# 1/4 — Plumbing fitting and pipe thread standards

For 1/4, thread standard is BSP; threads per inch (tpi) is 19; thread pitch (mm) is 1.337; major (outside) diameter (mm) is 13.157; minor diameter (mm) is 11.445, recorded from its source on 2026-08-12.

- **Thread standard:** BSP _(our reading, not quoted from the source)_
- **Nominal size:** 1/4 _(verified: appears in the quote below)_
- **Threads per inch (TPI):** 19 _(verified: appears in the quote below)_
- **Thread pitch (mm):** 1.337 _(verified: appears in the quote below)_
- **Major (outside) diameter (mm):** 13.157 _(verified: appears in the quote below)_
- **Minor diameter (mm):** 11.445 _(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

> 1/4 | 19 | 1.337 | 0.856 | 13.157 | 12.301 | 11.445 |

## 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.

### Major (outside) diameter (mm)

amesweb.info says major (outside) diameter (mm) is **13.157**, as of 2026-08-12.

> 1/4 | 19 | 1.337 | 0.856 | 13.157 | 12.301 | 11.445 |

Source: https://amesweb.info/Screws/BSP-Thread-Chart.aspx

htsspares.com says major (outside) diameter (mm) is **13.16**, as of 2026-08-12.

> 1/4 Inch BSP | 13.16 mm / 1/2" | 19

Source: https://www.htsspares.com/blog/how-to/how-to-measure-your-bsp-fittings

trimantec.com says major (outside) diameter (mm) is **13.2**, as of 2026-08-12.

> -04 (1/4) | 19 | 13.2 | 0.52 | 11.9 | 0.47

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


## Source

- https://amesweb.info/Screws/BSP-Thread-Chart.aspx
- https://www.htsspares.com/blog/how-to/how-to-measure-your-bsp-fittings
- 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/).