I'm trying to purchase the correct toilet flange but having difficulty.

The bathroom was roughed in years ago in ABS and we are trying to finish it out. As a I measure the toilet waste pipe, it measures 3 1/2" inches from outside edge to outside edge. The inner diameter (inner edge to inner edge) is more like 3 1/8"+. So, we are close to 3", but it is still slightly wider than 3".

The last flange I purchased advertised itself as "able to fit INTO a 3" ABS waste pipe", however it was in actuality the exact same diameter, so it did not insert into the pipe... it just sat on top and was of course no help.

Before I make yet another trip back to the store, I wasn't sure if there was any wisdom out there on the best product to go with given my situation and pipe measurements. Or perhaps I'm not understanding the terminology


1 Answer 1


The outside diameter is more important with PVC pipe because that's how the fittings attach. For instance, DWV (drain) pipe and SCH40 pipe have way different wall thicknesses (so different inside diameters) but the same outside diameter.

That said, you have a 3" pipe there, and should get a flange made for 3" pipe. What you will typically find will be a "3x4 toilet flange" that will fit over a 3" pipe (flange has a 3.5" inside diameter) or into a 4" pipe.

You may have purchased a flange made to go into a 3" fitting previously.

Your comments indicate this is a concrete slab, and you can't connect around the 3" ABS pipe, so a 4x3 flange won't work as it sits. You have two options in that case.

The easiest is to get a flange designed to fit inside ABS pipe specifically, like the Sioux Chief 888-A. This is simple, but some people scoff at the push-to-fit flanges as unprofessional.

The other option is to chip out concrete around the pipe and use a standard 4x3 flange with a stainless steel ring. This will securely glue to the pipe and be more like what a plumber would have done if they were installing the toilet from new.

  • In this case, we want the flange to fit into the drain pipe though, so I need a flange with a fitting that will drop into the pipe since there really isn't any room to join things with a fitting. The pipe comes out of the concrete slab and above the tile. I was just going to cut the pipe flush with the tile and drop the flange with fitting into the ABS pipe. I was looking at this: lowes.com/pd/Oatey-3-in-ABS-Twist-N-Set-Flange/50315023
    – Martin
    Feb 5, 2020 at 21:37
  • Oh, I see - didn't realize you couldn't put a flange around the pipe. Here is a detailed drawing of that flange you linked to. It looks like the max inside diameter of the pipe it supports is 3.03" (gasket OD). That is for cast iron pipe that has an inside diameter of 2.96".
    – JPhi1618
    Feb 5, 2020 at 21:49
  • Also, from your measurements, it seems like you have what is called SDR26 ABS pipe with an inside diameter of 3.126".
    – JPhi1618
    Feb 5, 2020 at 21:52
  • Looks like this Sioux Chief 888-A might be what you want, but I can't find a detailed drawing to confirm the dimensions.
    – JPhi1618
    Feb 5, 2020 at 21:55
  • Thanks for the input. I'll check out that flange option. Sorry I may not have explained it properly enough to begin with. Bottom line, it sounds like I'm looking at the right products, I just need to really try a few things out since I might have an uncommon diameter.
    – Martin
    Feb 5, 2020 at 22:07

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