The "Standard Specification for Poly(Vinyl Chloride) (PVC) Plastic Pipe, Schedules 40, 80, and 120 guide," also listed as the ASTM D1785, lists the outside and inside diameters and wall thicknesses for PVC and CPVC pipes between 1/2-inch to 16-inches in size. The most common are Schedule 40 and 80 pipes.
The differences in pipe sizes are not only based on their diameter. The wall thickness for the PVC pipes actually varies as well, with pipe walls growing thicker as the pipes grow larger. This is to allow more support for the pipes as more material is likely to flow through their lengths.
The outside diameters remain consistent between Schedule 40 and Schedule pipes. For example, a 1/2-inch Schedule 40 and 80 pipe both have an outside diameter of 0.840 inches, while the 16-inch sized pipes both have an outside diameter of 16 inches. The inside diameter differs by pipe type, however, with the inside diameter of the 1/2-inch Schedule 40 pipe being 0.622 inches while the inside diameter of the 1/2-inch Schedule 80 pipe is only 0.546 inches. The inside diameter of Schedule 40 pipes is always larger than that of the Schedule 80 pipes of the same size.
Another difference between pipe sizes is the thickness of the pipe walls. The 1/2-inch Schedule 40 pipe has a wall thickness of 0.109 inches, but the thickness of the Schedule 80 pipe at 1/2-inch is 0.147 inches. With a Schedule 40 16-inch pipe the thickness is 0.500, while a Schedule 80 16-inch pipe has a thickness of 0.843. All wall thicknesses listed in the Schedules are the minimum allowed.