The plumbing in my new house is from the 1950s and I need to replace a section of it, a few feet of what I think is size 3/8 inch NPS. I figured this would be a simple matter, but I am completely unable to find this size pipe available in a consumer quantity. It seems like it is only made for industrial purposes? Where do owners of older homes find this type of pipe?

EDIT Apparently my situation is odd (?) so I thought I would add some pictures. You can see that the pipe I am replacing has OD of ~0.68" and the main pipe has OD ~0.88". For reference you can see that a nearby 2x4 measures accurately on the calipers.

pipe to replace main pipe 2x4 reference

  • Where are you on the planet? You seem to be mixing up pipe size conventions. "DN" is metric European, "NPS" is American. Also, "s" as in 10s you reference is a pipe wall thickness designation for pipes too thin for threads. 3/8" NPS pipe being used in an American home water supply system would be very unusual unless used for an appliance or fixture connection of some sort. 10s does not exist at all for American home plumbing. How certain are you that it is 3/8" NPS pipe (remembering that the 3/8" is not an actual measurement you will find on the pipe itself, hence the "nominal" in NPS. Jun 10, 2020 at 4:16
  • You say "schedule 40" which, to my knowledge is only used on the drain/vent (unpressurized) side of things. Odds are good you're going to have to get some flexible rubber couplings that will compress with a hose clamp to accommodate the old pipe size and transition to modern pipe sizes for your replacement pieces. If this is a pressurized supply side line, please edit to indicate that.
    – FreeMan
    Jun 10, 2020 at 12:29
  • That said, if you're planning/thinking about/expecting/it's-even-a-remote-possibility-of upgrade fixtures, you're going to run into this issue at every connection, and you may want to consider doing one big upgrade of all the plumbing to modern standards in one shot instead of piecemealing it one frustrating adapter at a time.
    – FreeMan
    Jun 10, 2020 at 12:31
  • @JimmyFix-it Interesting. I'm in the US. I have measured it with a variety of precision calipers and the outside diameter is definitely between 0.675" and 0.685", which as far as I can tell corresponds to 3/8" NPS pipe. Likewise, the Main pipe is ~0.87" - 0.88". When I went searching, I only saw similar values in tables of NPS pipe. I guess this is some sort of anomaly? Jun 10, 2020 at 13:09
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    @FreeMan Thank you. As you can tell, I'm no plumber - however I have considerable experience with electricity, enough to know to keep proper grounding. But thank you very much for the warning! So am I correct in assuming that these aren't standard sizes any more? Jun 10, 2020 at 13:42

1 Answer 1


It's copper, so it is rigid tube, not pipe, as specified in ASTM B88 (Standard Specification for Seamless Copper Water Tube). The actual OD is 1/8" larger than the nominal size. So, your .88 diameter is 3/4" and your .68 diameter is a slightly fat 1/2".

  • TBH, the 0.68" tubing looks like "black pipe" or iron pipe, but I'll absolutely yield to someone who is more sure.
    – FreeMan
    Jun 10, 2020 at 14:10
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    It looks like all solder joints to me. The short pup piece between the reducing tee and the valve may be leaking or leaked in the past causing some mineral deposits and corrosion of the copper.
    – Jim S.
    Jun 10, 2020 at 14:17
  • Good point. Thought those looked like the ends of threads.
    – FreeMan
    Jun 10, 2020 at 14:18
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    Concur - the 0.88" is spot-on for 3/4" (should be 0.875 but the caliper is limited to 2 decimal places). The 0.68" is a little high for the 0.625" that 1/2" should be, but it looks most likely that the extra 0.05" is coming from a ball of solder/corrosion, not an oddball pipe size.
    – nobody
    Jun 10, 2020 at 14:25
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    Yes, the pictures make it quite clear that this is bog-standard soldered copper, and the "slightly fat" is probably from measuring on a sloppy (poorly cleaned up after soldering) solder joint on a short stub between fittings.
    – Ecnerwal
    Jun 10, 2020 at 14:25

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