It will typically be either 1¼” or 1½” for sump pumps, but may be larger.
If you are not sure, or the pipe itself is not marked, try to measure the diameter. In plumbing pipes, the rated diameter is the inside diameter. This means a 1¼" PVC pipe will have an outside diameter of around 1.5" Likewise, a 1½" diameter PVC pipe will measure about 2" outside diameter.
It is important that we always look at the inside diameter when determining the pipe size.
Installing new pipe for the sump pump
If you are installing a sump pump where there was none before, or you are replacing the discharge pipe, then skip this step of determining pipe size. Once you have chosen the best pump for you based on the rest of the steps, you should install piping that is the same size as, or one size larger than, the discharge size of the pump you chose.
A pump that is pushing water through a narrow pipe will not move water as quickly, or efficiently, as a pump that is moving water out a pipe that is larger in diameter.
Always use a discharge pipe that is AT LEAST as large as the discharge size of the pump.
If you need to push water out through a long run of pipe (more than 20' or so) then you may need to increase pipe size even more. The friction of moving water through a narrow pipe is more than moving it through a wider one. If the pipe is too long and too narrow, your sump pump's performance can drop off greatly. Don't be afraid to increase pipe size by two or three steps for a longer run.
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