# 3/8 — Plumbing fitting and pipe thread standards

For 3/8, thread standard is BSPT; threads per inch (tpi) is 19; thread pitch (mm) is 1.337; major (outside) diameter (mm) is 16.66; minor diameter (mm) is 14.95, recorded from its source on 2026-08-12.

- **Thread standard:** BSPT _(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):** 16.66 _(verified: appears in the quote below)_
- **Minor diameter (mm):** 14.95 _(verified: appears in the quote below)_
- **Thread angle (degrees):** 55 _(our reading, not quoted from the source)_
- **Governing standard:** ISO 7-1 _(our reading, not quoted from the source)_

## What the source says

> <https://www.machiningdoctor.com/threadinfo-bspt/?tid=4503> BSPT R 3/8 | 3/8" | 19 | 1.337 | 6.4 | 1.0 / 1.3 | 16.66 | 15.81 | 14.95 |

## Source

- https://www.machiningdoctor.com/charts/bsp/

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/).