# 1¼ — Plumbing fitting and pipe thread standards

For 1¼, thread standard is BSP; threads per inch (tpi) is 11; thread pitch (mm) is 2.309; major (outside) diameter (mm) is 41.91; minor diameter (mm) is 38.953, recorded from its source on 2026-08-12.

- **Thread standard:** BSP _(our reading, not quoted from the source)_
- **Nominal size:** 1¼ _(verified: appears in the quote below)_
- **Threads per inch (TPI):** 11 _(verified: appears in the quote below)_
- **Thread pitch (mm):** 2.309 _(verified: appears in the quote below)_
- **Major (outside) diameter (mm):** 41.91 _(verified: appears in the quote below)_
- **Minor diameter (mm):** 38.953 _(verified: appears in the quote below)_
- **Thread angle (degrees):** 55 _(our reading, not quoted from the source)_
- **Governing standard:** ISO 228-1 _(our reading, not quoted from the source)_

## What the source says

> 1¼ | 11 | 0.0909 | 2.309 | 1.65 | 41.91 | 1.5336 | 38.953 | 0.5 | 12.7

## Source

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

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/).