General Pipe and Tube Information
Rigid pipe sizes are designated by a nominal diameter, meaning by name only. For example, a 1/2" pipe does not measure 1/2" on the outside nor on the inside, it is just a nominal distinction. There are various wall thicknesses available in pipe that allow the pipe to be used in different applications. This is designated by the schedule of the pipe. The OD of a pipe does not change between schedules, only the ID. Therefore, the OD of a 1/2" pipe is the same whether it is schedule 20 or schedule 80. Pipe connections are often made with threaded fittings but can also be made with socket fittings (welded for iron pipe, solvent-weld for PVC). A 1/2" coupling will fit on ANY 1/2" pipe, regardless of its schedule.
Tubing is designated by both the OD and ID and are actual measured dimensions. If only one dimension is given, it will be the OD. For example, 1/2" PVC tubing will have an outside diameter of 1/2". Most frequently though, both the OD and ID are given and these values are important to know when selecting various tubing connectors.
Tubing can either be rigid or flexible and both types have associated fittings. Compression fittings and quick-connect fitting are both used on rigid tubing and are sized according to the OD of the tube. Barb fittings are used on flexible tubing and are selected according to the ID of the tube.