The most common mistake people make when ordering air fittings is that they measure the width of the thread in millimeters (mm) and convert it to imperial, this will get you the wrong size thread and the wrong size air fitting. Air fittings are sized on the internal bore of the passage that carries the air, the outside thread size is actually larger.

Please see the reference chart below to help you find the correct size air fitting.

Air fittings in Australia and most of the world (except the USA) are generally ordered using British Standard Pipe (BSP)  in sizes as shown in the example above 1/8" BSP, 1/4" BSP, 3/8" BSP and so on.

In the USA they have to be different, so they use the National Pipe Thread Taper (NPT) system. You can however push a BSP air fitting into an NPT thread and vise versa but it will feel a little tight and it may only work on smaller sizes such as 1/8", 1/4" and 3/8" etc.

To determine which thread system your tool or equipment uses is as simple as finding out where it was made, if it is made in the USA then it will generally be NPT, otherwise it would normally be BSP. Please consult the manufacturer of your equipment or tool if you are unsure.