Ho do you measure end mill diameters?

[blackburn mark]

is it just a case of rounding up?

i wouldnt do that ? you want to be telling your cam software exact sizes

with vernier calipers i use the full length of the faces of the calipers to pinch as much of the cutting section of the tool as i can

i guess makes more sense to use a micrometer but that can be a pain as there are peaks and troughs when measuring with such a small surface aria that micrometers have so in that situation i revolve the tool in the mic until it starts to grip (revolve the tool away from the cutting direction)

youll get a feel for it

[Jonathan]

I think a micrometer is the best bet as callipers can easily be fooled by positioning it wrong etc. If the cutter has an odd number of flutes then neither will get far...

As they were from eBay the cutters may have been reground, which would make them slightly under the original standard (metric or imperial) dimension. I would start with using the nearest round number (unless it's way off) on something that's not critical, measure the part and compensate from that. It all depends on just how accurate you want to be.