Adult penis size varies considerably from person to person .A penis is considered abnormally small only if it measures less than 3 inches when erect (a condition called micropenis)
Also here are some points of interest from some of the international studies on this subject .
A study published in the September 1996 Journal of Urology concluded that average erect length of the adult penis was 12.9 cm (5.08 in) (Measured by staff) .
A study published in the December 2000 International Journal of Impotence Research found that average erect penis length was 13.6 cm (5.35 in) (Measured by staff) .
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