BSP, NPT and Metric Threads — Which Fits What?
BSP and NPT look alike but do not fit — one is 55° and the other 60°. This page covers the differences between the three thread standards, how to tell them apart and where each is used.
4 min read · 9-row dimension table · 4 frequently asked questions
Three separate thread worlds
Three thread families turn up in hydraulic connections, and none of the three fits the others:
- BSP (British standard) — 55° thread angle, Whitworth form. The most common family in Europe and Türkiye.
- NPT (American standard) — 60° thread angle, tapered. Found on American-built equipment.
- Metric (DIN/ISO) — 60° thread angle, parallel. Expressed as M16×1.5 and so on.
BSPP or BSPT — parallel and tapered
BSP itself divides in two, and that division determines how the seal is made:
BSPP (marked G) — parallel thread. The diameter is constant along the thread. The thread does not make the seal; that is done either by a bonded seal or by a 60° internal cone face. It suits joints that are broken and remade, giving a repeatable seal.
BSPT (marked R) — tapered thread. The diameter reduces along the thread. As it is tightened the threads wedge together and the thread itself makes the seal. It is normally used with thread tape or liquid sealant, and the thread wears slightly with every remake.
Why BSP and NPT get confused
Their diameters are very close. Measure G 1/2" BSP and 1/2" NPT with a caliper and they come out almost identical. The difference is the thread angle: 55° against 60°. Forced together, the first few threads will engage and it may even tighten — but:
- the joint never becomes fully leak-tight,
- it can blow out under pressure,
- and the thread on both parts is permanently damaged.
The easiest way to separate them is threads per inch. G 1/4" BSP has 19 threads while 1/4" NPT has 18, so in the smaller sizes you can tell them apart by counting. At 1/2" and 3/4" the thread counts are identical and the only difference is the angle — which is exactly where the risk of confusion is greatest.
Size table
Note: the inch figure in a BSP size refers to the nominal bore of the pipe, not the diameter of the thread. That is why the outside diameter of a G 1/2" thread is roughly 21 mm rather than 1/2" (12.7 mm).
Practical summary
- If the part is marked G it is parallel BSP; it seals with a bonded seal or a cone.
- If it is marked R it is tapered BSP; fit it with thread tape.
- If it says NPT it is an American tapered thread and will not fit a BSP connection.
- If it starts with M it is a metric thread (M22×1.5 and so on).
- If you are not sure, do not force it — a thread forced once cannot be repaired.
BSP – NPT thread size comparison
| Nominal size | BSP OD | BSP TPI | NPT OD | NPT TPI | Will they fit? |
|---|---|---|---|---|---|
| 1/8" | 9.7 mm | 28 | 10.3 mm | 27 | No |
| 1/4" | 13.2 mm | 19 | 13.7 mm | 18 | No |
| 3/8" | 16.7 mm | 19 | 17.1 mm | 18 | No |
| 1/2" | 21.0 mm | 14 | 21.3 mm | 14 | No |
| 3/4" | 26.4 mm | 14 | 26.7 mm | 14 | No |
| 1" | 33.2 mm | 11 | 33.4 mm | 11.5 | No |
| 1.1/4" | 41.9 mm | 11 | 42.2 mm | 11.5 | No |
| 1.1/2" | 47.8 mm | 11 | 48.3 mm | 11.5 | No |
| 2" | 59.6 mm | 11 | 60.3 mm | 11.5 | No |
The table scrolls horizontally. Values are for guidance; we recommend confirming the size before ordering.
Frequently Asked Questions
Do BSP and NPT threads fit together?
No. BSP threads are cut at 55° and NPT threads at 60°. Because the diameters are so close the first few threads may engage and it may even tighten, but the joint will not seal, it can blow out under pressure, and the thread on both parts is permanently damaged.
What is the difference between BSPP and BSPT?
BSPP (G) is a parallel thread with constant diameter, and the seal is made by a bonded seal or a 60° internal cone. BSPT (R) is tapered, the diameter reduces, and the thread itself makes the seal — normally with thread tape.
Why is a G 1/2 thread not 12.7 mm across?
The inch figure in a BSP size refers to the nominal bore of the pipe, not the diameter of the thread. That is why the actual outside diameter of a G 1/2" thread is around 21 mm. It is worth knowing this before reaching for the caliper.
How can I tell whether my thread is BSP or NPT?
Count the threads per inch and measure the diameter. In smaller sizes the difference is clear: G 1/4" BSP has 19 threads while 1/4" NPT has 18. At 1/2" and 3/4" the counts are identical, so the only distinction is the thread angle (55° or 60°) — if you are not certain, do not force it.