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The nominal pipe diameter DN, which is also known as the nominal connection diameter or simply the nominal diameter, indicates the approximate internal diameter of a pipe in millimetres (mm). The nominal pipe diameter is given without a unit in DN (French: diamètre nominal). Many companies also use the specification in inches. The nominal sizes for medium and heavy threaded pipes are specified in DIN EN 10255. The outside diameter of a pipe is used for direct measurement on site, as shown in Table 1.
Note: When measuring the outside diameter of the pipe, there may be slight deviations from the nominal outside diameter. DIN EN 10255 therefore also specifies possible deviations from the nominal outside diameter. Example: For DN 10, a nominal outside diameter of 17.2 mm is specified. The pipe can then have a minimum outside diameter of 16.7 and a maximum outside diameter of 17.5 mm and is still considered a DN 10 pipe. As a guide, you can assume +/- 5 mm.
Following you will find the most important nominal diameters with thread size and outer diameter for orientation:
| Nominal Diameter DN | Thread size in inches | Nominal outer diameter in mm |
| DN 6 | 1/8″ | 10,2 |
| DN 8 | 1/4″ | 13,5 |
| DN 10 | 3/8″ | 17,2 |
| DN 15 | 1/2″ | 21,3 |
| DN 20 | 3/4″ | 26,9 |
| DN 25 | 1″ | 33,7 |
| DN 32 | 1 1/4″ | 42,4 |
| DN 40 | 1 1/2″ | 48,3 |
| DN 50 | 2″ | 60,3 |
| DN 65 | 2 1/2″ | 76,1 |
| DN 80 | 3 | 88,9 |
| DN 100 | 4 | 114,3 |
| DN 125 | 5 | 139,7 |
| DN 150 | 6 | 165,1 |
