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Estimate skeletal muscle mass, muscle mass percentage, and your Skeletal Muscle Mass Index (SMI) using the validated Lee (2000) equation.
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Skeletal muscle mass (SMM) is the weight of the muscles attached to your skeleton — the tissue you build with resistance training and lose when sedentary or underfed. It is a subset of lean body mass, which also includes bone, organs, and water, so your muscle mass is always a smaller number than your lean mass.
This calculator uses the Lee (2000) equation, which predicts total-body skeletal muscle from height, weight, age, sex, and ethnicity. It was developed and cross-validated against whole-body MRI measurements in 244 adults across a wide range of body sizes (R² = 0.86, standard error ≈ 2.8 kg). Treat the result as a solid population-level estimate rather than an exact individual measurement.
The Skeletal Muscle Mass Index (SMI) divides muscle mass by height squared, the same way BMI normalizes weight, and is the value clinicians screen against for low muscle mass (sarcopenia). The Janssen (2004) cut-points used here — below 8.50 kg/m² for men or 5.75 kg/m² for women — were derived from a study of over 14,000 US adults and linked to elevated disability risk.
On average, skeletal muscle accounts for about 40% of total body weight in healthy young men and roughly 30–35% in healthy young women. These numbers decline with age: adults who do not resistance-train lose approximately 3–8% of their muscle mass per decade after age 30, a process called sarcopenia that accelerates after 60.
Unlike BMI, which cannot distinguish muscle from fat, SMI directly captures the muscle component. Two people can share the same BMI yet have very different muscle mass — an athlete and an inactive office worker at the same height and weight will get different SMI scores, giving you a far more useful picture of body composition.
If you have access to a DEXA scan or bioelectrical impedance analysis (BIA), those methods provide a more direct measurement. However, the Lee equation requires nothing beyond a tape measure and a scale, making it a practical first-pass screening tool that you can repeat at home to track changes over months of training.
SMM (kg) = 0.244 × weight(kg) + 7.80 × height(m) + 6.6 × sex − 0.098 × age + ethnicity − 3.3 sex = 1 (male) or 0 (female); ethnicity = 0 White/Hispanic, −1.2 Asian, +1.4 African American SMI = SMM (kg) / height (m)²
No. Lean body mass is everything that is not fat — muscle plus bone, organs, and water. Skeletal muscle mass is only the muscle portion, so it is a smaller number. Skeletal muscle is roughly 30–40% of total body weight in healthy adults, while lean mass is typically 60–90%.
Enter your height, weight, age, and sex into the calculator above. It uses the Lee (2000) equation — a peer-reviewed formula validated against MRI — to estimate total skeletal muscle in kilograms. For a more precise number, get a DEXA scan or whole-body MRI.
The Lee equation correlates strongly with MRI (R² = 0.86) with a standard error of about 2.8 kg. It is excellent for tracking changes over time, but a DEXA or MRI scan is more precise for a single snapshot.
Commonly used reference ranges are 8.51–10.75 kg/m² for men and 5.76–6.75 kg/m² for women. Below 8.50 (men) or 5.75 (women) indicates low muscle mass that may warrant a fuller sarcopenia assessment.
For men, a healthy skeletal muscle mass percentage is roughly 36–45% of total body weight; for women, roughly 28–38%. Athletes and regular weight-trainers typically sit at the higher end. If your percentage falls below the lower bound, increasing protein intake and starting resistance training are the two most effective interventions.
Progressive resistance training 2–4 times per week, 1.6–2.2 g of protein per kg of body weight per day, a small calorie surplus, and adequate sleep are the levers with the strongest evidence. Consistency over months matters more than any single workout or supplement.
Yes. Without resistance training, adults lose 3–8% of skeletal muscle per decade after age 30, accelerating after 60. This age-related muscle loss (sarcopenia) increases fall risk, slows metabolism, and raises the risk of disability. Regular strength training and adequate protein intake can substantially slow or reverse this decline.
Skeletal muscle is the voluntary muscle you control during movement — biceps, quads, glutes. Smooth muscle lines your organs and blood vessels and works involuntarily. This calculator estimates only skeletal muscle because it is the component you can actively build through training and diet.