NPT and BSP describe different thread systems. Some are tapered, some are parallel, and nominal pipe size is not the measured outside diameter in the way a beginner may expect.
Why the straight-thread rule fails
Pipe threads involve taper, truncation, gauge planes and engagement rules that are not represented by nominal diameter minus pitch. The nominal pipe label also refers to a pipe system, not a direct measured major diameter.
- Identify NPT, NPTF, BSPT or BSPP.
- Confirm the exact tap and material.
- Use the tap manufacturer’s current drill recommendation.
- Gauge the thread at the required plane.
What ThreadSizeGuide does today
The calculator deliberately returns unavailable for NPT and BSP until the exact standard-specific datasets, editions and verification cases have been reviewed. This is safer than a plausible-looking extrapolation.
Important limitation
A thread can start and still be the wrong standard. Do not judge NPT/BSP identity or sealing quality by hand fit alone.
A thread can start and still be the wrong standard. Do not judge NPT/BSP identity or sealing quality by hand fit alone.