Well here in the US we don't exactly use millimeters, we're a little behind everyone else on that...
But for you, I'll tell you.
3/8" is 9.525mm
1/2" is 12.7mm
and just for future reference, pretty much everyone on this board is in the US, so you'll get English units all the time. Just go to google.com and type the conversion. To get your measurements, I went to google and typed "1/2 inch to mm"