creatine dilution test, measures only the contractile portion of muscle by tracking how your body processes a small dose of labeled creatine. In postmenopausal women, D3-creatine muscle mass correlated only moderately with DEXA lean mass, yet it was a stronger predictor of physical function. Women with high D3-creatine muscle mass had roughly five times the odds of scoring well on a standard physical performance battery compared with women who had low muscle mass, a relationship stronger than what DEXA-based measures showed.3PubMed Central. The Association of Muscle Mass Measured by D3-Creatine Dilution Method With Dual-Energy X-Ray Absorptiometry and Physical Function in Postmenopausal Women The practical takeaway: if you are trying to understand how much functional muscle you carry, DEXA and BIA give you a rough guide, but they blend contractile muscle with non-contractile tissue in ways that can be misleading, especially as you age.
How Age Reshapes the Picture
Muscle mass does not follow the same aging curve in women as in men. A large UK study of nearly 400,000 adults found that men’s skeletal muscle mass index (muscle mass adjusted for height) declines steadily across the lifespan, while women’s stays remarkably flat through middle age. At age 50, the median for women was about 6.8 kg/m², and it barely budged into later decades for most women, only dropping slightly in those who started with above-average muscle.4PubMed Central. Reference values for skeletal muscle mass and fat mass measured by bioelectrical impedance in 390 565 UK adults That does not mean women are immune to muscle loss. It means the decline is subtler and shows up more in muscle quality (strength per unit of muscle) than in raw mass.
Menopause accelerates the decline in muscle quality. Estrogen plays a direct role in how forcefully muscle fibers can contract, influencing processes like the chemical tagging that helps muscle filaments grip and slide against each other, as well as the function of satellite cells, the repair crew that muscle tissue relies on to recover from damage and grow.5PubMed Central. Aging of the musculoskeletal system: How the loss of estrogen impacts muscle strength This is why a postmenopausal woman and a premenopausal woman can have similar amounts of muscle on a scan but perform very differently on strength tests.
Ethnicity and Body Size Change the Baseline
The 46-pound average was drawn from a mixed but predominantly North American sample, and it does not apply uniformly across ethnic groups. Research tracking skeletal muscle across the lifespan found that Black women carry the highest average skeletal muscle mass, followed by White, Hispanic, and Asian women. The rate of age-related decline also differed: Black women showed the steepest drop in muscle after peak values, while Hispanic women lost muscle most slowly.6PubMed Central. Ethnicity-related skeletal muscle differences across the lifespan
These are not trivial differences. When researchers compared reproductive-age women at the same BMI, Black women carried about 2.1 kg (roughly 4.6 pounds) more total lean mass than White women and about 4.8 kg (10.6 pounds) more than Hispanic women.7PubMed Central. Racial difference in lean mass distribution among reproductive-aged women The discrepancy was even larger in the legs. Height also matters, of course. Researchers typically divide muscle mass by height squared to create a muscle mass index, which allows fairer comparisons between a woman who is 5’2″ and one who is 5’10”.8PubMed Central. Reference Values for Skeletal Muscle Mass – Current Concepts and Methodological Considerations Even after that adjustment, ethnic differences persist, reinforcing that population-specific reference values are needed for clinical thresholds like sarcopenia diagnosis.9PubMed. Ethnic differences in fat and muscle mass and their implication for interpretation of bioelectrical impedance vector analysis
How Much Muscle Can Women Realistically Gain?
A common misconception is that women cannot build significant muscle. In an early controlled study comparing men and women doing the same upper-body resistance program, women increased the cross-sectional area of their arm muscles by about 23%, while men gained about 16%. The difference in relative gains was not statistically significant for most measures. Men did gain more in absolute size, simply because they started with larger muscles.10PubMed. Muscle hypertrophy in men and women In practical terms, women’s muscles respond to resistance training with a similar percentage increase as men’s. The lower starting point means the absolute pounds gained are smaller, but the capacity for growth is very much present.
How much total muscle a woman can add over months or years of training is harder to pin down, because studies rarely run long enough and individual variation is enormous. A reasonable expectation for a previously untrained woman is somewhere in the range of several pounds of muscle over her first year of consistent strength training, with gains slowing each subsequent year. Genetics, nutrition, sleep, and hormonal status all modulate the ceiling.
What Muscle Mass Does for Metabolism
You have probably heard that muscle “burns calories at rest.” That is true, but the effect is far smaller than gym lore suggests. Validated research puts the resting metabolic rate of skeletal muscle at about 13 calories per kilogram per day, which converts to roughly 6 calories per pound per day.11PubMed Central. Comparing DXA and MRI body composition measurements in cross-sectional and longitudinal cohorts For a woman carrying 46 pounds of muscle, that works out to about 276 calories a day from muscle tissue alone. Adding five pounds of muscle to your frame would raise your resting burn by roughly 30 calories a day, about the energy in a single bite of a banana. Muscle’s metabolic value is real but modest at rest; its bigger metabolic contribution comes during and after exercise, when active muscle tissue burns far more fuel.
