For example, an erect penis of inches is in the 95th percentile. That means that out of men, only five would have a penis longer than An erection is a physiological phenomenon in which the penis becomes firm, engorged, and enlarged. Penile erection is the result of a complex interaction of. The average erect penis was inches (cm), while the average flaccid penis was inches (cm). But maybe take this with a pinch of salt – the " study.