BACKGROUND Although body mass index (BMI) is currently being utilized frequently as an indicator of obesity, it provides little information concerning body composition; key components such as fat and muscle cannot be differentiated. It is especially non-sensitive in identifying muscle mass, which can be challenging to examine without the use of radiologic methods. We sought to identify whether biometric values such as upper arm subcutaneous fat thickness/circumference could provide an adequate indicator of muscle mass. MATERIAL AND METHODS Patients admitted to our clinic for various causes were retrospectively studied in 95 consecutive cases. Physical parameters including upper arm subcutaneous fat thickness, upper arm circumference, weight, and height were measured. Then, values such as limb muscle weight (LMWDXA) and total fat weight (FWDXA) were obtained from dual-energy X-ray absorptiometry. Pearson's correlation coefficients were calculated and linear regression analysis was conducted. RESULTS Neither upper arm subcutaneous fat thickness nor upper arm circumference was correlated with LMWDXA. FWDXA also showed a correlation with BMI (r=0.823, P<0.001). LMWDXA also significantly correlated with measured body weight (BWm)-BMI (r=0.719, P<0.001). CONCLUSIONS From our analytic data we propose an equation for calculating muscle mass, designated the Simple Muscle Weight (SMW): SMW=289.2×(BWm-BMI)+3631. SMW calculation has potential for use as an easy and simple first-line diagnostic tool to identify diminished muscle mass.