Human penis size - Wikipedia en.wikipedia.org/wiki/Human_penis_sizePrivate View Studies Distribution of penis sizes by length. 45% of erect penises are between 12 and 14 cm long. Distribution of penis sizes by circumference. 81% of erect penises (green) are between 10 and 13 cm in circumference. Percentile of penile length Percentile of penile circumference. While results vary slightly across reputable studies, the consensus is that the mean human penis, when erect, is in ...