NPT vs BSPT Threads: American vs British

NPT (National Pipe Taper) and BSPT (British Standard Pipe Taper) are the two dominant tapered pipe thread standards used globally in oil and gas piping. Both create a seal through thread interference as the tapered male thread wedges into the tapered female thread, but they differ in thread angle, pitch, and sealing characteristics. NPT and BSPT threads are not interchangeable despite similar nominal sizes.

NPT vs BSPT Comparison Table

Feature	NPT (ASME B1.20.1)	BSPT (BS 21 / ISO 7-1)
Thread taper	1:16 (1 deg 47 min)	1:16 (1 deg 47 min)
Thread angle	60 deg	55 deg
Thread form	Sharp “V” with truncated crests	Rounded crests and roots
Pitch designation	TPI (threads per inch)	TPI (threads per inch)
Sealing method	Thread deformation + sealant	Thread interference + sealant
Common regions	North America, Middle East	Europe, Asia, Australia
Standard	ASME B1.20.1	BS 21 / EN 10226-1
Parallel counterpart	NPS (ANSI B1.20.3)	BSP (BS 2779 / ISO 228)

The taper rate is identical for both systems (1:16), but the thread form is fundamentally different. NPT uses a 60-degree thread angle with flattened crests and roots, while BSPT uses a 55-degree Whitworth form with rounded crests and roots.

Thread Pitch by Size

Nominal Size	NPT TPI	BSPT TPI
1/8”	27	28
1/4”	18	19
3/8”	18	19
1/2”	14	14
3/4”	14	14
1”	11.5	11
1-1/4”	11.5	11
1-1/2”	11.5	11
2”	11.5	11
2-1/2”	8	11
3”	8	11
4”	8	11

The TPI values differ at most sizes, confirming that NPT and BSPT are not compatible even when they share the same nominal pipe size.

Why They Cannot Be Mixed

Attempting to connect an NPT male fitting into a BSPT female fitting (or vice versa) causes several problems:

1. Thread angle mismatch: the 60-degree NPT thread does not seat properly against the 55-degree BSPT form, leaving spiral leak paths
2. Pitch mismatch: at most sizes, TPI values differ, so the threads bind after only a few turns
3. No reliable seal: even with thread sealant (PTFE tape or pipe dope), the joint cannot achieve the metal-to-metal interference needed for pressure integrity

Common Mistake

At 1/2” and 3/4”, both NPT and BSPT share the same TPI (14). Field crews sometimes force these threads together and assume the connection is valid. It is not; the 5-degree difference in thread angle creates a helical leak path that may pass an air test but will fail under hydrotest or operating pressure.

When to Use Each

Application	Recommended Thread
North American piping per ASME B31.3	NPT
Threaded flanges per ASME B16.5	NPT
European piping per EN 13480	BSPT
Instrumentation connections (global)	NPT (most common)
Field instruments	NPT or BSPT (per manufacturer)
Hydraulic systems (European)	BSP (parallel, not tapered)

Thread Sealant Requirements

Both NPT and BSPT require thread sealant to achieve a reliable seal. The sealant fills the voids between imperfect thread surfaces:

• PTFE tape: wrap 3-5 turns in the direction of thread tightening (clockwise viewed from the male end)
• Anaerobic sealant: suitable for high-vibration or high-temperature applications
• Pipe dope (thread compound): used with or instead of PTFE tape, provides lubrication during assembly

For connections involving stud bolts and flanges, tapered pipe threads are irrelevant; flanged joints seal via gasket compression. Pipe threads apply only to threaded (screwed) connections such as small-bore instrument taps, drain valves, and socket-weld to threaded transitions.

Read the full guide to stud bolts for flanges