Understanding pipe diameters

There is often confusion amongst new users of industrial plastic pipe as to the actual measurements of the internal bore and external diameter. Over the decades, and indeed centuries, there have been many adapted and modified standards, some of which have been long abandoned, but others which have remnants in today’s specifications.

To understand how plastic pipe is measured, the user must first understand the methods of production and consider the way that industrial plastic pipes are generally assembled:

The machines that manufacture lengths of plastic pipe heat plastic pellets and then squeeze them through a die. This allows the wall thickness and diameter to be controlled. The problem comes in the cooling process, where, like all materials, the hot plastic contracts. If this were left unchecked, the finished product would have a varying external diameter and bore, which would result in a poor fitting product that is next to useless. The pipe manufacturers concentrate on maintaining a fixed external diameter, and let all the shrinkage happen to the internal diameter / bore of the pipe. It is here that we need to appreciate how industrial plastic pipe is assembled. In general, and this is true for both solvent weld pipework such as pvc pipe and electrofusion welded materials such as polypropylene, assembly is by inserting a pipe into a socket. The external diameter of the pipe is therefore critical, as any variation will make a poor joint with the socket. The internal diameter is less critical as it does not have to perfectly match anything else.

The plastic pipe manufacturers therefore control the outside diameter of the pipe. The external diameter of the pipe therefore remains constant, and the internal diameter varies depending on the pressure rating (due to a thicker wall). It is important to note that even with the same manufacturer, wall thicknesses for the same specification of pipe can vary slightly, resulting in a slight bore variation (usually less than 0.5mm). This is certainly the case between different manufacturers and the internal bore should not be relied on for machining purposes.

With metric pipe, and this knowledge, determining the pipe dimensions is simple; te outside diameter of the pipe measures the same as the stated size; so a 50mm pipe has a 50mm outside diameter, a 90mm pipe a 90mm outside diameter etc. Unfortunately the same cannot be said for imperial (inch) pipe. This is an area of great confusion as the size of the pipe refers to the “nominal bore” (or to put it another way “approximate bore”) of the pipe. So, a 2 inch pipe has an approximate bore of 2”. As the outside diameter of the pipe needs to remain fixed, to allow for fitting into sockets etc. the wall thickness of the plastic pipe changes with pressure rating and so, as a result, does the bore…which is why it is referred to as “nominal”. So a 2” pipe does not actually measure 2” anywhere! Many inexpereicnced users will order a 2 inch pipe and expect it to have an outside diameter of 50.8mm but in fact the OD is 60.3mm.

The general rules of thumb are:

Metric pipe – will measure the stated diameter as an external diameter

Imperial pipe – Will not measure the stated diameter anywhere so the user must check first in the table below:

Pipe size (inches)	Outside diameter (mm)
3/8	17.1
½	21.4
¾	26.7
1	33.6
1 1/4	42.2
1 1/2	48.3
2	60.3
2 ½	75.2 (Note that for North American Pipe 2 ½” = 73mm)
3	88.9
4	114.3
5	140.2
6	168.3
8	219.1
10	273.0
12	323.9