NPT / NPTF (ASME B1.20.1)
The American tapered pipe thread. 60 degree flanks, taper 1 in 16, sealed on the thread.
60 degree flanksTaperedTapered thread, sealed on the thread
Confusion pair
NPT vs BSPT
Two tapered pipe threads that look identical and are five degrees apart on the flank. At 1/2 and 3/4 they share a thread count and sit within half a millimetre on diameter, so they start on each other.
BSPT, R external (ISO 7-1)
The British tapered pipe thread. 55 degree Whitworth flanks, taper 1 in 16, sealed on the thread.
55 degree flanksTaperedTapered thread, sealed on the thread
The three answers
What actually happens if you put them together
What interchanges
- Nothing. Two publishers describe the pair as not generally interchangeable and note that the thread pitches differ at most sizes and that the thread form and diameters are close but not identical.
What threads together and will still leak or fail
This is a safety statement, so it is only made where it is sourced. Each item below either cites a published statement that the two are not interchangeable, or rests on the figures on the size pages showing an identical thread with a different seat.
- At 1/2 and 3/4 in trade size the two run the same threads per inch - 14 - and their basic diameters sit 0.39 mm and 0.23 mm apart. An NPT male will start and pull up in a BSPT or Rc female at those two sizes. It is the classic pipe-thread failure: the 60 degree and 55 degree flanks bear on their crests instead of on the flanks, so the wedge that is supposed to make the seal never forms, and sealant is left holding the pressure on its own.
- The two are close enough at 1/8, 1/4 and 3/8 that a male will engage the first turn or two of the wrong female before binding. That is enough to damage a soft port and enough to convince somebody the parts match.
What will not fit at all
- At 1/8, 1/4, 3/8, 1, 1 1/4, 1 1/2 and 2 the thread counts differ: 27 against 28, 18 against 19, and 11 1/2 against 11. Beyond the first couple of turns they will bind.
The check that settles it
Gauge the thread form, not the diameter. A 60 degree blade rocks on a 55 degree thread and a 55 degree blade rocks on a 60 degree thread; the one that seats flat is the answer. If you only have a rule, count threads: 18 per inch at about 13.5 mm is NPT 1/4, and 19 per inch at about 13.1 mm is R 1/4.
Size by size
Where the threads match, and where they do not
2 of the 10 sizes below run the same thread count within half a millimetre on diameter. Those are the rows that will screw together. Built from the published dimensions at page-build time, not from a hand-kept list.
| Size | NPT / NPTF | BSPT, R external | Will they assemble? |
|---|---|---|---|
| 1/16 in | 1/16-27 NPT · 7.94 mm | R 1/16 (28 TPI, 55 degree, taper 1:16) · 7.72 mm | No - 27 TPI against 28 TPI |
| 1/8 in | 1/8-27 NPT · 10.29 mm | R 1/8 (28 TPI, 55 degree, taper 1:16) · 9.73 mm | No - 27 TPI against 28 TPI |
| 1/4 in | 1/4-18 NPT · 13.72 mm | R 1/4 (19 TPI, 55 degree, taper 1:16) · 13.16 mm | No - 18 TPI against 19 TPI |
| 3/8 in | 3/8-18 NPT · 17.15 mm | R 3/8 (19 TPI, 55 degree, taper 1:16) · 16.66 mm | No - 18 TPI against 19 TPI |
| 1/2 in | 1/2-14 NPT · 21.34 mm | R 1/2 (14 TPI, 55 degree, taper 1:16) · 20.95 mm | Yes, and it is the wrong joint |
| 3/4 in | 3/4-14 NPT · 26.67 mm | R 3/4 (14 TPI, 55 degree, taper 1:16) · 26.44 mm | Yes, and it is the wrong joint |
| 1 in | 1-11 1/2 NPT · 33.4 mm | R 1 (11 TPI, 55 degree, taper 1:16) · 33.25 mm | No - 11.5 TPI against 11 TPI |
| 1 1/4 in | 1 1/4-11 1/2 NPT · 42.16 mm | R 1 1/4 (11 TPI, 55 degree, taper 1:16) · 41.91 mm | No - 11.5 TPI against 11 TPI |
| 1 1/2 in | 1 1/2-11 1/2 NPT · 48.26 mm | R 1 1/2 (11 TPI, 55 degree, taper 1:16) · 47.8 mm | No - 11.5 TPI against 11 TPI |
| 2 in | 2-11 1/2 NPT · 60.33 mm | R 2 (11 TPI, 55 degree, taper 1:16) · 59.61 mm | No - 11.5 TPI against 11 TPI |
Diameters shown are the basic major diameter of the male thread. On a tapered thread that figure is defined at the gauge plane, which is not where a caliper naturally sits - the size pages carry the span published charts actually measure.
