BSPP, BSPT and NPT threads compared by nominal bore, and which of our threaded products are cut to each.
Find a size
BSPP (parallel, sealed on a washer or O-ring), BSPT (taper, seals on the thread) and NPT look alike at a glance and are not interchangeable — an NPT male will start into a BSPT female and cross-thread rather than seat. Pick a bore to see all three side by side, and to see which of our own products are cut to each.
BSPP, BSPT and NPT compared
| Bore | Inch | BSPP (G) TPI | BSPP OD mm | BSPT (R) TPI | BSPT OD mm | NPT TPI | NPT OD mm |
|---|---|---|---|---|---|---|---|
| 8 mm | 1/8" | 28 | 9.73 | 28 | 9.73 | 27 | 10.29 |
| 15 mm | 1/4" – 3/8" | 19 | 13.16 | 19 | 13.16 | 18 | 13.72 |
| 20 mm | 1/2" | 14 | 20.96 | 14 | 20.96 | 14 | 21.34 |
| 25 mm | 3/4" | 14 | 26.44 | 14 | 26.44 | 14 | 26.67 |
| 32 mm | 1" | 11 | 33.25 | 11 | 33.25 | 11.5 | 33.40 |
| 40 mm | 1 1/4" | 11 | 41.91 | 11 | 41.91 | 11.5 | 42.16 |
| 50 mm | 1 1/2" | 11 | 47.80 | 11 | 47.80 | 11.5 | 48.26 |
| 65 mm | 2" | 11 | 59.61 | 11 | 59.61 | 11.5 | 60.33 |
| 80 mm | 2 1/2" | 11 | 75.18 | 11 | 75.18 | 8 | 73.03 |
| 100 mm | 3" | 11 | 87.88 | 11 | 87.88 | 8 | 88.90 |
| 125 mm | 4" | 11 | 113.03 | — | — | 8 | 114.30 |
| 150 mm | 5" | 11 | 138.43 | — | — | — | — |
A blank cell is not a rounding error — that size is not a standard cut in that thread form (BSPT and NPT both stop before 5", for instance).
Telling them apart on site
- BSPP (marked G) is straight-sided and seals on a bonded washer, O-ring or flat gasket at the shoulder — the thread itself carries no seal.
- BSPT (marked R external, Rc internal) tapers at 1:16 and seals on the thread flanks with PTFE tape or jointing compound — no separate washer.
- NPT also tapers at 1:16 and looks close enough to BSPT to start hand-tight on the wrong one — but the flank angle differs (60° against 55°), so a mismatched pair backs off under pressure rather than sealing. Never mix BSPT and NPT on the same joint.
Common questions
What is the difference between BSPP and BSPT?
BSPP (parallel BSP, marked G) seals on a washer or O-ring; BSPT (taper BSP, marked R external / Rc internal) seals on the thread flanks itself, with PTFE tape or jointing compound — no separate washer.
Can NPT and BSPT be mixed on the same joint?
No. They look close enough to start hand-tight on each other, but NPT's flank angle is 60° against BSPT's 55° — a mismatched pair backs off under pressure instead of sealing.
Is this thread chart verified against the actual standards?
No — it is transcribed from BS 21/ISO 228, ISO 7-1 and ASME B1.20.1 rather than read from a current copy of each standard. Confirm before cutting a thread against it, the same caution as the flange and pipe-weight tables.