The penis also grows bigger during puberty. At the start of puberty, the average length of the penis is 6 cm (2.4 in). The penis reaches adult size about five years later. A study done in 1996 found the mean length of an adult man's penis is 3.5 in (8.9 cm) when it is flaccid (, ) (not erect or hard). The average length of an erect penis is about 12.9 to 15 cm (5.1 to 5.9 in).