Nominal size, pipe size and thread sizes - We enlighten you!
The inner diameter of a pipe, hose or the connection dimension of a fitting is also called nominal size / nominal diameter. The material steel is often assumed for the specifications of the nominal size, but not always mentioned.
If you want to convert the dimension 1" into centimeters, you will get a result of 25.4mm. However, this is not quite correct! Thus, the indication of 1 inch does not correspond to the correct measurement of the diameter. Why inches are not equal to inches is historical. In the early 20th century, an inner diameter (= nominal size) of approx. 25.4mm was measured for a steel pipe with a wall thickness of 3.8mm. This resulted in an outer diameter of approx. 33mm. As a result, suitable fittings and adapters as well as corresponding tools were developed for this outside diameter. The quality of steel has improved over the years and this has reduced the wall thickness of the pipes. However, to keep the pipe thread dimension compatible with the existing fittings and adapters, the inner diameter of the pipe was finally increased. The outer diameter remained the same.
However, there is an exception for thick-walled pipe as well as also for pipes made of glass fiber-reinforced plastic, as here the outer diameter is slightly larger.
According to EN ISO 6708, the nominal size is indicated by the designation DN ("Diamètre Nominal") in conjunction with a number corresponding to the approximate inside diameter in millimeters. If you now want to connect pipes from different manufacturers, you must ensure that the nominal size DN is specified with reference to the same DIN standard.
Threaded pipe DIN EN 10255 - medium heavy, steel | boiling pipe
| Nominal size (DN) | Pipe thread R | Outer diameter | Wall thickness | Inner diameter |
| 8 | 0.25" | 13,5mm / 0.53" | 2.35mm / 0.09" | 8.8mm / 0.35" |
| 10 | 0.375" | 17,2mm / 0.68" | 2.35mm / 0.09" | 12.5mm / 0.5" |
| 15 | 0.5" | 21,3mm / 0.84" | 2.65mm / 0.1" | 16mm / 0.63" |
| 20 | 0.75" | 26,9mm / 1.06" | 2.65mm / 0.1" | 21.6mm / 0.85" |
| 25 | 1" | 33,7mm / 1.33" | 3.25mm / 0.13" | 27.2mm / 1.07" |
| 32 | 1.25" | 42,4mm / 1.67" | 3.25mm / 0.13" | 35.9mm / 1.41" |
| 40 | 1.5" | 48,3mm / 1.9" | 3.25mm / 0.13" | 41.8mm / 1.65" |
| 50 | 2" | 60,3mm / 2.37" | 3.65mm / 0.14" | 53mm / 2.09" |
| 65 | 2.5" | 76.1mm / 3" | 3.65mm / 0.14" | 68.8mm / 2.71" |
| 80 | 3" | 88.9mm / 3.5" | 4.05mm / 0.16" | 80.8mm / 3.18" |
| 100 | 4" | 114.3mm / 4.5" | 4.5mm / 0.18" | 105.3mm / 4.15" |
| 125 | 5" | 139.7mm / 5.5" | 4.85mm / 0.19" | 130mm / 5.12" |
| 150 | 6" | 165.1mm / 6.5" | 4.85mm / 0.19" | 155.4mm / 6.12" |
Normally, the nominal size is given in inches for threaded tubes, and the outside diameter is given in millimeters for copper tubes.
The tube wall thickness is from
- 10 - 22mm = 1mm / 0.39 - 0.87" = 0.04"
- 22 - 42mm = 1.5mm / 0.87 - 1.65" = 0.06"
- 54 - 64mm = 2mm / 2.13 - 2.52" = 0.08"
Threaded pipe DIN EN 10255 - heavy, steel | gas pipe
| Nominal size (DN) | Pipe thread R | Outer diameter | Wall thickness | Inner diameter |
| 10 | 0.375" | 17.2mm / 0.68" | 2.9mm / 0.11" | 11.4mm / 0.45" |
| 15 | 0.5" | 21.3mm / 0.84" | 3.25mm / 0.13" | 14.8mm / 0.58" |
| 20 | 0.75" | 26.9mm / 1.06" | 3.25mm / 0.13" | 20.4mm / 0.8" |
| 25 | 1" | 33.7mm / 1.33" | 4.05mm / 0.16" | 25.6mm / 1" |
| 32 | 1.25" | 42.4mm / 1.67" | 4.05mm / 0.16" | 34.3mm / 1.35" |
| 40 | 1.5" | 48.3mm / 1.9" | 4.05mm / 0.16" | 40.2mm / 1.58" |
| 50 | 2" | 60.3mm / 2.37" | 4.5mm / 0.18" | 51.3mm / 2.02" |
| 65 | 2.5" | 76.1mm / 3" | 4.5mm / 0.18" | 67.1mm / 2.64" |
| 80 | 3" | 88.9mm / 3.5" | 4.85mm / 0.19" | 79.2mm / 3.12" |
| 100 | 4" | 114.3mm / 4.5" | 5.4mm / 0.21" | 102.5mm / 4.04" |
| 125 | 5" | 139.7mm / 5.5" | 5.4mm / 0.21" | 128.9mm / 5.07" |
| 150 | 6" | 165.1mm / 6.5" | 5.4mm / 0.21" | 154.3mm / 6.07" |