When selecting compatible fittings for your project it’s essential to know what you’re buying. There are key differences between imperial and metric, and understanding them is the first step to ensuring everything runs smoothly. When assembling pipe joints, the fitting always goes over the pipe, making the outer diameter of the pipe, and the inner diameter of the fitting crucial for compatibility. Metric pipes are identified by their outdoor diameter. For example, a 25mm fitting is designed for a 25mm pipe. Imperial pipes and fittings are not identified by their outer diameter. A 1” pipe does not have an outer diameter of 25.4mm as expected, but rather a 33.6mm. So a 1” fitting will not fit a 25mm metric pipe. Whilst imperial and metric generally don’t mix, adaptors are available to enable you to connect the two.
Imperial System:
Imperial pipes and components are specified by their nominal bore (NB), which often causes confusion when comparing them to metric pipes since they are measured differently. The term "nominal" indicates "in name only," meaning all 1/2" pipes must fit 1/2" fittings, regardless of the wall thickness or inner diameter. Because of these variations, it’s essential to verify actual dimensions unless you are well-accustomed to imperial measurements.
Metric System:
Metric pipes are identified by their outer diameter (O/D). For example, a 20mm pipe refers to its outer diameter. The wall thickness may vary to accommodate different pressure ratings (PN Rating can be PN10 = 10 bar, PN16 = 16 bar) but the outer diameter remains constant to ensure compatibility. Fittings designed for 20mm pipes will have an inner diameter slightly larger than 20mm to fit over the pipe, but they are still labelled as 20mm.
General Compatibility:
Metric and imperial pipes and fittings are typically not interchangeable. As shown in the diagrams and charts, there is a 1.4mm difference between the O/D of a 20mm metric pipe and a 1/2" pipe, which are close equivalents in the two systems. This difference means a 20mm pipe would be too loose when used with a 1/2" fitting, leading to leaks and pressure losses due to the improper fit. Adapters are necessary to convert between the two systems, making size comparison charts valuable.
Threaded fittings:
These fittings are always displayed in inches. Certain fittings are threaded on one end and metric on the other. The metric side is the part that allows you to solvent weld the pipe, or in some cases it is a barbed fitting where you can connect a flexible rubber pipe.
Summary:
Measurement Differences: Imperial and metric pipes are measured and referenced differently, making direct comparisons difficult.
Nominal Bore: This term indicates "in name only" and does not accurately reflect the inner diameter.
Identifying Pipes: Use the outer diameter (O/D) and a reference chart to identify existing pipes.
Accuracy: Always check measurements using a reliable reference chart, or contact us for assistance.
Tolerances: All pipes are produced within tolerances, with inner diameters sometimes varying slightly.
