# 1 ¼ — Plumbing fitting and pipe thread standards

For 1 ¼, thread standard is BSP; threads per inch (tpi) is 11; major (outside) diameter (mm) is 41.9, recorded from its source on 2026-08-12.

- **Thread standard:** BSP _(our reading, not quoted from the source)_
- **Nominal size:** 1 ¼ _(verified: appears in the quote below)_
- **Threads per inch (TPI):** 11 _(verified: appears in the quote below)_
- **Major (outside) diameter (mm):** 41.9 _(verified: appears in the quote below)_

## What the source says

> -20 (1 ¼) | 11 | 41.9 | 1.65 | 39.6 | 1.56

## Source

- 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/).