Have you ever wondered why they measure pipe in so many ways? If you ask for a piece of 1-3/4" pipe at the hardware store they would probably tell you that they didn't make 1-3/4" pipe. You know that they made 1/4", 3/4" and 1-1/4" pipe so why not 1-3/4" pipe? Also you may wonder why 3/8" water tubing is bigger than 3/8" refrigeration tubing and 3/8" galvanized pipe is bigger than either one. It is not because they use different size inches, but it is because they measure each product differently.
There are many standards used in the plumbing industry. The most common used are Iron Pipe Size (IPS), Tubular sizes measures by outside diameter (O.D.), Copper Tube Size (CTS), and many different Schedules (i.e. Sch 40 and DWV) of Pressure and Drain Lines. I will attempt to explain how these sizes are measured and why.
Iron Pipe Size (IPS) is measured by the nominal (remember this word, it means approximate not exact) inside diameter. This was the inside diameter (in inches) of Sch 40 Iron pipe when they first started making it (a very long time ago). Since that time steels have been made much stronger and the inside of the pipe has been made larger to reduce the wall thickness (save material), but still allow the use of the existing fittings. Most PVC pipe the you will normally use is measured in IPS. The nominal (there's that word again) outside diameter of black iron, galvanized or PVC pipe between the sizes of 1/2" and up to 2-1/2" Pipe size (Yes, there is 2-1/2" pipe size) is the size plus 3/8". Below 1/2" size you only add 5/16" to the size, and starting at 3" through 6" Sch 40 you add 1/2" to the size to get the outside diameter (O.D.) of the pipe.
Tubular Sizes is mostly used with refrigeration copper tubing and Tubular Traps (the kind under the kitchen sink). The outside diameter (O.D.) is the same as the size. Don't we wish it could all be this simple. The wall thickness of tubular size is determined by the application. Thus drains with no pressure involved are thin; while refrigeration tubing which has pressure involved are thicker. Generally the larger the size the thicker the wall. The wall thickness of most Refrigeration Tubing, in sizes 1/4" through 3/4", is about .030" to .035".
Copper Tube Size (CTS) is another one of those based on a nominal (there's that word again) inside diameter. The outside diameter is size plus 1/8". Most all specs for copper tubing has a larger I.D. than the size. For example 1/2" CTS Type L tubing has a wall thickness of .040, and an O.D. of .625" (5/8"), which leaves and I.D. of .545 (a little more than 1/32" of 1/2"). Type L is government Spec Heavy. There are also Type K, Spec Extra Heavy (.049" wall) and Type M, Spec Common (.028" wall). The above specs are for 1/2" CTS, larger sizes have thicker walls.
Schedules and Specifications: By far this must be the most misunderstood and confusing part of of pipe sizing terms. Schedules are a group of Specifications apply to a certain type of pipe. Most that you see is ASTM (American society of Tube Manufactures), NSF (National Sanitation Foundation), and UPC (Uniform Plumbing Code). These are by no means all of the organizations that make Schedules or Specifications for the plumbing industry. These can be schedules that deal with Iron pipe, Copper tube, PVC Pipe, PVC Glue, Fittings of all types and even Pipe Joint compound. Some of the most common specs ar D????. These are ASTM Specifications.
Sch 80 Heavy Wall Pipe in both Iron and PVC. O.D. same as Sch 40, the I.D. gets smaller.
Sch 160 Extra Heavy Wall Pipe in both Iron and PVC. O.D. same as Sch 40, the I.D. gets even smaller.
D 2620 PR 160 Pressure Rated Pipe. Rated 160 psi working pressure at 73 degrees Fahrenheit (remember the temperature rating it is very important). This pipe is often called Sch 20 or Sch 160 in error. Also commonly called Thin-Wall PVC. It's O.D. is also the same as Sch 40 Pipe. This is an ASTM Specification.
D 2665 (ASTM) and DWV (NSF) are very similar to Sch 40 and sometimes pipe will be marked with all three specifications. The Outside Diameter of course is the same as Sch 40.
D 2729 is also commonly called S & D (Sewer & Drain). This is a thin-wall drain pipe for non-pressure applications. It is not to be used in a foundation, in a wall, or under a house or trailer. It may be run from the house to the septic tank. The O.D. is considerably smaller than DWV and cannot use the same fittings. Fewer fittings are available for it because of its limited applications. The O.D. size is plus 1/4".
D 3031 & D3034 has the same O.D. as D 2729 but has thicker walls and uses the same fittings.
D 3031 is a very strange pipe because even it's O.D. is less than its size. Its size was determined by the flow of sewage. Since a smooth wall PVC pipe will flow more sewage than a rough cast iron pipe, it was reasoned that the PVC pipe could be reduced in size until it flowed the same sewages like cast iron pipe. Thus came about D 3031. The municipal sewer system in the City of Parker has D 3031. This pipe has now been discontinued to the best of my knowledge.
D 2846 is CPVC pipe. It is measured the same as CTS and is rated at 100 psi at 180 degrees Fahrenheit. This pipe is made for hot water plumbing.
The only other common pipe left is Black Polyethylene to be honest with you I don't know how they arrived at the sizes for it. The O.D. & the I.D. both run larger than the size of the pipe. Maybe we can find out in the future.
I hope this article has been useful to you. This article was composed Oct, 1993 and included in a newsletter we mailed that month. The information is still valid today.