| Thread size (in) | NPT / NPTF (mm) | BSP / BSPT (mm) | ||
|---|---|---|---|---|
| Inside | Outside | Inside | Outside | |
| 1/8 | 8.50 - 8.90 | 9.40 - 9.70 | 8.50 - 8.90 | 9.40 - 9.70 |
| 1/4 | 11.40 - 11.90 | 12.90 - 13.10 | 11.40 - 11.90 | 12.90 - 13.10 |
| 3/8 | 14.90 - 15.40 | 16.30 - 16.60 | 14.90 - 15.40 | 16.30 - 16.60 |
| 1/2 | 18.30 - 18.70 | 20.70 - 21.10 | 18.60 - 19.00 | 20.50 - 20.90 |
| 3/4 | 23.70 - 24.10 | 26.30 - 26.70 | 24.10 - 24.50 | 26.10 - 26.40 |
| 1 | 30.30 - 30.80 | 32.90 - 33.40 | 30.30 - 30.80 | 33.00 - 33.20 |
| 5/4 (1 1/4) | 39.20 - 39.60 | 41.40 - 42.00 | 39.00 - 39.50 | 41.50 - 41.90 |
| 6/4 (1 1/2) | 45.10 - 45.50 | 47.30 - 47.90 | 44.80 - 45.30 | 47.40 - 47.80 |
Pipe thread calculator helps you identify the thread type and size used in water installations - especially for fittings, filter housings, tees, reducers and valves where different standards (BSP/NPT) can be mixed.
This tool is useful for water filter servicing and plumbing work: it reduces wrong purchases, speeds up troubleshooting and lowers the risk of leaks after installation. For best results, confirm with caliper measurements and a thread pitch gauge.
- Thread identification when documentation is missing.
- Choosing the right fitting for imported parts.
- Checking whether the issue is the thread or the sealing method.
FAQ - Pipe thread calculator
How do I tell BSP from NPT?
BSP and NPT differ in thread form and pitch. In practice, compare pitch and how the part engages after a few turns - ideally with a thread gauge.
How should I measure thread diameter?
Use a caliper and measure the outside diameter for male threads or the inside diameter for female threads. Take a few readings and average them.
What does 1/2" mean in plumbing?
It is a nominal pipe/thread designation, not always the actual diameter in mm. Combine the inch label with real measurements.
Are pipe threads the same as metric threads?
No. Metric threads are marked with M (e.g., M20). Pipe threads use inch-based designations (1/2", 3/4", etc.) and different profiles.
Can BSP and NPT be connected directly?
It is not recommended. They may partially engage but often leak. Use a proper BSP-to-NPT adapter when needed.
Which sealing method should I use?
Common options are PTFE tape, sealing cord or paste. Tapered threads typically require careful sealing to avoid leaks.
Why does it leak even if the thread seems correct?
Possible causes include mixed standards (BSP vs NPT), damaged first threads, cross-threading, missing gasket or an unsuitable sealing method.