1/4-18 NPT vs R1/8: specs compared
Updated 05 September 2026.
13.72 mm
1/4-18 NPT wins on major diameter
On major diameter, 1/4-18 NPT leads: 13.72 mm versus 9.728 mm.
| Spec | 1/4-18 NPT | R1/8 |
|---|---|---|
| Designation | 1/4-18 NPT | R1/8 |
| Major diameter | 13.72 mm | 9.728 mm |
| Threads per inch / pitch | 18 | 28 |
| Thread form | tapered, 60 degree | tapered, 55 degree |
| Seals on | on the thread flanks with PTFE tape | on the thread flanks with PTFE tape or paste |
Questions and answers
Which has the higher major diameter, 1/4-18 NPT or R1/8?
On major diameter, 1/4-18 NPT leads: 13.72 mm versus 9.728 mm.
How do 1/4-18 NPT and R1/8 compare?
On major diameter, 1/4-18 NPT leads: 13.72 mm versus 9.728 mm.