What is the actual diameter of our current bicycle chain's rollers? Wikipedia states that it's 5/16" (~7.9mm) while two or three sites list it as 7.75mm.

I'm a little bit confused. Does it differ in different internal width? Does it differ between singlespeed and derailleur chains? If so, I'd like to know the diameter of the derailleur chain's rollers.
I've measured three well-used chains with vernier calipers.  Below are the method and measurements.  I'm assuming you mean External diameter of the roller in the chain.

TL,DR 7.66mm +0.04mm -0.02mm measured.


KMC X10 10speed chain with 4223 km 7.64mm

Old Shimano CN-HG93 9 speed chain with 3800 km 7.70mm

8 speed folder with cheap Clark chain, around 2000 km  7.64mm





Method - my calipers were binding on the inner plates, so to get a good reading I had to make the chain slack and then pull up a roller

BAD 

GOOD 

I used an old spoke as a hook to take the tension off while taking that photo.