Graphs Borrowed From Harvel

PVC is used by plumbers, pool installers, and so on, and local building and construction regulations set the safety and materials standards that such pipe must meet. Because different types of installations require different types of pipe, PVC pipe can be bought in various grades known as "schedules".

The most commonly used schedule is schedule 40, often referred to as "SCH 40". Usually schedule 40 pipe is white. In some areas of the country, like, and in Canada, SCH 40 is gray. The next schedule offered for sale is SCH 80, a type of pipe that is usually gray, and has a thicker wall making it better for high pressure applications. A sturdier variety, called SCH 120, is available as well, but usually hard to find.

Although the outside dimensions of any size pipe in schedules 40, 80 and 120 is the same, the inside diameter reduces as the schedule number increases. For example, 1" SCH 40 has a larger inside diameter than 1" SCH 80, which in turn is larger than 1" SCH 120. Another way to say it is that as the schedule number increases, the wall thickness also increases.

Schedules 40 and 80 are what you'll most often find at the hardware store. In addition, you should also find thin wall PVC pipe referred to as SDR pipe. You may find two or three SDR "schedules": SDR schedule 13.5 for 1/2 inch pipe, and SDR schedule 21 and/or 26 for larger diameter pipes.

SDR PVC pipe has the same outside diameter as Schedule 40 for the same nominal size, but its wall thickness is less (it's thinner and lighter).

Here's a comparison for 1/2 inch pipe:

Schedule 40: outside diameter: .840 in.; inside diameter: .608 in.; wall thickness: .232 in.
SDR PVC: outside diameter: .840 in.; inside diameter: .696 in.; wall thickness: .144 in.

There are other types of plastic pipe:
CPVC, use for high temperature, high pressure applications.
Clear PVC pipe, used in food and corrosive applications or to just make your gun look cool!

NOTE! These sizes are of Harvel Pipe, different makes should be very close.

SCH 40