30mm is 3cm diameter.
Which is for me 3 Fingers, as one of my fingers is about 1cm in diameter.
And my forearm is long enough.
From the ellbow to the hand O.o
Pinky tip to thumb tip when fingers are spread as far as possible.
No measurement available at work.
As an Army brat born in Germany and later mainly raised in the US, I still only do the conversions by having memorized some general conversions. The one I've always found easiest because it's a relatively straight-forward conversion is:
25mm = 1 inch
So, a 10mm round is the same thing as .401 (in bore diameter only: case lengths vary) and a .50 cal is 12.5mm; 5.56mm (i.e. M16)is a glorified .22LR.