Pipe dimensions are determined by the outside diameter (OD). The method used in the US is NPS or the nominal pipe size / nominal bore. It uses inches for measuring. In Europe it’s called DN (diameter nominal). It is measured in millimeters.
The NPS Scaling Conventions
If the pipe measure less than NPS 14 inches (equivalent to 350 mm) a rounded value is assigned. The actual outer diameter is not used. A 2 in NPS measures 2.375 but it is not specified as such. Only when the NPS size is above 14 in are the exact dimensions given. Assuming you have an NPS 14, that is equal to DIN 350 / 355.6 mm.
The Inside Diameter
While the outer pipe size is constant, the inner sections are subject to the thickness of the walls. For example, a Schedule 2” always has the same OD, but the Schedule 80 has thicker walls than a Schedule 40 so it’s tighter.
The standards vary depending on location and where the pipes are actually used. Early on, the pipe was measured by the inner diameter but that is longer used. Today pressure piping is determined by both the NPS and the SH (schedule). But the standards are numerous.
These include the ANSI/ASME, 10M and B35 in the US. In the United Kingdom the standards are the BS 1600 and BS 1387. When assessing the pipe size, remember that the wall variance has been determined to be at 12.5%.
Notes ID and DN
To avoid confusion, the inner diameter is labeled as ID in the US. In Europe is it DN. However their measurements are the same. For NPS 14 and higher, the DN will be equal if the multiple of 25 is applied. This is referred to as the ISO pipe.
Bear in mind that other countries may have their own measurements. For example, Japan uses the JPS system. Again this is due to the fact that pipes are used in different fields. The measuring system used by different countries is another reason for this.
The IPS (iron pipe size) is an old measuring system. However it is still referenced and referred to in some projects. The numbers used for the pipe size is similar to the NPS. However the schedules were more limited.
They included the STD (Standard Wall), XS (Extra Strong) and XXS (Double Extra Strong). The STD is equal to NPS 1/8 up to 10. The XS is the same as the SH 80. There is no exact equivalent to XXS. However it’s usually set to a schedule of 160 and higher.
DIPS and Others
Another legacy system, the DIPS used a different scale for measuring the OD. It’s also worth mentioning that PVC, CPVC and copper tubing utilize different measurements. Their standards are also different. The PIP pipes are used for irrigations.
Whatever standards are used for pipe size, they are strictly enforced. Given their importance (used in buildings, water systems etc), these need to be maintained regularly.