Average penis size is a little more than 5 inches long. With any average, the length of a penis varies from person to person.
On the small side of things, a guy is considered normal if his erect penis is at least 3 inches long. On the long end of things, some men have been found to have penises 8 inches long while erect. Longer penises certainly exist, however. In his book Penis Size and Enlargement: Facts, Fallacies, and Proven Methods, Garry Griffin found about 15 percent of men have a penis longer than 7 inches when erect; 3 percent of men are bigger than 8 inches when erect; one man in 10,000 has a 10-inch erection.
If you plan to measure your penis, wait until it is erect, as studies have shown penises that are relatively short when flaccid, tend to enlarge proportionately more during erection than larger penises.