Here are some links to sites that contain information about penile erections and related topics

These sites are listed because there are some useful topics on them. Some of them also contain advertisements and unsubstantiated claims that you should ignore.

 

penile length and circumference: a study of 3,300 young Italian males Published in 2001, this is a brief PubMed summary of a study by Ponchietti and his colleagues of penile dimensions of army draftees in Italy. This is an important study because of the large number of subjects and the fact that the subjects were not self selected (did not volunteer). The average stretched length (which correlates highly with erect length) was 12.5 cm. As in the study below, this equals about 5 inches.

penile length in flaccid and erect states

This is a brief PubMed summary of the 1996 article by Wessells, Lue, and McAninch. We consider this the most accurate USA measurement to date, with the best sample. Data are given in centimeters. For your information: 12.9 cm = 5.07 in.
foreskin retraction This is a nice animation of the foreskin and how it retracts. This animation is part of a site that opposes circumcision.
journal of human sexuality This is the site of the Electronic Journal of Human Sexuality. This site contains only a little on penile erections. One example is a paper, in Volume 2, based on a 1999 doctoral dissertation on penile augmentation surgery. This article is useful because it illustrates the wide variety of experiences men have with this procedure and the fact that complications can occur. Here you may find book reviews and other interesting topics on various aspects of sexuality.
altpenis This site has a variety of articles and topics relating to penises, flaccid and erect. Some sites on penis enlargement are referred to - and erectionphotos.com does not recommend these. However, the site has many other useful topics. It is recommended for these.
links to other penis articles This is a site that refers you to other sites. Many of its links are to articles that have some scientific basis. However, you should also know that some of the links, especially on other pages of the penissizelinks site refer you "size surveys" that have no scientific basis. There are ads for pornography on this site. Skip over what's at the top of the page. As long as you can separate fact from fun, this site is useful.