Choosing the wrong pneumatic fitting is one of the most common causes of leaks, pressure drop and wasted time on the workshop floor. A fitting has to match three things: the thread on the port it screws into, the outside diameter of the tube it connects, and the pressure and medium of the system. Get any one wrong and the joint either won’t seal or won’t assemble at all. This guide walks through how to size a pneumatic fitting correctly the first time.
Start With the Thread Type — BSP or NPT
Before you worry about size, identify the thread standard. BSP (British Standard Pipe) is the UK and European default and uses parallel threads sealed by an O-ring or bonded washer. NPT (National Pipe Thread) is the North American standard and uses tapered threads that seal on the thread form itself, usually with PTFE tape. The two look similar but are not interchangeable — forcing an NPT fitting into a BSP port will cross-thread and leak. Most of our threaded adaptors and compression fittings are BSP unless stated otherwise. If you are working on imported machinery, always check the port standard before ordering.
Getting the Thread Size Right
Thread sizes such as 1/4″, 3/8″ and 1/2″ are nominal — they refer to the old pipe bore, not a diameter you can measure. A 1/4″ BSP thread actually measures roughly 13mm across the outside. The quickest way to size an unknown port is to measure the male thread outside diameter with calipers and match it to a thread chart, or measure the female port and drop one size. Never judge by eye: 1/4″ and 3/8″ are easy to confuse, and a mismatched thread will appear to start before it binds and jams.
Sizing Tube OD for Push-In Fittings
For push-in (push-to-connect) fittings, the critical dimension is the outside diameter of the tube, not the bore. Metric push-in fittings are sized by tube OD in millimetres — 4, 6, 8, 10 and 12mm are the common sizes — while imperial fittings use 1/8″, 1/4″, 5/16″ and so on. A 6mm fitting will not grip 1/4″ tube reliably even though the two are close, so keep metric and imperial systems separate. Match the fitting to the tube OD and the tube’s pressure rating, and make sure the tube is cut square so the collet grips and seals cleanly.
Common Sizing Mistakes
The mistakes we see most often are: mixing BSP and NPT; ordering by tube bore instead of tube OD; assuming metric and imperial tube are interchangeable; and over-tightening a tapered thread to stop a weep, which splits the port. Another is ignoring the medium — a fitting rated for compressed air may not suit water, vacuum or coolant. When in doubt, note the size and standard of the fittings already on the machine before you replace them.
A Simple Sizing Guide
To size a fitting from scratch, work through it in order:
- Identify the thread standard on the port — BSP or NPT.
- Measure the thread OD and match it to a size (1/8″, 1/4″, 3/8″, 1/2″).
- Measure the tube OD for the push-in end (4, 6, 8, 10, 12mm, or imperial).
- Confirm the working pressure and the medium (air, water, vacuum).
- Choose the fitting shape — straight, elbow, tee or bulkhead — to suit the routing.
If you can tell us the port thread and the tube OD, our team can match the exact fitting from our range. Browse our push-in fittings and threaded adaptors, or get in touch and we’ll help you specify the right part.
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