This study clarifies the predicted subcutaneous shoulder depth and investigates the safety of the conventional (three-finger breadth method) and new (axillary method) intramuscular injection methods. The anatomical features of 245 volunteers who received the COVID-19 vaccination via the conventional method were investigated at the injection site (T point) and the hypothetical injection site using the new method (A point) via ultrasonography. The body mass index (BMI) and subcutaneous thickness at the T point (men: r = 0.75; women: r = 0.45) and the A point (men: r = 0.81; women: r = 0.55) were positively correlated. The upper arm circumference and subcutaneous thickness at the T point (r = 0.51) and the A point (r = 0.58) were correlated in women. Formulas to predict subcutaneous thickness using BMI and upper arm circumference were established: predicted subcutaneous thickness at the A point = 0.62 × BMI − 7.7 mm (R2= 0.66) in men and 0.658 × BMI − 5.5 mm (R2= 0.31) in women. This study demonstrates safe intramuscular injection sites and their depth.