Most men who believe their penises are too small are actually of normal size. According to the Mayo Clinic, the average penis is 3 to 5 inches, or 8 to 13 centimeters, long when flaccid and 5 to 7 inches, or 13 to 18 centimeters, long when erect. Your doctor may recommend surgery on your penis to correct a defect, treat an injury, or improve its function.