The average length of an erect penis

The study results were reported online in the Journal of Sexual Medicine.

For this sample of 1,661 men, the mean erect penile length was 14.15 cm (SD = 2.66; range = 4 to 26 cm), and the mean erect penile circumference was 12.23 cm (SD = 2.23; range = 3 to 19). 

The research team discusses their study in a column in Psychology Today, and there is commentary at Salon (and many other websites).

The data are consistent with previous reports.  Perhaps the cleverest aspect of the study was the method they used to ensure accurate self-reporting:

The data was taken from a larger study on condom fit — in other words, the research wasn’t framed as being about penis length, per se. The participants were told that they had to accurately measure their member in order to “receive condoms sized to fit their own erect penis.”

For Americans who still refuse to learn the metric system: 14.15 cm = 5.57 inches.