Detail
Everything each family carries
| Property | NPT / NPTF | BSPT, R external |
|---|---|---|
| Flank angle | 60 degrees | 55 degrees |
| Parallel or tapered | Tapered, 1 in 16 | Tapered, 1 in 16 |
| Seat | Tapered thread, sealed on the thread | Tapered thread, sealed on the thread |
| Where it seals | The thread flanks wedging together, normally with a thread sealant or PTFE tape. The seal is the thread. | The thread flanks wedging together, normally with a thread sealant or PTFE tape. The seal is the thread. |
| Defined by | ASME B1.20.1, ASME B1.20.3 | ISO 7-1, BS 21 |
| Sized by | Sized by trade size, which is neither the thread diameter nor the bore: 1/4 NPT measures about 0.54 in across the threads. | Sized by trade size, same nominal series as BSPP. |
Answers
Common questions
Will a NPT / NPTF screw into a BSPT, R external?
At 1/2 and 3/4 in trade size the two run the same threads per inch - 14 - and their basic diameters sit 0.39 mm and 0.23 mm apart. An NPT male will start and pull up in a BSPT or Rc female at those two sizes. It is the classic pipe-thread failure: the 60 degree and 55 degree flanks bear on their crests instead of on the flanks, so the wedge that is supposed to make the seal never forms, and sealant is left holding the pressure on its own.
Are NPT and BSPT (R) interchangeable?
Nothing. Two publishers describe the pair as not generally interchangeable and note that the thread pitches differ at most sizes and that the thread form and diameters are close but not identical.
How do I tell them apart?
Gauge the thread form, not the diameter. A 60 degree blade rocks on a 55 degree thread and a 55 degree blade rocks on a 60 degree thread; the one that seats flat is the answer. If you only have a rule, count threads: 18 per inch at about 13.5 mm is NPT 1/4, and 19 per inch at about 13.1 mm is R 1/4.
More pairs
Other fittings people mix up
Where these figures come from
Every value on this page was checked against 5 independent published sources:
- ANSI/ASME B1.20.1, Pipe Threads, General Purpose (Inch), Table 1 - ASME, ANSI/ASME B1.20.1-1983 (standard, read directly, read 2026-08-27)
- ISO 7-1, Pipe threads where pressure-tight joints are made on the threads, Table 1 - ISO, ISO 7-1:1994(E), third edition (standard, read directly, read 2026-08-27)
- Hydraulic coupling identification chart E2/50101 - Gates, Gates Europe, 2004 (manufacturer, read 2026-08-27)
- Fluid Ports and Connectors Identification Guide - McGill Hose & Coupling, SAE J512, J514, J1453, J1926-1, ISO 228, ISO 7, DIN 2353, JIS (distributor, read 2026-08-27)
- Thread and connector identification guide - Gerrard Hydraulics (RYCO), AS 1722.1, AS 1722.2, BS 21, ISO 7, ISO 228, ANSI/ASME B1.20.1 (distributor, read 2026-08-27)
Sources disagree: NPTF working pressure at 1/4 and 3/8 nominal pipe size.
- 5,000 psi at 1/4 and 4,000 psi at 3/8 (tompkins)
- 4,000 psi at 1/4 and 3,000 psi at 3/8 (brennan-pressure)
Both tables state the same basis - steel, 4 to 1 against minimum burst - and agree exactly from 1/2 upward, so the gap is a one-size offset in one of them rather than a difference of design intent. No working pressure is published at 1/4 or 3/8. Take it from the catalogue of the fitting you are installing.
Sources disagree: BSPT male thread diameter, every size.
- 12.8 mm at R 1/4, at the first full thread from the end (gerrard-id)
- 13.1 mm at R 1/4, near the gauge plane (adaptall-british)
- 13.6 mm at R 1/4, at the largest point (gates-idchart)
All three are correct measurements of the same fitting. A 1 in 16 taper loses 1.59 mm of diameter over 25.4 mm of length, so where the caliper sits decides the reading. The size page publishes the whole span, and the basic column carries the gauge-plane diameter from ISO 7-1 Table 1, which is the figure a thread gauge checks.