Measuring the diameter of a plastic pipe might seem quite straightforward at first – you just measure the diameter of the pipe. However, it's important that you measure the right diameter with the right unit, so you can tell exactly what size it is, and make sure that when you're reordering fittings or more pipe, that you choose the right size.
All plastic pipe is measured on the outside diameter of the pipe, not the inside diameter. This is because the pipe needs to fit into the socket. The pressure rating of the pipe has no significance. If the pipe has a heavy pressure rating and the pipe wall is very thick, the outside diameter is always going to remain the same, so that you always get that nice, snug fit inside a socket. As the pipe wall changes, it is the inside diameter that gets smaller. The outside diameter always stays the same. This is the same for metric sizes and for inch/imperial sizes.
The metric and inch/imperial sizes are two different systems. The metric system labels pipe diameters in millimetres, while the inch/imperial system labels pipe diameters in inches. The metric system is used in Europe and many other parts of the world, including the UK. The inch/imperial system is used in North America, Asia, and also in the UK. As both systems are used in the UK, it tends to make things a little complicated because we have to make sure we get the right measurement system because they're not interchangeable.
For example, an inch and a half pipe (inch/imperial system) measures just over 48mm on the outside diameter, and a 50mm pipe (metric system) measures 50mm on the outside diameter. Therefore, if you try and fit a 50mm pipe into an inch and a half socket, it won’t work because the socket is too small. Similarly, if you try and put an inch and a half pipe into a 50mm socket, the socket will be too big, and the pipe will rattle around inside it. Therefore, it is important the system is either fully metric or fully inch/imperial.
In the inch/imperial system, the size – for example, 2”, 3”, or ½” - relates to the approximate bore of the inside of the pipe; however, it is more effective to think of this size as the name for the pipe, rather than a measurement. A two-inch pipe does not measure two inches on the outside diameter, but rather measures 60.3mm on the outside diameter.
Conversely, in the metric system, the outside diameter of the pipe actually measures the same as the name of the pipe. For example, if you take a 63mm pipe, the outside diameter of the pipe measures 63mm. The name and measurement are the same in the metric system. This is how the two systems differ.
Some manufacturers use the same moulds for their metric fittings as they do for their inch/imperial fittings. For example if you take a Comer fitting, you’ll see it has two numbers on it. If the numbers read “90 – 3”, it is telling you that the fitting is a 90mm and it is a three-inch; but we know that it cannot be both, because they are different measurements. The manufacturers use the same mould and don't change the stamp on the fittings. For our “90 – 3” example, you would have to measure the outside diameter to figure out whether it was a 90mm or a three-inch.
Remembering the difference between the two systems and the ambiguity of the fitting sizes could save you time and money.
If you would like to watch a video explaining this a little more then you can visit our YouTube channel and watch the video!