Plastic Pipes Inch to Metric Sizing Guidelines
The most commonly used pipework sizing system in the UK has been the Imperial (inch) for many generations.
Imperial pipe sizes are designated to piping by the nominal bore (nb). The introduction of the metric sizing system was to bring the UK into line with the rest of Europe, and whilst accepted in many areas of industry, has had a slower uptake, resulting in both Imperial and Metric remaining readily available in the UK. However, this has proved confusing, as Metric is sized in millimeters and relates to the outside diameter (od) of the pipe, opposed to imperial, which is sized in inches and relates to the nominal bore(nb) (or inside diameter (id)).
As pipe sizes from the different groups cannot be mixed, a special conversion fitting must be used to change from one to another if necessary.
For example a, 1½” (nb) imperial pipe is the equivalent of a 50mm (od) metric pipe size. Similarly, a 2” (nb) tube has a 60.3mm od and is also known as 50mm nb, but the metric equivalent is actually 63mm od.
Please see the Comparison Chart for the full Imperial / Metric conversions from ¼” to 10”nb and 8mm to 280mm od.
|Inch Nominal Bore size||Outer Diameter||Plastic equivalent Outer Diameter Size|
|5"||140mm||125mm or 140mm|
|6"||168.3mm||160mm or 180mm (od)|
|8"||219.1mm||200mm or 225mm (od)|
|10"||273mm||250mm or 280mm (od)|