Please Note: All fractional sizes that are shown with our air fittings are BSP (They are not actual size - Also please note our fittings are not NPT which is American and not interchangeable due to the differences in thread forms)
We have a rough guide to working out your air fitting with some measurements below that will help.
1⁄8" BSP male outside thread diameter is approx. 3/8" (9.5 mm) in diameter
1⁄8" BSP female inside thread diameter is approx. 11/32" (8.8 mm) in diameter
1⁄4" BSP male outside thread diameter is approx. 1/2" (13 mm) in diameter
1⁄4" BSP female inside thread diameter is approx. 15/32" (11.8 mm) in diameter
I hope this helps as it causes great confusion
Also note that Paasche, Badger and Aztec have their own thread size off the airbrush so you will need either an adaptor or a hose that is that particular brand. Iwata, Sparmax and Harder & Steenbeck have a standard size of 1/8" BSP. Its confusing and many of our customers have problems getting their heads around it all. We get these questions every day so sometimes its best to give us the brand and let us work it out for you. You can also email and image to us against a ruler or a coin and this way we can most likely work out the size.