The average American man's penis is inches ( centimeters) long when erect, a new survey of 1, men finds. The study, published online July 10 in the Journal of Sexual Medicine, is relatively consistent with the results of prior surveys of penis size. Average Penis Size: What You Need to Know. According to a study published in the July 10, Journal of Sexual Medicine that looked at more than 1, men’s average penis size, researchers found that when erect, the average size penis is roughly about ” long (14 cm), ranging from to ” (4cm to 26 cm) long. How big is the average penis? By David Shultz Mar. 3, , AM but some reports suggest that only about 55% of men are satisfied with their penis size.