Tube I.D. mm (1/64”) 1/64 = .015
1.4mm---------(3.5) .0525
1.6mm --------(4.0) .060
1.8mm ------- (4.5) .0675
2.0mm --------(5.0) .075
2.2mm --------(5.5) .0825
2.4mm --------(6.0) .090
2.6mm --------(6.5) .0975

So if you mike your tip and find it to be 0.0825, you would need a 5.5 tip top.
To order you will have to use the (5.5) or whatever yours mikes out to be.
If the number you mike is not exactly the same size, just order one the next size larger.

I have done the math for those of you that did not understand the chart I put up.
I caution that sometimes when you buy tip tops you will find when you mike the ID they will be a bit smaller because of plating.
Not all manufactures clean them up as well as I feel they should.
Diane's idea of buying at least 3 tip tops when putting together a rod. I buy at least 12 each of all the sizes when I make an order.
When I reorder I just fill my inventory back up. Course I understand this is not a good idea for those that build only one or two rods per year.
It makes things easier to open my inventory and try fit a tip top until I find one that fits properly.