It used data from 1,661 men (and incentivised them to do so accurately by offering to match them with a better-fitting condom) to determine that the average penis length was 5.7 inches, and the average girth was 4.81 inches when erect. That's 14 centimetres long and 12.2 centimetres around.
The research also found that the means by which a man gets erect may influence his size – men who were aroused via oral seemed to get bigger than with manual stimulation.
Different findings were published in a BJU International paper, which compiled research from 17 studies that measured the penises of a combined 15,521 men. The men were all measured by health professionals using the same standard procedure to ensure the numbers were as accurate as possible. And they looked at both erect and flaccid penises.
From their findings, the average flaccid girth, was 3.67 inches. Erect penises averaged 4.59 inches around. For some size reference, that's about the width of a tube of toothpaste.
There are some caveats, though. Mainly, these studies didn’t control for things like temperature, level of arousal, and previous ejaculation.
But FYI – whether you're packing a chipolata or a bratwurst it should have no bearing on how well it performs between the sheets.