When choosing a product to secure a pipe, it’s easy to confuse nominal diameter and outer diameter. In this article, we’ll explain the difference between the two with reference values by size.
What you’ll learn in this article
- The difference between nominal diameter and outer diameter
- Correspondence between nominal diameter and outer diameter for various pipes
- Why you should choose based on outer diameter
What is "nominal diameter"?
Nominal diameter (A) is the size name used in piping standards, based mainly on a pipe’s "inner diameter." Since it is only a nominal size, the notation differs depending on the type of pipe.
· For PVC pipe
PVC pipe is generally written with VP (short for Vinyl Pipe) before the nominal diameter, and without "A" after it.
Examples: VP20, VP25
Copper pipe is also generally written with CU (the chemical symbol for copper) before the nominal diameter.
· For steel pipe
Steel pipe may be written using only the nominal diameter, or with GP (short for Gas Pipe) before the nominal diameter, omitting "A" in the same way as PVC pipe.
Examples: 20A or GP20
What is "outer diameter"?
The image above shows the "outer diameter" measurement of a PVC pipe (VP20). Outer diameter refers to the pipe’s "outside diameter." On the other hand, nominal diameter is a standard size name based on the inner diameter, so the outer diameter and nominal diameter often differ in value.
Below, we introduce a reference chart showing the correspondence between representative nominal diameters and outer diameters for each pipe type.
Nominal diameter and outer diameter for PVC pipe (VP)
| Nominal diameter | Outer diameter | Compatible products | |
| Saddle | Clamp | ||
| VP13 | 18mm | ||
| VP16 | 22mm | ||
| VP20 | 26mm | ||
| VP25 | 32mm | ||
For other sizes, please see Pipe size list | Quick reference chart for nominal diameter and actual outer diameter of PVC pipe (VP), etc..
Nominal diameter and outer diameter for steel pipe (GP)
| Nominal diameter | Outer diameter | Compatible products | |
| Saddle | Clamp | ||
| 10A | 17.3mm | ||
| 15A | 21.7mm | ||
| 20A | 27.2mm | ||
| 25A | 34.0mm | ||
For other sizes, please see Pipe size list | Quick reference chart for nominal diameter and actual outer diameter of PVC pipe (VP), etc..
Why is outer diameter important?
In general piping standards, nominal diameter is used, but our product sizes are displayed as "compatible diameter Ø" based on the actual outer diameter.
For that reason, if you choose a product based only on nominal diameter, problems may occur during installation, such as it not fitting due to a difference of just a few millimeters, or being too loose.
In particular, our vibration-damping products are designed with precision, so if there is a difference of "±0.4mm or more," they may not install properly. Therefore, please be sure to check the actual outer diameter, not the nominal diameter.
Checking points to avoid mistakes
- Do not judge by nominal diameter alone; measure the actual outer diameter
- If there is paint or coating, calculate or measure the outer diameter including its thickness
- Choose a product with the appropriate compatible diameter based on the actual outer diameter
Summary
Nominal diameter is a size name used in piping standards based on the inner diameter, so it differs from outer diameter. If you’re unsure which of our products to choose, first check or measure the outer diameter of the item. To ensure a secure pipe fit, selecting the compatible diameter based on the actual outer diameter is important.
Related articles
- How to measure pipe outer diameter | Explaining size checks with calipers and measuring tape
- Pipe size list | Quick reference chart for nominal diameter and actual outer diameter of PVC pipe (VP), etc.
- Size selection guide for resin saddle bands | Why choose by outer diameter, not nominal diameter
- Size selection guide for resin pipe clamps | How to choose based on pipe outer diameter