Which thread is this fitting?
BSP and NPT sit close enough that a fitting will start in the wrong port and then leak. Measure the thread, count it, and see everything it could be.
The test that actually settles it
Parallel or tapered. Run a vernier along a male thread: BSPP and metric stay the same diameter end to end, while BSPT and NPT get visibly smaller towards the nose. That single check separates the families faster than any measurement of diameter, and it is the one this page cannot do for you.
What this assumes
- The tables are the published standards - ISO 228 for BSP parallel, ISO 7 for BSP taper, ASME B1.20.1 for NPT and ISO 261 for metric. Nothing is interpolated: a size that is not in a standard does not exist in it.
- BSP is quoted by its major diameter and NPT by its nominal pipe size, which is why neither number matches the inch label on the fitting. A 1/4 inch BSP thread measures about 13.2 mm, not 6.4 mm.
- A worn, plated or over-tightened thread measures small, so the list is deliberately generous on diameter and strict on thread count.
- BSPP and BSPT share a thread form and therefore share every figure in this table. Both are always listed together; only looking at the taper tells them apart.
- This identifies a thread. It does not tell you what pressure a fitting is rated for, or that two parts are safe to join - that comes from the part, not from a measurement.