I bought a coupling to transition from a brass 1/2” FIP (female iron pipe) to 1/2” copper pipe. The only compression fittings that the store had did not have the traditional independent ferrule that I am accustomed to. Instead, it had a long sleeve inside for use inside plastic pipe, and a compression ring integrated into the nut. The exact part can be seen here.

I had thought that I could simply not use the insert that fits inside plastic tubing, since the specs say this fitting can be used on copper, but this compression ring that is integrated into the nut will not fit into 1/2" copper pipe. The copper pipe has an outside diameter of almost exactly 5/8”, but the inside diameter of the compression ring is only .57 (9/16”).

There is no way that this will go on, and this fitting obviously won't work, but I don't understand why it is sized the way it is. Neither Lowe's nor Home Depot stock the more traditional 5/8 compression fitting that I would expect. I could imagine that the spec could be wrong in mentioning copper, and that this could be designed for plastic only, but even so, how would any 5/8 OD tube or pipe ever fit through this thing with the integrated ferrule ID of .57? Why is it even called a 5/8?

  • Where did the copper pipe come from? Is it from a coil of bendable pipe that is normally used in refrigeration?
    – JPhi1618
    Mar 25, 2020 at 14:14
  • @JPhi1618 The pipe is standard 1/2” "hard copper" pipe, type L. 1/2” ID, 5/8” OD. This is not a bendable tubing.
    – Mike
    Mar 25, 2020 at 16:30
  • "Used to connect aluminum, copper and plastic tubing"
    – Mazura
    Dec 22, 2020 at 3:16

1 Answer 1


Pipe and tube are measured differently.
pipe is measured by inside diameter and tubing is measured by outside diameter. Depending on the fitting you may have the wrong one. I had a heck of a time remembering this when I went through my HVAC training as plumbing and electrical are normally pipe and hvac is normally tubing, back then the web was not as helpful as it is today for a full explanation and the 1/8” differences I believe that is your problem , look up pipe verses tubing measurements.

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