National Pipe Thread Taper (NPT) is a U.S. standard for tapered threads used on threaded pipes and fittings. In contrast to straight threads that are found on a bolt, a taper thread will pull tight and therefore make a fluid-tightseal.

The typical sizes you will see in medical oxygen filling are 1/4” and 1/2” NPT.  (The 1/4” and 1/2” names do not actually refer to the NPT sizing, but instead the names come from the pipe size that was originally used in the plumbing industry.) To determine what size you need, check out the table below.