The difference between "nominal diameter" and "outer diameter" is often confusing when choosing pipe fittings. In this article, we will explain the difference between the two and key selection points, along with examples of PVC pipes and steel pipes.
What you'll learn in this article
- Difference between nominal diameter and outer diameter
- Correspondence between nominal diameter and outer diameter for various pipes
- Reasons why you should choose based on outer diameter
What is "nominal diameter"?
Nominal diameter (A) is a nominal size in piping standards, primarily based on the "inner diameter" of the pipe. Since it is just a nominal size, the notation method differs depending on the type of pipe.
・For PVC pipes
For PVC pipes, it is common to add VP (abbreviation for Vinyl Pipe) before the nominal diameter, without adding an "A" after it.
Example: VP20, VP25
Similarly, for copper pipes, it is common to add CU (the element symbol for copper) before the nominal diameter.
・For steel pipes
Steel pipes may be labeled with just the nominal diameter, or with GP (abbreviation for Gas Pipe) added before the nominal diameter, omitting the "A" like PVC pipes.
Example: 20A or GP20
What is "outer diameter"?
The above is an image measuring the "outer diameter" of a PVC pipe (VP20). The outer diameter refers to the "dimension of the outer diameter" of the pipe. On the other hand, nominal diameter is a nominal size in standards based on the inner diameter, so the values of outer diameter and nominal diameter often differ.
Below, we introduce a correspondence table of typical nominal diameters and outer diameters for each type of pipe.
Nominal diameter and outer diameter of PVC pipe (VP)
| Nominal diameter | Outer diameter | Compatible products | |
| Saddle | Clamp | ||
| VP13 | 18mm | ||
| VP16 | 22mm | ||
| VP20 | 26mm | ||
| VP25 | 32mm | ||
Other sizes can be found inPiping size list | Quick reference table of nominal diameters and actual outer diameters for PVC pipes (VP) etc. please refer to it.
Nominal diameter and outer diameter of steel pipe (GP)
| Nominal diameter | Outer diameter | Compatible products | |
| Saddle | Clamp | ||
| 10A | 17.3mm | ||
| 15A | 21.7mm | ||
| 20A | 27.2mm | ||
| 25A | 34.0mm | ||
Other sizes can be found inPiping size list | Quick reference table of nominal diameters and actual outer diameters for PVC pipes (VP) etc. please refer to it.
Why outer diameter is important
Nominal diameter is used in general piping standards, but our product sizes are displayed as "compatible diameter Ø" based on the actual outer diameter.
Therefore, if you select a product based solely on nominal diameter, problems may occur during installation such as not fitting due to a difference of a few millimeters, or being too loose.
In particular, our anti-vibration type products have precise size designs, so if there is a difference of "±0.4mm or more", they may not install properly. Therefore, always check the actual outer diameter instead of the nominal diameter.
Key points to avoid mistakes
- Do not judge only by nominal diameter; measure the actual outer diameter
- If there is paint or coating, calculate and measure the outer diameter including that thickness
- Select a product with the appropriate compatible diameter based on the actual outer diameter
Frequently Asked Questions (FAQ)
Q. Do VP pipes, VU pipes, and HIVP pipes have different outer diameters?
There are PVC pipes with various properties, but the outer diameter is the same for the same nominal diameter. What differs is the wall thickness: VP pipes and HIVP pipes are thicker, while VU pipes are thinner. For more details, please refer toTypes and differences of PVC pipes | Features of VP, VU, HIVP and how to choose pipe fittings please refer to it.
Q. Is the outer diameter the same for 20A steel pipes and coated steel pipes?
Coated steel pipes have a coating applied to the outside of the pipe, so even if the nominal diameter is the same, the outer diameter will be larger than that of regular steel pipes. For example, for 15A, steel pipes have an outer diameter of 21.7mm, while coated steel pipes are about 22.9mm. For 20A, steel pipes have an outer diameter of 27.2mm, while coated steel pipes are about 28.4mm. Always check if the pipe has a coating on its surface.
Q. When the nominal diameter is the same, are there differences in outer diameter between manufacturers?
For JIS standard products manufactured in Japan, the outer diameter is basically the same. However, for ANSI standard products manufactured overseas or pipes for special uses, the dimensions may differ. Please check the markings on the pipe surface, and if they are unreadable, we recommend measuring the actual outer diameter.
Q. What are A notation and B notation?
A notation is the general piping size notation used domestically, while B notation is inch notation. For example, 15A equals 1/2B (2 bu), 20A equals 3/4B (6 bu), and 25A equals 1B (1 inch). In this article, we created the correspondence table based on A notation.
Q. Are there nominal diameters for pipes other than piping standards?
Yes, there are. However, for pipes other than piping standards, they are often managed by outer diameter (mm) for clarity. When attaching to stainless steel pipes, aluminum pipes, etc., always confirm the actual outer diameter when selecting.
Summary
Nominal diameter is a nominal size in piping standards based on inner diameter, so it differs from outer diameter. When you are unsure about choosing our products, first confirm or measure the outer diameter of the target object. To ensure secure pipe fixation, it is important to select a compatible diameter based on the actual outer diameter.
Related Articles
- Piping size list | Quick reference table of nominal diameters and actual outer diameters for PVC pipes (VP) etc.
- How to measure pipe outer diameter | Explained size confirmation methods using calipers and tape measures
- Types and differences of PVC pipes | Features of VP, VU, HIVP and how to choose pipe fittings
- Resin saddle band size selection guide | Reasons to choose by outer diameter instead of nominal diameter
- Resin pipe clamp size selection guide | Selection method based on pipe outer diameter