# 2½ — Plumbing fitting and pipe thread standards

For 2½, thread standard is BSP; threads per inch (tpi) is 11; thread pitch (mm) is 2.309; major (outside) diameter (mm) is 75.184; minor diameter (mm) is 72.227, recorded from its source on 2026-08-12.

- **Thread standard:** BSP _(our reading, not quoted from the source)_
- **Nominal size:** 2½ _(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):** 75.184 _(verified: appears in the quote below)_
- **Minor diameter (mm):** 72.227 _(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

> 2½ | 11 | 0.0909 | 2.309 | 2.96 | 75.184 | 2.8436 | 72.227 | 0.6875 | 17.463

## 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/).