# 1/2 — Plumbing fitting and pipe thread standards

For 1/2, thread standard is BSP; threads per inch (tpi) is 14; thread pitch (mm) is 1.814; major (outside) diameter (mm) is 20.955; minor diameter (mm) is 18.631, recorded from its source on 2026-08-12.

- **Thread standard:** BSP _(our reading, not quoted from the source)_
- **Nominal size:** 1/2 _(verified: appears in the quote below)_
- **Threads per inch (TPI):** 14 _(verified: appears in the quote below)_
- **Thread pitch (mm):** 1.814 _(verified: appears in the quote below)_
- **Major (outside) diameter (mm):** 20.955 _(verified: appears in the quote below)_
- **Minor diameter (mm):** 18.631 _(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

> 1/2 | 14 | 1.814 | 1.162 | 20.955 | 19.793 | 18.631 |

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

> 1/2 | 14 | 1.814 | 1.162 | 20.955 | 19.793 | 18.631 |

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

geminivalve.com says major (outside) diameter (mm) is **20.955**, as of 2026-08-12.

> 1/2 | 14 | 0.0714 | 1.814 | 0.825 | 20.955 | 0.7336 | 18.633 | 0.3214 | 8.164

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

htsspares.com says major (outside) diameter (mm) is **20.99**, as of 2026-08-12.

> 1/2 Inch BSP | 20.99 mm / 3/4" | 14

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

trimantec.com says major (outside) diameter (mm) is **20.8**, as of 2026-08-12.

> -08 (1/2) | 14 | 20.8 | 0.82 | 19.1 | 0.75

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


### Minor diameter (mm)

amesweb.info says minor diameter (mm) is **18.631**, as of 2026-08-12.

> 1/2 | 14 | 1.814 | 1.162 | 20.955 | 19.793 | 18.631 |

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

geminivalve.com says minor diameter (mm) is **18.633**, as of 2026-08-12.

> 1/2 | 14 | 0.0714 | 1.814 | 0.825 | 20.955 | 0.7336 | 18.633 | 0.3214 | 8.164

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/).