A couple of quick rules of thumb...
Roughly 2.5 centimeters to the inch (2.54 but 2 1/2's close enough for raw stock)
1 meter = 39 inches (again, a close enough)
And the golden number 0.03937 for converting millimeters to Imperial.
10mm x .03037 = .393
3/8 (or .375) / .03937 = 9.52mm