My previous answer appears difficult to interpret when copied. The following is from Wikipedia and refers to erect penises in American college students.
The CancĂșn LifeStyle study, in which the circumference of the shaft was measured at three points (base, mid-shaft, and just below the head) and then averaged, found an average of 12.63 cm (4.972 inches) with a standard deviation of 1.3 cm (0.5 in).[8]
S.A.Liroff, M.D.
This this the latest list of studies that I can readily access from home. It is hard to measure in a consistent manner
TABLE 1. Circumference measurements* of the penis (from Dickinson, 1933/1971)
Author Year Circumference (mm) Diameter (mm)
Flaccid Erect Flaccid Erect
Krause 1879 - - 27 40-45
Waldeyer 1889 90 120 30 40
Loeb 1899 80-105 - - -
Delbet 1901 90 120 30 40
Piersol 1907 75 83 25 -
White & Martin 1911 75 - - -
Testus 1931 90 120 30 40
Range (mm.) 75-105 83-120 25-30 40-45
Average (mm.) 85 110 28 41
S.A.Liroff, M.D.