Selecting the correct size fittings to match your PVC pipe can be confusing if you do not already know what size PVC pipe you have. All plumbing grade PVC should have size and rating information imprinted on the outside of the pipe. PVC pipe is named for the inside diameter, commonly referred to as "a nominal size". If not able to locate this information then simply measuring the inside or outside diameter will help you determine the correct pipe size using the chart below. PVC Pipe Size TableNominal PVC Pipe Size1/2"3/4"1"1 1/4"1 1/2"2"Average Inside Diameter0.622"0.824"1.049"1.380"1.610"2.047"Actual Outside Diameter0.840"1.050"1.345"1.660"1.900"2.375"Once you determine your PVC pipe size then fitting selection is easy - just match the nominal pipe size to the corresponding fitting size. Example: If your pipe measures 0.824" ID (inside diameter) or 1.050" OD (outside diameter), which is 3/4" PVC pipe, you will select 3/4" PVC fitting. This Schedule 40 PVC Slip x Slip Reducer Coupling is used to transition from one size PVC pipe to another in a straight connection. Slip connections are socket x socket and require primer/glue for leakproof connection.