by Eric, Technical Support for FlexPVC® a division of PVC Distributors LLC, Pahrump, Nevada
Why is it so hard to determine the correct pipe size?
This article is all about pipe sizes. I'm writing it because pipe and fitting size is probably the most confusing thing any new customer has to learn before they can purchase from this (or any) plumbing site. It's also the #1 reason for returns and exchanges.
I'm sure this article will be confusing to some people, but that's okay. Pipe sizes are confusing and it "doesn't make any sense" for most people and it surely does not follow "common sense." All of these are legitimate complaints. The problem is not you, but the standard. However we are not going to change standards that have been around for centuries so we'll learn it so get the right parts!
The first step is to forget what you think you know. Pipe size has nothing to do with common sense or how people normally think. Pipe size is based on engineers and tradition. It will help if you understand that pipe sizes are for not just pvc plastic pipe, but the same sizing also applies to metal (aluminum, steel, brass, copper, cast iron) and even clay pipe, some of it going all the way back to the Roman Empire!
The standard we use also has many applications and potential materials. For the purposes of this site, we'll just deal with plastic pipe sizing but understand there is crossover into other types of pipe.
Method #1. READ THE PIPE. If you are buying pipe to replace or repair existing pipe or buying fittings for existing pipe, just read the printing on the pipe. Sometimes it's not available, but this is the easiest and most accurate way to identify what you have. You will see a measurement, like 1", and a classification. Sch 40, Sch 80, Class 200, Class 315, CL 63 (Class 63), etc. You may also see SDR numbers. These are particularly confusing because there are different standards for SDR specifications. You will also likely see the letters IPS. IPS standard for Iron Pipe Size. Yes, you will see that printed on plastic pvc pipe. Remember these standards cross over into other materials. You might also see CTS. That stands for Copper Tube Size. Which is completely different from IPS. You will also see ASTM numbers but for the purpose of this discussion, they are essentially meaningless. ASTM tells you the type of plastic used, not the size, standard or classification.
So you are looking for the size, a number, always in inches or mm, and a standard. If you see IPS or CTS, then it's very easy. Just see our Specs and Dimensions page (http://dims.flexpvc.com) and look up the size you see printed on the pipe in and match the classification in see in the table.
Method #2. If you can't find the writing, then the next best thing is to measure the outside diameter (OD). This is the distance across the middle of the pipe. Not the distance around the pipe. And you must be accurate! Many people measure across the pipe, see "almost 2 inches," call it 2" pipe then order 2" fittings and get the wrong fittings! "Almost 2 inches" is not an engineering term. :-) (Remember I said this is all based on engineers and tradition.) What you may see as "almost 2 inches" is actually 1.900" which is the OD for 1.5" pipe. So you should order 1.5" pipe & fittings.
The same thing happens with 1.25" pipe. If you measure it, it'll measure 1.660" and many people will call that "1.5 inches" and so order 1.5" fittings and again get the wrong fittings. 1.66" is the OD for 1.25" IPS pipe.
What if you measure 1.625"? Many people will measure a pipe and without a digital caliper, they will see 1.66" as the same as 1.625". But that would be wrong. 1.66" is the OD for 1.25" IPS size pipe, but 1.625" is the OD for 1.5" CTS pipe! Remember engineers are precise people. To them, 1.625" is not the same as 1.660". ;-)
The plumbing world has tried to make it a little easier on you by using colors, however these colors cause just as much trouble as help. First, Sch 40 pipe is usually white. Sch 80 pipe is gray. In the plumbing world it's dark gray. However there is a light gray pipe that is Sch 40. It's electrical conduit. (Remember cross over? Yes, plumbing pipe standards cross over in to electrical standards.) So if you say "it's gray pipe" but without a standard of how gray is it it's not clarifying the situation. Is it light gray? A brownish gray? A dark gray? If it's light gray, it's probably Sch 40 Electrical Conduit. If it's a brownish gray that is an old UV rated Sch 40 plumbing pipe, if it's a dark, dark gray (almost black) then it's Sch 80 pvc pipe. Speaking of black ... there is black plastic pipe but it's not PVC, it's ABS (another type of pipe using in plumbing) and it's IPS Sch 40 size.
If you are completely overwhelmed by all this, you can try our Pipe & Tubing Identification Infographic
The example below is taken from our full pvc pipe specs and dims page.
Here are the specs and dims for 4" Nominal Pipe Size taken from the full chart just as an example. All of these pipes would be called, "4 inch nominal pipe" but you can see without knowing the details, you have no idea what pipe you are actually referring to:
Method #1 is represented by the light blue panels, and Method #2 is represented by the light green panels. Those are the two data points you need to know to identify your pipe. If using method #2, the only sure fire way to do this is buy a digital caliper. We sell them but you can also buy them for about $15-$25 at many hardware stores and local auto parts stores. They are sometimes in the bargain bins. All you need to do is turn it on, be sure it's closed, hit the "zero" button and then open them to measure the OD and wall thickness of the pipe you are going to use. Find OD listed in the 1st column and thickness in the 3rd and then you'll need the classification for your pipe. Once you know the classification and pipe size, you are set.
Also note: As for pressure ratings, the entire system will rated as the lowest rating of the lowest rated fitting and/or pipe in the system. Ie, "a chain is only as strong as it's weakest link." Something to be aware of if you are mixing and matching pipe and fittings. :-) Speaking of which, what about the fittings? That will be my next Tech Article. :-)
If you have any questions, just email us. :-)