Holley 12-879 pressure regulator

The below table gives the Threads Per Inch, Pithc and Major Diameter for NPT Threads.

Trade Size Threads per inch Pitch Major Diameter (O.D)
Inch mm Inch mm
1/8 27 0.03704 0.94082 0.405 10.29
1/4 18 0.05556 1.41122 0.54 13.72
3/8 18 0.05556 1.41122 0.675 17.15mm (major diameter OD)

Pipe is always measures in ID and tube is OD, so a 1/8 NPT fitting is huge compared to a 1/8 tube.