• National Pipe Thread (NPT) is a United States standard for tapered threads, originally used to join steel or brass pipe and fittings. The NPT sizes are commonly used and appear on both the pipe and fittings in most United States hardware stores.


• These are the two most common sizes:
  
  1/4" NPT measures to about 1/2" across.
  3/8" NPT measures about 5/8" across.