NPT / NPTF (ASME B1.20.1)
Tapered, 60 degree flanks, sealed by the flanks wedging together with sealant or tape.
60 degree flanksTaperedTapered thread, sealed on the thread
Confusion pair
NPT vs BSPP
One is tapered and seals on the thread; the other is parallel and physically cannot. Mixing them is the most common cause of a port that will not stop weeping.
BSPP, G (ISO 228-1)
Parallel, 55 degree Whitworth flanks, sealed on a bonded washer, an O-ring or a cone at the face. Never on the thread.
55 degree flanksParallelParallel thread, sealed on a face gasket
The three answers
What actually happens if you put them together
What interchanges
- Nothing. One guide states that the BSPP male is similar to, but not interchangeable with, the American NPSM male, and that the pitches differ at most sizes.
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.
- An NPT male wound into a BSPP female port. The diameters are close, and at 1/2 and 3/4 in trade size the two even share a thread count of 14 per inch with basic diameters 0.39 mm and 0.23 mm apart, so it goes in and feels tight as the taper jams. It will still leak, and it is worse than a leak: the taper is wedging a parallel port open from the inside, and a soft aluminium or cast port bells out and is then scrap. This one shows up on imported machinery constantly.
- A BSPP male in any port, with the bonded seal washer left off. A parallel thread cannot seal on itself no matter how hard it is pulled up, and tightening further only strips the port. The washer is the seal.
What will not fit at all
- A BSPP male will not seal into an NPT female under any circumstance - there is nothing at the face for it to seal against, and the parallel thread will not wedge.
- At 1/8, 1/4, 3/8, 1, 1 1/4, 1 1/2 and 2 the thread counts differ outright and the two will bind within a turn or two.
The check that settles it
Measure across the crests at both ends of the male thread. Same reading at both ends means parallel, so BSPP; smaller at the entering end means tapered, so NPT. Then check the flank angle with a pitch gauge to confirm 60 against 55 degrees. A BSPP male also carries a flat shoulder or a chamfer behind the thread for its seal washer; an NPT male has nothing there.
Size by size
Where the threads match, and where they do not
2 of the 12 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 | BSPP, G | Will they assemble? |
|---|---|---|---|
| 1/16 in | 1/16-27 NPT · 7.94 mm | G 1/16 (28 TPI, 55 degree) · 7.72 mm | No - 27 TPI against 28 TPI |
| 1/8 in | 1/8-27 NPT · 10.29 mm | G 1/8 (28 TPI, 55 degree) · 9.73 mm | No - 27 TPI against 28 TPI |
| 1/4 in | 1/4-18 NPT · 13.72 mm | G 1/4 (19 TPI, 55 degree) · 13.16 mm | No - 18 TPI against 19 TPI |
| 3/8 in | 3/8-18 NPT · 17.15 mm | G 3/8 (19 TPI, 55 degree) · 16.66 mm | No - 18 TPI against 19 TPI |
| 1/2 in | 1/2-14 NPT · 21.34 mm | G 1/2 (14 TPI, 55 degree) · 20.95 mm | Yes, and it is the wrong joint |
| 3/4 in | 3/4-14 NPT · 26.67 mm | G 3/4 (14 TPI, 55 degree) · 26.44 mm | Yes, and it is the wrong joint |
| 1 in | 1-11 1/2 NPT · 33.4 mm | G 1 (11 TPI, 55 degree) · 33.25 mm | No - 11.5 TPI against 11 TPI |
| 1 1/4 in | 1 1/4-11 1/2 NPT · 42.16 mm | G 1 1/4 (11 TPI, 55 degree) · 41.91 mm | No - 11.5 TPI against 11 TPI |
| 1 1/2 in | 1 1/2-11 1/2 NPT · 48.26 mm | G 1 1/2 (11 TPI, 55 degree) · 47.8 mm | No - 11.5 TPI against 11 TPI |
| 2 in | 2-11 1/2 NPT · 60.33 mm | G 2 (11 TPI, 55 degree) · 59.61 mm | No - 11.5 TPI against 11 TPI |
| 5/8 in | No such size | G 5/8 (14 TPI, 55 degree) · 22.91 mm | Only one family has this size |
| 7/8 in | No such size | G 7/8 (14 TPI, 55 degree) · 30.2 mm | Only one family has this size |
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 | BSPP, G |
|---|---|---|
| Flank angle | 60 degrees | 55 degrees |
| Parallel or tapered | Tapered, 1 in 16 | Parallel |
| Seat | Tapered thread, sealed on the thread | Parallel thread, sealed on a face gasket |
| Where it seals | The thread flanks wedging together, normally with a thread sealant or PTFE tape. The seal is the thread. | A bonded seal washer or O-ring at the face. The parallel thread cannot seal on itself. |
| Defined by | ASME B1.20.1, ASME B1.20.3 | ISO 228-1, BS 2779 |
| 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 like NPT, and like NPT the number is not the thread diameter: G 1/4 measures about 13.16 mm across the threads. |
Answers
Common questions
Will a NPT / NPTF screw into a BSPP, G?
An NPT male wound into a BSPP female port. The diameters are close, and at 1/2 and 3/4 in trade size the two even share a thread count of 14 per inch with basic diameters 0.39 mm and 0.23 mm apart, so it goes in and feels tight as the taper jams. It will still leak, and it is worse than a leak: the taper is wedging a parallel port open from the inside, and a soft aluminium or cast port bells out and is then scrap. This one shows up on imported machinery constantly.
Are NPT and BSPP (G) interchangeable?
Nothing. One guide states that the BSPP male is similar to, but not interchangeable with, the American NPSM male, and that the pitches differ at most sizes.
How do I tell them apart?
Measure across the crests at both ends of the male thread. Same reading at both ends means parallel, so BSPP; smaller at the entering end means tapered, so NPT. Then check the flank angle with a pitch gauge to confirm 60 against 55 degrees. A BSPP male also carries a flat shoulder or a chamfer behind the thread for its seal washer; an NPT male has nothing there.
More pairs
Other fittings people mix up
Where these figures come from
Every value on this page was checked against 4 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 228-1, Pipe threads where pressure-tight joints are not made on the threads, Table 1 - ISO, ISO 228-1:2000(E), fourth edition (standard, read directly, 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.