The threads on metric fittings look the same as those on NPT fittings and a 1/4" NPT fitting will have a similar diameter to a 1/4" Metric fitting. I assure you, they are not the same.
Metric fittings have a different thread configuration, with differences in the thread tip-to-tip distance.
As a result, metric thread sizes are incompatible with their NPT same-sized counterparts.
Some firms do suggest they offer a universal fitting. Check and be sure before buying into this concept. If you find a true universal fitting, do let me know and I'll note it here. I have seen some that claimed to be, but they did not provide adequate thread-depth penetration or sealing.
It is not always easy to tell them apart. Some manufacturers use different color release-rings for their metric fittings. Unless someone clues you in to the color guide though, how will you know which are which?
The threads on this style will likely be shorter in length than other types of metric fittings. The reason for this is that the G-type fitting is designed to screw right down into a boss.
In other words, you will screw this fitting thread right down until the sealing ring or O-ring squishes between the body of the fitting wrench flats and the boss into which you are threading it, sealing in the compressed air.
G-style fittings come in a variety of thread sizes, the commonest of which are:
Larger sizes in the G-style metric fitting are certainly available, yet the sizes shown seem to be the highest volume in industrial compressed air applications.
Since the R-type threaded fitting has a taper, it too seals by penetrating into the threads of the boss until sufficient threads are in contact with each other (the fitting is tight) to prevent the escape of compressed air.
R-type metric fittings come in a variety of thread sizes, the commonest of which are:
As with other styles of compressed air fittings, larger fitting sizes in the R-type are available. The sizes shown above are the most common for industrial compressed air valves and cylinders.
Fittings, whether metric or NPT have to connect to something...right?