# 1/8 — Plumbing fitting and pipe thread standards

For 1/8, thread standard is BSP; threads per inch (tpi) is 28; thread pitch (mm) is 0.907; major (outside) diameter (mm) is 9.728; minor diameter (mm) is 8.566, recorded from its source on 2026-08-12.

- **Thread standard:** BSP _(our reading, not quoted from the source)_
- **Nominal size:** 1/8 _(verified: appears in the quote below)_
- **Threads per inch (TPI):** 28 _(verified: appears in the quote below)_
- **Thread pitch (mm):** 0.907 _(verified: appears in the quote below)_
- **Major (outside) diameter (mm):** 9.728 _(verified: appears in the quote below)_
- **Minor diameter (mm):** 8.566 _(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

> 1/8 | 28 | 0.907 | 0.581 | 9.728 | 9.147 | 8.566 |

## 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 **9.728**, as of 2026-08-12.

> 1/8 | 28 | 0.907 | 0.581 | 9.728 | 9.147 | 8.566 |

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

geminivalve.com says major (outside) diameter (mm) is **9.728**, as of 2026-08-12.

> 1/8 | 28 | 0.0357 | 0.907 | 0.383 | 9.728 | 0.3372 | 8.565 | 0.1563 | 3.97

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

htsspares.com says major (outside) diameter (mm) is **9.52**, as of 2026-08-12.

> 1/8 Inch BSP | 9.52 mm / 3/8" | 28

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

trimantec.com says major (outside) diameter (mm) is **9.7**, as of 2026-08-12.

> -02 (1/8) | 28 | 9.7 | 0.38 | 8.9 | 0.35

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


### Minor diameter (mm)

amesweb.info says minor diameter (mm) is **8.566**, as of 2026-08-12.

> 1/8 | 28 | 0.907 | 0.581 | 9.728 | 9.147 | 8.566 |

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

geminivalve.com says minor diameter (mm) is **8.565**, as of 2026-08-12.

> 1/8 | 28 | 0.0357 | 0.907 | 0.383 | 9.728 | 0.3372 | 8.565 | 0.1563 | 3.97

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


## Source

- https://amesweb.info/Screws/BSP-Thread-Chart.aspx
- https://www.geminivalve.com/bsp-thread-vs-npt-fitting/
- 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/).