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Pipe Threads And Size Identification
We sell a lot of plumbing/pneumatic products which are sized to engineering standards that are commonly used in engineering, however for people not familiar with engineering, size identification can be difficult, with the result of purchases being returned as customers have purchased the wrong size. The aim of this page is to remove any doubt when ordering plumbing and pneumatic fittings.

I'm not going to go in to great detail on types of threads, cutting angles etc. my aim is to remove doubt when sizing of products.

  • Plumbing pipe (solid pipe, not flexible or copper) size is NOT calculated on the outside diameter as show by 'B' below

  • Plumbing pipe size is calculated on its inside diameter, and this dimension remains imperial, for example 1/2", 3/4" etc

  • Plumbing fitting sizes are known by their size and the type of thread. All water plumbing threads are known as BSP (British Standard Pipe Thread) occasionally you'll come across BSPT which indicates the thread is tapered ('T'), rather than parallel. So you'll get 1/2"BSP, 3/4"BSP etc.

The image below represents a cross section of a pipe with threads on the end.

By measuring your pipe thread outside diameter ('B') and relating your actual measurement to the closest dimension in either the centre column or the right hand column of the table below, this will give you the thread size in the left hand column

Pipe & Thread Size 'B' Outside Diameter
Inch mm
1/8" 0.3830 9.728
1/4" 0.5180 13.157
3/8" 0.6560 16.662
1/2" 0.8250 20.955
5/8" 0.9020 22.911
3/4" 1.0410 26.441
7/8" 1.1819 30.020
1" 1.3090 33.249
1 1/4" 1.6500 41.910
1 1/2" 1.8820 47.803
2" 2.3470 59.614
2 1/2" 2.9600 75.184
3" 3.460 87.884
4" 4.45 113.030

The figures in the left hand column will give you a rough idea of the inside diameter, but shouldn't be relied up on, as the pipe wall thickness do vary considerably.

The above information is good for most water pipes, however pneumatic air lines used for air suspension systems is different, as these are 'sized' by their outside diameter. The American kits generally use 1/4" outside diameter air lines, where as everyone else uses metric! Dunlop's original air lines are 5mm, however if supplied with a basic gauge kit, the air lines can be either 4mm or 6mm. The threads used on either the American or the Dunlop kits are mostly 1/8"BSP and 1/4"BSP, however some threads on the Dunlop fittings can be M5 !

Remember, if in doubt, ASK


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