# 3/8 — Plumbing fitting and pipe thread standards

For 3/8, thread standard is NPT; threads per inch (tpi) is 18; thread pitch (mm) is 1.4111; major (outside) diameter (mm) is 17.145; thread angle (degrees) is 60, recorded from its source on 2026-08-12.

- **Thread standard:** NPT _(our reading, not quoted from the source)_
- **Nominal size:** 3/8
- **Threads per inch (TPI):** 18
- **Thread pitch (mm):** 1.4111
- **Major (outside) diameter (mm):** 17.145 _(verified: appears in the quote below)_
- **Thread angle (degrees):** 60 _(our reading, not quoted from the source)_
- **Governing standard:** ASME B1.20.1 _(our reading, not quoted from the source)_

## What the source says

> 0.675 | 17.145 |

## Sources disagree

More than one authority states this, and they do not state the same
thing. Both are reproduced with the source each came from.

### Thread pitch (mm)

en.wikipedia.org says thread pitch (mm) is **1.4111**, as of 2026-08-12.

> 0.675 | 17.145 |

Source: https://en.wikipedia.org/wiki/National_pipe_thread

geminivalve.com says thread pitch (mm) is **1.41122**, as of 2026-08-12.

> 3/8 | 18 | 0.05556 | 1.41122 | 0.675 | 17.15

Source: https://www.geminivalve.com/bsp-thread-vs-npt-fitting/


### Major (outside) diameter (mm)

en.wikipedia.org says major (outside) diameter (mm) is **17.145**, as of 2026-08-12.

> 0.675 | 17.145 |

Source: https://en.wikipedia.org/wiki/National_pipe_thread

geminivalve.com says major (outside) diameter (mm) is **17.15**, as of 2026-08-12.

> 3/8 | 18 | 0.05556 | 1.41122 | 0.675 | 17.15

Source: https://www.geminivalve.com/bsp-thread-vs-npt-fitting/

trimantec.com says major (outside) diameter (mm) is **17.3**, as of 2026-08-12.

> -06 (3/8) | 18 | 17.3 | 0.68 | 15.7 | 0.62

Source: https://trimantec.com/blogs/t/thread-identification-guide


## Source

- https://en.wikipedia.org/wiki/National_pipe_thread
- https://www.geminivalve.com/bsp-thread-vs-npt-fitting/
- https://trimantec.com/blogs/t/thread-identification-guide

Last verified: 2026-08-12. Review by: 2028-08-11.
Part of [Plumbing fitting and pipe thread standards](https://referencesource.org/plumbing-fitting-thread-standards/).