What is the average penis size? The average girth is 3. Girth is the circumference of the penis at its widest section. The BJUI research used data from 17 studies, and the study participants totaled more than 15, average health man penis size. In addition to the averages listed previously, the analysis charted sizes and placed them into percentiles. For example, an erect penis of 6. That means that out of men, only five would have a penis longer than 6.