NPT vs BSP Threads for Filter Housings: How to Tell and Which to Order
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Short answer. NPT and BSP are two different pipe thread systems. NPT is the usual standard in the United States and Canada. BSP is the usual standard in the UK, Europe, Australia, New Zealand and much of Asia. They are not interchangeable: even where the thread count matches, the thread angle is different, so a mixed joint will leak or damage the threads. Order the housing port that matches the fittings already on your pipe. Every threaded housing in our stainless steel filter housings range is listed in both NPT and BSP/G as separate options.
NPT vs BSP: the main differences
| NPT | BSP | |
|---|---|---|
| Full name | National Pipe Taper (ASME B1.20.1) | British Standard Pipe, Whitworth thread form |
| Thread angle | 60° | 55° |
| Crests and roots | Flattened | Rounded |
| Taper | Always tapered (1 in 16 on diameter) | Two kinds: parallel (BSPP, marked G) or tapered (BSPT, marked R / Rc) |
| How it seals | On the threads, with PTFE tape or thread sealant | BSPP: with a gasket, O-ring or bonded washer. BSPT: on the threads, with sealant |
| Where it is common | USA, Canada | UK, EU, Australia, New Zealand, much of Asia |
Threads per inch by size
The nominal size is the same name in both systems, but the thread pitch is not always the same:
| Nominal size | NPT threads per inch | BSP threads per inch |
|---|---|---|
| 1/4 in | 18 | 19 |
| 3/8 in | 18 | 19 |
| 1/2 in | 14 | 14 |
| 3/4 in | 14 | 14 |
| 1 in | 11.5 | 11 |
| 1-1/2 in | 11.5 | 11 |
| 2 in | 11.5 | 11 |
| 2-1/2 in | 8 | 11 |
The 1/2 in and 3/4 in sizes are where most mistakes happen. Both systems use 14 threads per inch there, so an NPT fitting may start into a BSP port and turn a few times. The 60° and 55° thread flanks still do not match, and the joint is likely to weep or leak under pressure. At 1 in and above the pitch is different as well, and the threads cross after a turn or two.
"1 inch" is not 1 inch across
Pipe thread sizes are names, not measurements. A 1 in pipe thread measures about 33 mm across the outside of the male thread. Use this table to identify a fitting with a caliper:
| Nominal size | Approx. outside diameter of male thread |
|---|---|
| 1/2 in | about 21 mm |
| 3/4 in | about 26.5 mm |
| 1 in | about 33 mm |
| 1-1/2 in | about 48 mm |
| 2 in | about 60 mm |
| 2-1/2 in | about 73–76 mm (NPT and BSP differ more at this size) |
The outside diameters of NPT and BSP at the same nominal size are close, usually within half a millimetre, so diameter alone tells you the size, not the system.
How to tell which thread you have
- Look for markings. NPT fittings are often marked NPT. BSP parallel threads are marked G (for example G1). BSP taper threads are marked R (male) or Rc (female). Rp is a parallel female thread used with R male threads.
- Measure the outside diameter of the male thread and compare it with the table above to get the nominal size.
- Count threads per inch with a thread gauge, or count the crests over 1 in (25.4 mm). 11.5 means NPT; 11 or 19 means BSP. At 1/2 in and 3/4 in the count is 14 in both systems, so use a gauge with 55° and 60° leaves or check the markings.
- Check the profile. Sharp, flattened crests suggest NPT; rounded crests suggest BSP.
- Think about where the equipment came from. Pumps, valves and fittings bought in the UK, EU, Australia or New Zealand are usually BSP. Equipment bought in the USA is usually NPT. This is a hint, not proof.
Which port should you order on a filter housing?
- Match the fittings already on your pipe. If the pump outlet, valve or hose tail that the housing connects to is BSP, order BSP/G. If it is NPT, order NPT.
- Do not rely on tape to fix a mismatch. PTFE tape can mask a small leak for a while, but a mixed NPT and BSP joint is not a reliable seal, especially on a housing that is opened for cartridge changes and sees pressure changes.
- If you must join the two systems, use a proper NPT-to-BSP adapter rather than forcing the threads.
- Parallel or taper BSP. Our BSP/G ports are G (BSPP) parallel threads, the ISO 228-1 form. They are not NPT, and they are not PT, R or Rc taper threads. A G port pairs with a G fitting that seals on a gasket, O-ring or bonded washer. It will not seal directly against an R, Rc or PT taper fitting, which is common on equipment from Japan and parts of Asia; use a fitting with a sealing washer or an adapter, or email us the fitting details before ordering.
- Sanitary lines are different. Food, beverage and brewing pipework often uses tri-clamp ferrules instead of threads. See sanitary tri-clamp filters.
Port options on our housings
| Housing | Threaded port options |
|---|---|
| 2.5 in cartridge housings (threaded lid, clamp lid, 76 mm slim body) | 1/2 in, 3/4 in, 1 in, each in NPT or BSP/G |
| 4.5 in jumbo housing | 1 in, 1.5 in, 2 in, each in NPT or BSP/G; or 50.5 / 64 / 77.5 mm tri-clamp ferrules |
| Large-bore housing | 2.5 in NPT or BSP/G |
| Y-strainer | 1 in NPT or BSP/G |
| #3 / #4 bag filter housing | 1 in NPT or BSP/G |
| #1 / #2 bag filter housing | Usually 1 in, 1.5 in or 2 in, NPT or BSP/G, confirmed by email after the order |
NPT and BSP/G are separate options on each product page, so choose the thread before you add to cart. The thread does not change which cartridge fits; for that, see the filter cartridge size guide. For the housing material, see 304 vs 316 stainless steel filter housings.
FAQ
Can I screw an NPT fitting into a BSP port?
Not reliably. At 1/2 in and 3/4 in it may start, because the pitch is the same, but the thread angle is different and the joint is likely to leak. Use an adapter.
I am in the UK, EU or Australia. Which should I choose?
Usually BSP/G, but check the fittings you are connecting to. Some imported equipment uses NPT.
I am in the USA. Which should I choose?
Usually NPT.
Is "BSP/G" the same as BSPP?
Yes. G is the standard marking for British parallel pipe thread (BSPP). Our BSP/G ports are parallel G threads, not taper.
I ordered the wrong thread. What now?
Email us before fitting the housing so we can look at the options with you. Do not force the threads.
Not sure which thread you have? Email a photo of the fitting with a ruler next to it to alicia@ssfilterhousing.com, or request a quote for a custom port.