# 1 — Plumbing fitting and pipe thread standards

For 1, thread standard is NPT; threads per inch (tpi) is 11 1/2; thread pitch (mm) is 2.2087; major (outside) diameter (mm) is 33.4010; 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:** 1 _(verified: appears in the quote below)_
- **Threads per inch (TPI):** 11 1/2
- **Thread pitch (mm):** 2.2087
- **Major (outside) diameter (mm):** 33.4010 _(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

> 1.315 | 33.4010 |

## 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.

### Threads per inch (TPI)

en.wikipedia.org says threads per inch (tpi) is **11 1/2**, as of 2026-08-12.

> 1.315 | 33.4010 |

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

trimantec.com says threads per inch (tpi) is **11½**, as of 2026-08-12.

> -16 (1) | 11½ | 33.3 | 1.31 | 31.5 | 1.24

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


### Major (outside) diameter (mm)

en.wikipedia.org says major (outside) diameter (mm) is **33.4010**, as of 2026-08-12.

> 1.315 | 33.4010 |

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

trimantec.com says major (outside) diameter (mm) is **33.3**, as of 2026-08-12.

> -16 (1) | 11½ | 33.3 | 1.31 | 31.5 | 1.24

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


## Source

- https://en.wikipedia.org/wiki/National_pipe_thread
- 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/).