Where muscle mass becomes genuinely important for metabolic health is its role in blood sugar regulation. Skeletal muscle is the largest site for glucose disposal in the body. Analysis of national health data found that across the full range, higher relative muscle mass was associated with better insulin sensitivity and a lower risk of prediabetes. Each 10% increase in the ratio of skeletal muscle to total body weight corresponded to roughly an 11% improvement in insulin resistance and a 12% reduction in prediabetes risk.12The Journal of Clinical Endocrinology & Metabolism. Relative Muscle Mass Is Inversely Associated with Insulin Resistance and Prediabetes. Findings from The Third National Health and Nutrition Examination Survey That association held for both sexes, though a separate study in younger adults with overweight found the link between muscle mass and insulin sensitivity was significant only in men, not in women, suggesting the relationship may be more nuanced at different life stages.13PubMed Central. Association between muscle mass and insulin sensitivity independent of detrimental adipose depots in young adults with overweight/obesity
The Muscle-Bone Connection
Muscle and bone are not just neighbors; they are biochemical partners. Muscle contractions apply mechanical load to bones, stimulating the cells that build and maintain bone density. In a nationally representative U.S. sample, skeletal muscle mass index was positively associated with lumbar spine bone mineral density in both men and women.14PubMed Central. Correlation of muscle mass and bone mineral density in the NHANES US general population, 2017–2018 This is why the loss of muscle and bone often travel together after menopause, a condition researchers now call osteosarcopenia. When both conditions coexist, the risk of falls, fractures, disability, and death rises beyond what either condition alone would predict.15PubMed. A pas de deux of osteoporosis and sarcopenia: osteosarcopenia Resistance training is one of the few interventions that addresses both simultaneously.
Protein Intake and Preserving Muscle After Menopause
Among postmenopausal women, higher protein intake is independently associated with greater skeletal muscle mass index, even after accounting for age, time since menopause, and total calorie intake. Women eating less than about 0.93 grams of protein per kilogram of body weight had lower muscle mass indexes and higher waist circumference, body fat percentage, and trunk fat compared with those eating above 1.3 grams per kilogram.16PubMed. Skeletal muscle mass is associated with higher dietary protein intake and lower body fat in postmenopausal women: a cross-sectional study Physical activity was also positively correlated with muscle mass in the same analysis. Neither protein nor exercise alone tells the whole story, but the combination seems to matter most for women trying to hold onto muscle in a hormonal environment that no longer favors it.
Pregnancy, Lactation, and Temporary Muscle Changes
Pregnancy itself does not appear to cause significant muscle loss. An animal model looking at musculoskeletal changes across pregnancy and lactation found that muscle mass stayed stable during pregnancy but dropped by about 23% during lactation, likely because the metabolic demands of producing milk redirect protein and energy away from muscle tissue.17PubMed Central. Effects of pregnancy and lactation on muscle-tendon morphology Human data on this question are sparse, but clinically, many women report feeling weaker postpartum, and strength testing supports that experience. Both adolescent and adult women showed significantly higher odds of hip weakness in the third trimester and postpartum compared to earlier in pregnancy.18PLoS ONE. Muscle strength during pregnancy and postpartum in adolescents and adults The good news is that this loss appears to be recoverable with appropriate activity after the postpartum period.
Oral Contraceptives and Muscle Gains
Whether hormonal birth control affects muscle building has become a lively debate in exercise science, and the evidence is genuinely mixed. One study in young women doing a 10-week resistance training program found that oral contraceptive users gained about 1.0 kg of lean mass compared with 1.6 kg in non-users, a statistically significant difference. The pill users also had lower levels of anabolic hormones like DHEA and IGF-1 and higher cortisol.19PubMed. Oral Contraceptive Use Impairs Muscle Gains in Young Women That sounds alarming if you are someone who trains seriously and uses the pill.
But other research tells a less dramatic story. A 12-week strength training study comparing oral contraceptive users and non-users found no significant difference in muscle thickness gains or strength improvements between the groups. Both groups got substantially stronger and thicker in the muscles tested.20PubMed Central. Effects of oral contraceptive use on muscle strength, muscle thickness, and fiber size and composition in young women undergoing 12 weeks of strength training: a cohort study And a study examining muscle protein synthesis at the cellular level found that the active and inactive phases of the pill cycle produced identical rates of muscle protein building at rest and after exercise.21PubMed. Oral contraceptive pill phase does not influence muscle protein synthesis or myofibrillar proteolysis at rest or in response to resistance exercise The honest read is that oral contraceptives might slightly dampen the magnitude of lean mass gain in some contexts, but they do not prevent meaningful muscle growth, and the effect, if it exists, is probably small enough to be overshadowed by differences in training quality, nutrition, and genetics.
Menstrual Cycle and Day-to-Day Strength Fluctuations
If you have ever felt noticeably weaker on some training days than others, your menstrual cycle may be part of the explanation. In female soccer players, lower-limb muscle strength varied across the cycle: most muscle groups produced their highest force values during the late follicular phase (the days just before ovulation, when estrogen peaks) and their lowest during menstruation.22PubMed Central. Phase-Specific Variations in Lower-Limb Muscle Strength Across the Menstrual Cycle in Female Soccer Players The researchers flagged that phases were identified by calendar tracking rather than blood tests, so the precision is limited. Still, the effect sizes were moderate to large, which suggests this is not just statistical noise. For women who track their cycles, scheduling heavy lifting sessions during the late follicular phase and lighter recovery work during menstruation could be a reasonable experiment, though the research is not yet strong enough to call it a firm recommendation.
These fluctuations affect acute performance rather than long-term muscle mass. Training through every phase of the cycle still builds muscle. The question is whether periodizing around your cycle squeezes out a small extra benefit, and the jury is genuinely still out on that.