The study, published online in the Journal of Sexual Medicine, also found the average penile girth was about 4.8 inches.
Debby Herbenick, a researcher at Indiana University, and colleagues asked the 1,661 men to measure their penis's length and girth for the purposes of fitting for a condom. The research was part of a larger study dealing with condom fitting.
The findings were relatively consistent with the results of prior surveys of penis size, but the study found different methods of obtaining an erection might lead to different size measurements. At the time of their measurement, some men were with partners, but most employed a do-it-yourself method for attaining an erection, Herbenick said.
The study found men aroused via oral sex reported larger penis size on average than those who attained erections through fantasizing, Herbenick said.
"We don't know if that means that when men have oral sex that it's more arousing and they get a bigger erection, or means that men who have bigger penises could be getting more oral sex in the first place," Herbenick told LiveScience.
Herbenick said next she would like to analyze data on female genitalia.
Richter Rubber Technology, a condom company in Kedah, Malaysia, provided condoms to the study participants.