# 1 1/4 — Plumbing fitting and pipe thread standards

For 1 1/4, thread standard is BSPP; threads per inch (tpi) is 11; thread pitch (mm) is 2.309; major (outside) diameter (mm) is 41.910; minor diameter (mm) is 38.953, recorded from its source on 2026-08-12.

- **Thread standard:** BSPP _(our reading, not quoted from the source)_
- **Nominal size:** 1 1/4 _(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):** 41.910 _(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

> BSPP G 1 1/4 | 1 1/4" | 11 | 2.309 | 41.910 | 40.431 | 38.953

## Source

- https://www.machiningdoctor.com/charts/bsp/

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/).