Common copper pipe diameter size comparison table
| Nominal Pipe Diameter (DN) / Inch | Average Outer Diameter | |
| – | 1/4″ | 9.52 mm |
| – | 3/8″ | 12.70 mm |
| 15A | 1/2″ | 15.88 mm |
| – | 5/8″ | 19.05 mm |
| 20A | 3/4″ | 22.22 mm |
| 25A | 1″ | 28.58 mm |
| 32A | 1-1/4″ | 34.92 mm |
| 40A | 1-1/2″ | 41.28 mm |
| 50A | 2″ | 53.98 mm |
| 65A | 2-1/2″ | 66.68 mm |
| 80A | 3″ | 79.38 mm |
| 90A | 3-1/2″ | 92.08 mm |
| 100A | 4″ | 104.78 mm |
| 125A | 5″ | 130.18 mm |
| 150A | 6″ | 155.58 mm |
| 200A | 8″ | 206.38 mm |
| 250A | 10″ | 257.18 mm |
| 300A | 12″ | 307.98 mm |
# For reference only. Please refer to the actual measurement for pipe diameter.
Copper pipe diameter size conversion and comparison
The nominal diameter of copper pipes is often expressed in inches or DN. For example: DN 100 is a 4″ (inch) pipe. Standard pipe diameters comes with corresponding average outer diameter and approximate inner diameter, which are marked in mm. When determining the nominal pipe diameter, you can first measure the outer diameter of the pipe, and then use the above comparison table to find out the nominal pipe diameter of the pipe.
For example, if you want to convert a copper pipe with an outer diameter of 79.38 mm, you can compare it to DN 80, or 3″ (inch) pipe.
Measuring the diameter of copper pipes
If you need to measure the diameter of a copper pipe, for smaller sizes, you can use a caliper to directly measure the outer diameter of the pipe and then convert it to the nominal pipe diameter. If the pipe diameter is too large and there is no suitable caliper, you can also use a measuring tape to measure the circumference of the pipe, and then divide it by 3.14 to get the approximate pipe diameter. Example: A copper pipe with a circumference of 807.5 mm should be a DN 250 or 10” pipe.