Pipe thread identifier: NPT vs BSP chart
Pipe sizes are not the real thread size: a 1/2 in. NPT thread measures about 0.84 in. across. Measure the outside of a male thread and count threads per inch, and this tool tells you what it is. The chart below crosses NPT and BSP sizes.
NPT and BSP cross chart
| Nominal size | NPT / NPTF / NPS (60°) | BSPT / BSPP (55°) | Will they mate? | ||
|---|---|---|---|---|---|
| TPI | OD, in. (mm) | TPI | OD, in. (mm) | ||
OD = thread major diameter at the gauge point. A tapered male end usually measures a little smaller at the tip. NPT and BSPT taper 1:16 on diameter. BSPP (G) is parallel and seals with a washer or O-ring, not on the threads.
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Questions & answers
Can you connect NPT to BSP?
Not reliably. The thread angles differ (60° vs 55°) and most sizes have a different thread count, so they cross-thread or leak. In 1/2 and 3/4 in. both use 14 TPI and may start to thread together, but they still will not seal properly. Use an NPT-to-BSP adapter.
Why is 1/2 in. pipe thread bigger than 1/2 in.?
Pipe sizes are nominal. They came from the old inside diameter of the pipe, not the thread. A 1/2 in. NPT thread is about 0.84 in. (21.3 mm) across the outside.
What is the difference between NPT and NPTF?
They share size and thread count, so they fit together. NPTF (dryseal) has a tighter form at the crests and roots and is made to seal without sealant. NPT normally needs tape or pipe dope.
BSPT vs BSPP: how do I tell?
BSPT (R or Rc) is tapered and seals on the threads, like NPT. BSPP (G) is straight and seals with a bonded washer, O-ring or face seal. Lay a straightedge along the threads: a taper narrows toward the end.