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People often call us confused when they're shopping for tubing or trying to size pipe fittings for tubing. When they measure their pipe dimension, it reads 1-inch, but the 1-inch fitting ordered is too large and doesn't fit the pipe or tubing. John Woodard, our Master Water Specialist, explains nominal pipe size and how to find the right fittings based on pipe dimensions.

How to determine fitting size from pipe dimensions

In our industry, pipes and tubing are sized in two ways:

  1. By the outer diameter (OD) for smaller fittings measuring 1/4, 3/8, or 1/2 inch
  2. By the inner diameter (ID). The ID size is referred to as CTS, which stands for copper tube size. CTS is based on the nominal size of a pipe.

What is nominal pipe size?

Nominal pipe size is the measurement based on the inner diameter rather than the actual diameter. When you're planning the plumbing for a house or another building, you calculate how much flow rate you need through the inner diameter of the tube or the pipe. The fittings used with those pipes measure larger than their labeled size because they connect over the outside of a plumbing pipe.

The size of OD tubing is typically marked in fine print on the side of the tube. If the tubing is sized by the OD, then you should look for fittings sized appropriately. For example, a 1/2 OD push-to-connect fitting goes with a 1/2 inch OD tube. But if you buy a 1/2 inch fitting designed for CTS (measured by ID) by mistake, then it won't fit properly. The CTS pipe fits nicely into a 1/2 inch CTS fitting.

When to use ID vs. OD fittings

CTS is actually available all the way down to 1/4 inch, although it's not used frequently. The most common CTS fittings are 1/2 inch, 3/4 inch, and larger sizes because they're used for household plumbing more frequently than ID nominal size parts. Smaller appliances, like under counter filters that run to your refrigerator or ice maker, use OD sized parts. 

OD fitting

How to measure pipe fittings

Measuring thread size may also be confusing. Threads are sized just like the CTS fittings. They're sized by the nominal size of the pipe, or the inner diameter. If you buy a 1/2 inch thread and measure the diameter, it will measure over 1 inch because it's designed to have a nominal pipe fitting. The female fitting should be sized just as large as the male fitting to get the right fit.

If you measure the actual thread size of a 1/2 inch fitting, then it will not actually measure 1/2 of an inch, even though that's what the part says.

To recap:

  • ID tubing is based on the nominal size of the pipe rather than the actual measurement. 
  • OD tubing is based on the outer diameter (the one you measure with a ruler) just like it sounds and fits into OD size tubing.

Learn more about pipe size and thread types from John.


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