Sizing PEX pipe


February 13, 2014 by

Part of making sure that your new “car wash” shower will work is providing adequate water flow. During the planning phase, we discovered that all dimensional plumbing is not created equal.

When I ordered my shower components, I asked what size supply plumbing we’d need. I’d not yet started demolition and was unsure what would be inside of my walls. I wanted to make certain that I was not planning for more shower heads than my home plumbing could handle.

Our plumbing supplier checked all of the specifications for our components and found that we’d need at least 1/2-inch plumbing supply lines to provide enough water for our configuration. (Water, not necessarily hot water… we’ll discuss water heaters in another posting.) They told me that, in our area, most bathrooms are plumbed with either 1/2-inch or 3/4-inch PEX supply lines, so our configuration would be valid either way.

Yay! Order the rough plumbing parts and let the demolition begin!

Once the plumbing was exposed, we found that we had 3/4-inch PEX supply lines coming to the bathroom, which were split inside the shower wall to two sets of 1/2-inch PEX lines: one set going to the shower, the other set going to the tub and sinks.


Great! Or, was it…

I’d planned to run the hot/cold supply lines to my shower controls using PEX. All of the plumbing past the initial connection to the first control valve would be done using copper. I have worked with copper before and you are able to do a lot more with it in a confined space. Copper is more expensive for materials and labor; however, it would not be that bad, since I was doing all of the labor myself.

There was 1/2-inch PEX in the shower area… check! All I’d need to do is reroute it. I went out and bought the necessary 1/2-inch PEX pipe and fittings.

Before I started laying out the shower control plumbing, I decided to take a look in the documentation that came with it, to see if there was anything special that I needed to worry about. The shower valve documentation specified the supply line requirements as “1/2-inch I.D., minimum,” where “I.D.” stands for “inside diameter.” The word minimum concerned me.

I looked at my 1/2-inch PEX pipe and fittings. While the PEX pipe looked like it had an I.D of about 1/2-inch, there was no way that the fittings which fit inside the pipe were 1/2-inch I.D.


See! It always pays to read the documentation before you start!

What I’ve discovered is that the fittings for 1/2-inch PEX are actually more like 3/8-inch I.D.


The fittings for 3/4-inch PEX are closer to 1/2-inch I.D. (actually just a bit bigger than 1/2-inch).


Note: These are the brass PEX fittings that are commonly available where I live. I have seen PEX fittings that look more like copper, which appear to have thinner walls. In either case, they’ll still reduce the I.D. of your pipes below what you’d expect.

Where 1/2-inch and 3/4-inch ratings for PEX (and copper) plumbing pipe comes from is from the I.D. of the pipe. The PEX pipes are made with an I.D. that matches the I.D. sizing of its copper-pipe counterpart. The difference is that PEX joints go inside the pipe, reducing the inside diameter at the joint, while copper joints go outside the pipe, increasing the inside diameter at the joint.

So, what did I do?

Since I had 3/4-inch PEX supply lines coming to the shower area already, I continued the 3/4-inch PEX up to my shower valves, giving me the minimum 1/2-inch I.D. that I need for my shower supply lines. I left the 1/2-inch PEX lines running to the tub and sinks as they were, since they’d never given me any flow problems in the past.


Interestingly, the plumbers and plumbing suppliers I’ve spoken with don’t consider that PEX fittings cause a restriction when they are sizing plumbing. The guy we hired to help us fix a leak in my new shower manifold (more on that in another upcoming post) was like “wow…”

A few words about copper pipe sizing…

I’ve discovered that 1/2-inch copper pipe isn’t really 1/2-inch I.D., either: it is nominally 1/2-inch I.D. For copper plumbing pipe, the O.D (outside diameter) is 1/8-inch larger than the listed nominal size. For 1/2-inch copper pipe, the O.D. is 5/8-inch.

There are several types of copper pipe for each nominal size, with different wall thickness for use in different types of applications. All types of pipe for a specific nominal size are designed to use the same junction fittings (that go outside the pipe): this is why you see several types of pipe for each size at the plumbing supplier, but only one assortment of junction fittings for each size of pipe. The differences in wall thickness between copper pipe types have minimal effect on the pipe’s I.D., so considering the copper pipe type is not normally part of calculating its water flow capacity.

You can read more about the types of copper pipe here: Remember to consult your local building codes for information about which type of pipe to use for each plumbing application in your locality.


  1. Josh says:

    Just so that I clearly understand what you did. You connected the 3/4 in pex pipe to the valve WITHOUT the fitting? If that’s what you did, isn’t weeping a concern because of the threading on the valve? I’m about to purchase a shower valve and the ID of the 1/2 in fitting was a concern for me too

    • says:

      Hi, Josh.

      I got a fitting that was threaded to match the valve assembly on one side, with 3/4-inch PEX on the other side. The 1/2-inch ID of the fitting meets the 1/2-inch plumbing requirement.

      Threaded fittings are not made as well as they used to be. I did the standard 3 wraps of Teflon tape and it still leaked. A plumber said to start with 6 wraps: if it was tight, peels some off – if it was loose, put more on. I did and it fixed the leak.

      I recommend pressure testing the whole thing with air before applying water. Make sure it does not leak down for a couple days.

      Good luck!

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