What are the sizing differences between PVC pipe, and PVC rigid tubing, and PVC flexible tubing?

PVC rigid tubing is measured and listed by the actual dimensions of the inside and outside diameter measured. For instance: 1/2” rigid PVC tubing is 1/2” outside diameter and 3/8” inside diameter. The fittings that can be used for this type of tubing are compression fittings or flexible fittings that can be clamped onto the PVC tube.

PVC flexible tubing is measured and listed in the same way as the rigid tubing; you would measure the outside and inside diameters. The difference is the type of fittings that you would use. Hose barbs or insert fittings should be used, and they get inserted into the tubing. That is why you would go by the inside diameter of the tubing to determine the size fitting you need.

For instance: 1/2” outside diameter tubing with a 3/8” inside diameter would require a 3/8” hose barb or insert fitting.

PVC pipe sizes are nominal, so the pipe is neither the outside nor the inside diameter for most pipe sizes. PVC pipe is socket fittings (schedule 40) and socket or threaded fittings (schedule 80). All socket fittings are designed to go on the outside of the pipe, not the inside.