Quick guide to Npt pipe thread measurements for fittings

Did you know plumbing fittings use a special Npt call out size system that requires conversion. We have the chart for easy identification.

Npt itself is not a measured size, it is an industry call out size.

Similar to dimensional lumber being a bit smaller than its call out. Try measuring a modern 2" x 4" wood framing stud and you'll be surprised to find its smaller profile wise. 

Ok back to Npt thread sizing. Lets take the below example a 3/8" Npt threaded fitting has a nominal thread (actual thread) diameter of 5/8" (0.675 decimal inches if you use a caliper).

When ordering a new brass fitting to fit towards an existing fitting always be sure to measure your existing thread diameter and convert it to the npt pipe size.

Also note that Npt and Compression (that fine thread with sleeve and nut you find on those hoses under the sink or on the bathroom shut off) are not the same. Npt threads seal thru wedging / taper. Compression seals with the ferrule deforming around the copper tubing. We have crossover fittings for Npt to compression should you ever need them for your project.