You mean the plastic ones, right? They're standard in their OD for the cylindrical portion of the body of the tube, but not the cap. FYI, the 2 ml ones are the same OD, but have a longer cylindrical portion to make the extra 1/2 ml.
Anyway, I'd guess 10 mm. I'll edit later, if I can find a caliper to measure one.
I couldn't find a caliper, but eyeballing one vs. a plastic ruler shows 10 mm.
Reason: add "eyeballing" data