One of the most common sources of confusion in the piping industry is thread standards. Using the wrong thread type can result in leaks, thread damage, or complete system failure. This article explains the three most common pipe thread standards and how to tell them apart.
What Are Pipe Threads?
Pipe threads are helical ridges cut into the ends of pipes and fittings that allow them to be screwed together to form a sealed joint. There are two basic categories:
- Tapered threads: The diameter decreases along the length of the thread. The seal is achieved by the wedging action of the tapered threads.
- Parallel threads: The diameter remains constant. Sealing requires a separate gasket or O-ring.
For malleable iron pipe fittings, tapered threads are the standard.
NPT (National Pipe Taper)
NPT is the US standard for tapered pipe threads, governed by ASME B1.20.1. It is widely used in North America.
Key characteristics:
- Taper: 1 in 16 (3/4 inch per foot)
- Thread angle: 60°
- Flattened crests and roots (truncated)
- Seal: Thread-to-thread interference (requires thread sealant)
- Sizes: 1/8" to 6" and larger
⚠️ Important: NPT threads are NOT interchangeable with BSPT threads, despite looking similar. The thread angle and profile are different.
BSPT (British Standard Pipe Taper)
BSPT is the British standard for tapered pipe threads, governed by BS EN 10226 (formerly BS 21). It is used in Europe, Asia, Australia, and most of the world outside North America.
Key characteristics:
- Taper: 1 in 16 (same as NPT)
- Thread angle: 55° (different from NPT's 60°)
- Rounded crests and roots
- Seal: Thread-to-thread interference with sealant
- Sizes: 1/8" to 6" (denoted as Rc, R2)
BSPP (British Standard Pipe Parallel)
BSPP is the parallel (straight) version of BSP. It has the same 55° thread angle and profile as BSPT but no taper. Sealing is achieved by a bonded washer or O-ring at the face of the fitting, not by the threads themselves.
BSPP is commonly used in hydraulic systems and instrumentation applications.
Comparison: NPT vs BSPT
| Feature | NPT | BSPT |
|---|---|---|
| Thread angle | 60° | 55° |
| Crest & root | Flattened (truncated) | Rounded |
| Taper | 1 in 16 | 1 in 16 |
| Standard | ASME B1.20.1 | BS EN 10226 |
| Pitch (1/2") | 14 TPI | 14 TPI |
| Pitch (3/4") | 14 TPI | 14 TPI |
| Pitch (1") | 11.5 TPI | 11 TPI |
| Primary region | North America | Europe, Asia, Rest of world |
| Interchangeable? | NOT interchangeable | |
💡 Pro Tip: While NPT and BSPT have the same taper (1:16) and similar pitch for smaller sizes, the 5° difference in thread angle means they will not form a proper seal. Forcing them together can damage both the fitting and the pipe.
How to Identify NPT vs BSPT
If you are unsure which thread standard you have, here are three quick checks:
- Check the thread angle: Using a thread gauge, measure the angle. 60° = NPT, 55° = BSPT.
- Look at the profile: NPT has flat crests and roots; BSPT has rounded crests and roots.
- Size comparison (1" and above): On a 1" fitting, NPT measures approximately 1.315" OD, while BSPT measures approximately 1.309" OD — a subtle but measurable difference.
Which Thread Should You Use?
Your choice of thread standard is largely determined by your geographic region and the existing piping system:
- NPT: USA, Canada, Mexico, some Middle Eastern projects following US standards
- BSPT: UK, Europe, Australia, New Zealand, China, Southeast Asia, Africa, South America
- BSPP: Hydraulic systems, instrumentation, and some European industrial applications
At JIELI, we offer both NPT and BSP/BSPT threaded fittings to match your project requirements. When ordering, make sure to specify the thread standard clearly.
JIELI Advantage: All our fittings are available in both NPT and BSPT threads. We clearly mark thread types on every quotation and package to avoid confusion. Contact us for thread gauges and technical support if you need assistance identifying your thread standard.
Have questions about thread compatibility? Get in touch with our technical team — we are happy to help.
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