You can get endpin measurement devices from Lemur. They come in metric 8mm, 9mm, 10mm and 11mm, or you can opt for the British system 3/8", 7/16". etc if you have a British bass. Of course, I am not serious... You could measure the circumference and divide by pi, 3.14159... The other thing you could do is set the endpin down, point up, on a piece of paper and trace the circumference of the blunt end, then measure the diameter. The Lemur online catalog
http://lemur-music.com/catalog/Lemur...talogue_hs.pdf
sells wheels for the following endpin sizes:
5/16" or 8mm
10mm
5/8"
3/4"
9/16"
There is a tool that has calipers for measuring things like this, but I forget what it is called (sorry). It ought to be simpler than that though...
Jim