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 by their 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 N.B., but the metric equivalent is actually 63mm OD

Please see the below Comparison Chart for the full Imperial / Metric conversions from ¼” to 10” NB and 8mm to 280mm OD.

We sell pipe in both Imperial and Metric sizes (in 5.8 metre or 6 metre lengths). We also stock a variety of fittings should you be looking to convert metric to imperial or vice versa.

Nominal Bore Size Actual O.D. (Outer Diameter) Equivalent O.D. (Outer Diameter)
1/4" N/A 8mm
3/8" 17.1mm 16mm
1/2" 21.3mm 20mm
3/4" 26.7mm 25mm
1" 33.4mm 32mm
1 1/4" 42.4mm 40mm
1 1/2" 48.3mm 50mm
2" 60.3mm 63mm
2 1/2" 75mm 75mm
3" 88.9mm 90mm
4" 114.3mm 110mm
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)