Ah, but Copper Tube Size (CTS) is ID! A 3/4" type K copper pipe is .875" OD. That is not 3/4". The ID of 3/4" type K is .745", that's 3/4" to most people.
However, since there is also type L, and Type M, with the wall thickness being the difference between types, "they" kept the OD the same for all three types, and let the ID change. Type M ID is .811". The OD of CTS pipe/tubing is .125" greater than the size listed. Even for DWV and gas rated copper.
Then "they" made PEX and PB to the same CTS standard OD measurements. So these (CTS pipes/tubing) are all standardized to an OD, but sized by number to close to their ID. My head is spinning....
The only OD sized tubing I know of is in the refrigeration field, ACR (Air-Conditioning and Refrigeration) copper tubing. If you work with that, most of the time you need to visit the HVAC supply store to get fittings.
Now what was the original question?:?
Frank DuVal