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Comparing different types of water piping is confusing. It seems that a 1/2" copper pipe is not the same size as a 1/2" PEX tube. I've read different things in different places (e.g. forums).

Which type of copper tube is equivalent to which type of PEX tubing?

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    Are you asking from an engineering (ex. head loss calculation) perspective or from a plumbing (ex. fittings and adapters) perspective? (Also just noticed you answered your own question, but it'd be good to clarify for future readers) – Hari Ganti Feb 28 '18 at 1:54
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I'm not sure about metric sizes, but this applies to US customary units.


The "1/2" designation is just a name - it is not a measurement of anything although in some cases it may be more or less approximately the same value.

For some applications, the outer diameter of the pipe is what is important. For instance, even though 1/2-in copper and 1/2-in PEX are not actually 0.5", they do have the SAME outer diameter and so will both fit many of the same push-fit fittings.

In other cases, such as making sure the desired amount of water flow can be achieved, the inner diameter is what is important. Consult the table below to compare among different types of pipe / tubing.

 Nominal Pipe Size   O.D.    I.D. Type K Copper   I.D. Type L Copper   I.D. Type M Copper   I.D. Pex A, B, or C   I.D. PEX-AL-PEX  
 ------------------- ------- -------------------- -------------------- -------------------- --------------------- ----------------- 
  1/4                 0.375   0.305                0.315                n/a                  n/a                                    
  3/8                 0.5     0.402                0.43                 0.45                 0.36                  0.346"           
  1/2                 0.625   0.527                0.545                0.569                0.485                 0.500"           
  5/8                 0.75    0.652                0.666                0.569                0.584                 0.637"           
  3/4                 0.875   0.745                0.785                0.811                0.681                 0.806"           
  1                   1.125   0.995                1.025                1.055                0.875                 1.032"           
  1-1/4               1.375   1.245                1.265                1.291                1.054                 No data          
  1-1/2               1.625   1.481                1.505                1.527                1.244                 No data          
  2                   2.125   1.959                1.985                2.009                1.653                 No data          
  2-1/2               2.625   2.435                2.465                2.495                No data               No data          
  3                   3.125   2.907                2.945                2.981                No data               No data          
  3.5                 3.625   3.385                3.425                3.459                No data               No data          
  4                   4.125   3.857                3.897                3.935                No data               No data          
  5                   5.125   4.805                4.875                4.907                No data               No data          
  6                   6.125   5.741                5.845                5.881                No data               No data          
  8                   8.125   7.583                7.725                7.785                No data               No data          

So, for example the closest PEX tubing size to a nominal 1/2-in copper pipe (any type) would be a 5/8-in PEX A, B, or C types.

Sources:

  • Normal plumbing and conduit sizes are measured by inside diameter not outside diameter. HVAC plumbing is measured by outside diameter the inside diameter is 1/8" smaller not so with plumbing sizes. So the numbers do mean something. – Ed Beal Feb 28 '18 at 15:07

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