This may sound like a stupid question (I'm new to plumbing terms), but for something like this. What diameter PVC pipe can fit into this?

For example, does this mean the diameter of the coupling itself is 2 1/2" and therefor can snuggly fit a 2 1/8" diameter pvc in it?

Or does this mean a 2 1/2" diameter PVC pipe can fit in it?

  • The trick with pipe is the outside diameter is what is standardized. The number is only the "trade size". At the time the pipe was invented, the trade size was the actual I.D. For instance when iron pipe was invented. As materials got stronger, the pipe walls get thinner but the OD is kept the same, so that it would continue to work with standard fittings (i.e. The threads are on the outside). You can look up trade vs actual OD on the web. Nov 16, 2017 at 8:36

1 Answer 1


If it is what we call a standard reducing coupling then the fitting you are talking will accept a standard 2" PVC on one side and 2.5" on the other.

There are some other odd ball coupling type fittings out there but for that size of PVC they would be extremely rear.

When we have a 1" PVC coupling then it is designed to couple together two sections of 1" PVC pipe. This is basically true for all the individual sizes. P.

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