Honestly form I hear, accurately measuring average penis size is very difficult to do. I think ranges are the best we're going to be able to do for many years. Until some massive scientific breakthrough or something anyways. Those tend to completely re-arrange the entire field!
There definitely have been a lot of studies that used a clearly flawed methodology, the big three problems being:
1 - self selection (if asked to volunteer to have someone measure your dick, you'd definitely get fewer small guys.)
2 - self measurement (easily the worst one - studies that allow this are way, way higher than ones that don't.)
3 - erectile quality (EQ - the only in the list that would bring the number down.)
The first one can be controlled by using a group with a composition where size wouldn't be a factor - e.g. clinical studies for something unrelated that also gathers this metric (most common). I think the best was the Ponchietti et al study, which was on Italian conscripts as part of their physical. Young men (~18 - after the age where the dick stops growing for nearly everyone) getting their physicals for mandatory military service were measured, so this is about as good a cross sampling as you could possibly hope for. Also, at that age, they wouldn't carry as much fat so the whole bone-pressed vs non-bone-pressed thing becomes less of an issue.
The second brings the number way up - often these studies are at least 0.5" higher. When you combine 1 + 2, such as an internet survey (anonymous people just saying how big their dick is) the average skyrockets, often to something ridiculously high compared to medical studies, usually around 6.5".
EQ is a big issue, because we all know how an erection can vary day-to-day, and probably your weakest erection would be in a clinical setting where you're being monitored and pressured in to getting aroused. One way around this is to pharmacologically induce the erection (just inject something in to the penis) which ensures that everyone in the study has a consistently hard erection. The Chen et al study proved that maximal stretched length is an extremely good predictor of erect length. It varies slightly based on arousal state, but not much since you've already stretched it as far as it goes (usually erect is ~0.2-0.3" longer than fully-stretched.)
Every study that controls on those 3 factors has always, always put the average (or median, depending on what data they released) length in the low 5"s. I think the Ponchietti study is probably the best ever done, and they put 50th percentile stretched length at 12.5cm -- approx 4.9". You could add a little bit on there to extrapolate the erect length, and you'll get low 5"s like every other good study gets.
I just pulled out my tape measure (been wall-mounting some stuff today
) and measured using their outlined methodology - maximal stretch, and measure along the top from the tip to the skin (ie no bone pressing), and got 18cm (about 7.1"). I'm normally around 18.5-19cm (about 7.3-7.5") so the stretch predictor works pretty well for me at least. The Ponchietti study puts me above the top 1% with that, so definitely not complaining. I think of the distribution like this: that's about where the average NBA player is in terms of their height percentile. Most NBA players are extremely tall, but if you're the average NBA player height, about 50% of NBA players are taller than you. I think guys with porno-sized dicks compare against porno, and if 50% of the guys there (or on this site) are bigger than you it can really damage the ego of someone who has attached their entire self-worth to having the biggest dong.
Of course, if I pull my dick towards the floor and jam in the measure, then suddenly I'm 8.5". I start the measure from 1" (as way more guys do than you would believe), that's 9.5"; start measuring from the side instead of the top and that's another 1.5" inches, and then WOOAAHHH I'm 11" long! Crazy. That's about as silly a measurement as someone could do and legitimately think that they're actually measuring it right. If I put a ruler down on a table, hold my dick above it and take a picture, I can be 14" long. If I do the wide-angle lens thing from below my dick and take that picture between my legs facing up, my dick can be the size of my torso.
Hmm I don't want this thread to be where everyone writes their manifesto, but I hope I was able to illustrate how much of a difference using an established methodology can make.
You need to purchase the full Ponchietti study, but they did publicly publish many of the results in a followup article. This one was interestingly about how the vast majority of guys who think they have little dicks actually have average dicks - a bunch of them even had pretty big ones and thought they needed surgery. If there's any weight against that whole 'who does it hurt?' line it's in this article. This also covers the measurement methodology and includes a percentile distribution of their results:
Penile length is normal in most men seeking penile lengthening procedures
