A pitch gauge cannot tell 1/2 NPT from 1/2 BSP. Both cut 14 threads per inch. The gauge reads 14, you write down 14, and you still do not know which fitting is in your hand.
That is the trap in every NPT vs BSP chart. The first column people check is TPI, and TPI is the column that lies.
Here are the numbers side by side, both standards, all nine sizes we ship.
| Nominal | NPT TPI | BSP TPI | NPT major (mm) | BSP major (mm) | NPT minor (mm) | BSP minor (mm) |
|---|---|---|---|---|---|---|
| 1/8″ | 27 | 28 | 10.29 | 9.73 | 8.48 | 8.57 |
| 1/4″ | 18 | 19 | 13.72 | 13.16 | 11.00 | 11.45 |
| 3/8″ | 18 | 19 | 17.15 | 16.66 | 14.42 | 14.95 |
| 1/2″ | 14 | 14 | 21.34 | 20.96 | 17.81 | 18.63 |
| 3/4″ | 14 | 14 | 26.67 | 26.44 | 23.13 | 24.12 |
| 1″ | 11.5 | 11 | 33.40 | 33.25 | 29.06 | 30.29 |
| 1-1/4″ | 11.5 | 11 | 42.16 | 41.91 | 37.79 | 38.95 |
| 1-1/2″ | 11.5 | 11 | 48.26 | 47.80 | 43.86 | 44.85 |
| 2″ | 11.5 | 11 | 60.33 | 59.61 | 55.87 | 56.66 |
NPT sizes come from ASME B1.20.1, thread angle 60°, taper 1:16. BSP comes from ISO 228-1 for parallel and ISO 7-1 for taper, thread angle 55°. The minor diameters are the bore of the female thread, which is what a plug has to fill.
Three rows survive a pitch gauge. 1/8″ is 27 against 28, one thread off across the whole length. 1/2″ and 3/4″ are 14 against 14. On those two sizes, a gauge tells you nothing.
That is where the angle does the work. NPT cuts 60°, BSP cuts 55°. Five degrees of difference. A 55° gauge seats on BSP and rocks on NPT. Two gauges settle the question in about two seconds. Most shops carry one gauge. That is why this error keeps showing up on the bench.
Now check the major diameter at the first full thread. At 1/2″ the split is 21.34 mm against 20.96 mm. Callipers read that easily. At 1″ it is 33.40 against 33.25, a margin of 0.15 mm. Most callipers will not resolve it. The angle is the decisive test, so do not lean on one diameter reading alone.
For 1-1/4″ and 1-1/2″, the pitch parts again, but the two diameters sit only 0.25 mm and 0.46 mm apart. Measure both ends here. A single reading taken at the wrong point on a tapered thread will point you the wrong way.
Do not identify a thread by measuring the hex or the body. Those numbers track nothing in either standard. Do not trust the bag label on an unmarked fitting either. And do not buy a “1/2 inch” fitting from a listing that never says NPT, BSP or G. Half the returns we see start there.
Write down three things before you order. Nominal size, TPI from your gauge, and major diameter in mm. Send all three. We will tell you the standard and the exact part.
Our brass push-in fittings and brass connectors ship in NPT, BSPT and BSPP. If you are still working out the tube end, the mm size chart covers that side, and the measuring guide walks through the callipers.