# 3/8 — Plumbing fitting and pipe thread standards

For 3/8, thread standard is BSP; threads per inch (tpi) is 19; thread pitch (mm) is 1.337; major (outside) diameter (mm) is 16.662; minor diameter (mm) is 14.950, recorded from its source on 2026-08-12.

- **Thread standard:** BSP _(our reading, not quoted from the source)_
- **Nominal size:** 3/8 _(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):** 16.662 _(verified: appears in the quote below)_
- **Minor diameter (mm):** 14.950 _(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

> 3/8 | 19 | 1.337 | 0.856 | 16.662 | 15.806 | 14.950 |

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

> 3/8 | 19 | 1.337 | 0.856 | 16.662 | 15.806 | 14.950 |

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

geminivalve.com says major (outside) diameter (mm) is **16.662**, as of 2026-08-12.

> 3/8 | 19 | 0.0526 | 1.337 | 0.656 | 16.662 | 0.5886 | 14.95 | 0.25 | 6.25

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

htsspares.com says major (outside) diameter (mm) is **16.66**, as of 2026-08-12.

> 3/8 Inch BSP | 16.66 mm / 5/8" | 19

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

trimantec.com says major (outside) diameter (mm) is **16.5**, as of 2026-08-12.

> -06 (3/8) | 19 | 16.5 | 0.65 | 15.2 | 0.60

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


### Minor diameter (mm)

amesweb.info says minor diameter (mm) is **14.950**, as of 2026-08-12.

> 3/8 | 19 | 1.337 | 0.856 | 16.662 | 15.806 | 14.950 |

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

geminivalve.com says minor diameter (mm) is **14.95**, as of 2026-08-12.

> 3/8 | 19 | 0.0526 | 1.337 | 0.656 | 16.662 | 0.5886 | 14.95 | 0.25 | 6.25

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/).