Really neither for the smaller diameters we deal with that is why it is called a nominal diameter. If you look at the size chat that I linked in post #2 it shows nominal and OD (outside diameter) and ID (inside diameter). Once you get to PVC, sch 40 or 80, larger than 14" or so the OD is typically the same as the nominal pipe size and the ID varies based on the schedule of the PVC (40 or 80).
IF you are using different PVC piping (other than Sch 40 or 80, typically used in municipal water or wastewater situations) have varying OD and ID depending on the different dimensions ratios used and wall thickness required for the specific situation.
IF you are using different PVC piping (other than Sch 40 or 80, typically used in municipal water or wastewater situations) have varying OD and ID depending on the different dimensions ratios used and wall thickness required for the specific situation.