R3/8 vs 3/8-18 NPT: specs compared
Updated 05 September 2026.
17.15 mm
3/8-18 NPT wins on major diameter
On major diameter, 3/8-18 NPT leads: 17.15 mm versus 16.662 mm.
| Spec | R3/8 | 3/8-18 NPT |
|---|---|---|
| Designation | R3/8 | 3/8-18 NPT |
| Major diameter | 16.662 mm | 17.15 mm |
| Threads per inch / pitch | 19 | 18 |
| Thread form | tapered, 55 degree | tapered, 60 degree |
| Seals on | on the thread flanks with PTFE tape or paste | on the thread flanks with PTFE tape |
Questions and answers
Which has the higher major diameter, R3/8 or 3/8-18 NPT?
On major diameter, 3/8-18 NPT leads: 17.15 mm versus 16.662 mm.
How do R3/8 and 3/8-18 NPT compare?
On major diameter, 3/8-18 NPT leads: 17.15 mm versus 16.662 mm.