From Wikipedia – Nominal Pipe Size:
- For NPS ⅛ to 12, the NPS and OD values are different. For example, the OD of an NPS 12 pipe is actually 12.75 inches. To find the actual OD for each NPS value, refer to the tables below. (Note that for tubing, the size indicates actual dimensions, not nominal.)
- For NPS 14 and up, the NPS and OD values are equal. In other words, an NPS 14 pipe is actually 14 inches OD.
The following chart was created using data from The Engineering ToolBox – Pipes – Nominal Wall Thickness. I have only used data for STD pipe. The link shows dimensions for Schedule 10 through XXS.