There's a lot of hoopla about penis size on a site like this. There's propaganda that 'black' mens penis is longer, there's poor research results that say averages not only vary per ethnicity and race but also country. Sadly all of that is bullshit.

Fact is most US based media is of dark skinned porn models. So well above average size. Otherwise they just don't get selected. And the vast majority of the poor dark skinned dudes miss out on well getting their dick sucked.

Most men also report their own measurements. Whatever their skin colour. Which unfortunately means they add a few yahoo inches, or worse, if they're sensitive about it, never report on their sizes. So you don't normally hear about below average size penises, except for a few who have learned to get off on it. Not many have unfortunately.

So the most reliable studies or surveys are done by professionals such as urologists. Around the world. Wikipedia has a great article on it: https://en.wikipedia.org/wiki/Human_penis_size

 And the British Journal Of Urology (BJU) also wrote an article about it on Dec 2014: https://onlinelibrary.wiley.com/doi/full/10.1111/bju.13010

So the average is 5.17 inches, or 13.12 cm in length.