Pipe Thread Chart — NPT Pipe Thread Size

Every pipe thread size in the table above is a national pipe thread (NPT) dimension. The label — 1/8 NPT, 1/4 NPT, 1/2 NPT — is a trade name. The measured outside diameter is always larger: 1/8 NPT diameter is 0.405 in across the crests, and 1/2 NPT is 0.840 in. If you are matching a fitting, measure the outside diameter and count the threads per inch, then read the size back out of the chart.

Because tapered NPT threads seal on the flanks, only part of the thread actually engages. Hand tight engagement is roughly 3 to 4 turns for small sizes and 3 turns for 1 in and up. Wrench makeup adds another 2 to 3 turns. The effective thread length column is the usable engagement length; anything past it runs out of taper and stops tightening. Over-tightening past that point splits a female thread instead of improving the seal.

NPT is not a self-sealing thread on its own. A thread sealant — PTFE tape or a paste rated for the service fluid — fills the small spiral gap at the thread roots. Two to three wraps of tape in the direction of the thread is normal practice; more tape acts as a lubricant and encourages over-tightening. Dry-seal NPTF threads are the exception and mate crest-to-root without sealant.

British standard pipe threads (BSPT and BSPP) look similar and share some nominal sizes, but the thread form is 55 degrees rather than 60, and BSPP is parallel rather than tapered. A male NPT will start into a British standard female thread and then bind, so verify the standard before assembly. For electrical work, rigid metal conduit and IMC use the same tapered NPT geometry at each trade size.