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. Here, the women opted for, on average, a penis that was inches/ cm long and with a circumference of inches/ cm. But in a long-term relationship, size was slightly less of an issue, with the average preference being a length of inches/ cm and circumference of inches/ cm. Average Penis Size and things Related to it. Penis size is a topic which is discussed a lot when it comes to sexual relationships and their importance.