NPS - American National Standard Straight Pipe Threads size chart
NPS (National Pipe Straight), governed by the ANSI/ASME B1.20.1 standard, is the American technical benchmark for straight (parallel) pipe threads. Unlike tapered NPT threads, NPS threads maintain a constant major diameter along the entire length of the pipe, allowing for a free-fitting mechanical assembly.
Key Engineering Characteristics:
- • Mechanical Precision: Primarily utilized for mechanical and structural connections, including couplings, flanges, and locknut joints, where axial alignment is critical.
- • Sealing Logic: While the thread form itself does not create a pressure-tight seal (unlike NPT), NPS joints achieve leak-proof integrity when combined with a secondary sealing element, such as a gasket or O-ring.
- • Serviceability: The parallel structure significantly simplifies frequent installation and removal, preventing thread galling and maintaining connection integrity over multiple maintenance cycles.
Available in nominal sizes ranging from 1/8" to 4" (and beyond), NPS threads are indispensable in high-precision plumbing, HVAC systems, and CNC-machined structural components. Understanding the nuances between NPS vs. NPT is essential for ensuring system compatibility and operational safety in industrial piping design.
Technical Tip: While NPS threads are ideal for mechanical joints, if your project involves high-pressure fluid systems requiring a tapered interference seal, please refer to our comprehensive NPT Thread Size Chart & Torque Specifications to ensure system safety and ASME compliance.
1.NPS vs. NPT: Functional Comparison
While NPS (Straight) and NPT (Tapered) threads share a similar 60° thread angle and pitch, their sealing mechanisms and application logic differ significantly:
NPT (Tapered)
Designed for pressure-tight sealing through thread-to-thread interference. The 1:16 taper allows the threads to "wedge" together, typically requiring only PTFE tape or thread sealant and hand-tightening to prevent leaks in high-pressure systems.
NPS (Straight)
Designed for mechanical alignment. Since the diameter remains constant, the threads do not wedge together. Sealing is achieved via a gasket or O-ring compressed against the shoulder. These are ideal for low-pressure applications requiring frequent assembly.
Industry Insight: Why NPS is Preferred for Sanitary & Precision Fittings
In sanitary and high-finish applications, NPS threads serve as critical tensioners for two primary reasons:
1. Surface Integrity: Unlike NPT, which often requires heavy-duty wrenches to achieve a wedge-seal, NPS can often be hand-tightened against a seal. This prevents scratches, tool marks, and surface damage on polished aesthetic or food-grade fittings.
2. Efficient Maintenance: The parallel structure allows for rapid assembly and disassembly. Because the threads do not deform under pressure, the components can be reused indefinitely without compromising thread integrity or fitment speed.
While NPS and NPT threads often overlap in industrial use, NPT remains the primary choice for pressure-containment systems. The 1:16 taper of NPT allows for a metal-to-metal wedge seal, which is superior for preventing fluid and gas migration. However, the selection between NPS and NPT is strictly dictated by the specific sealing integrity and mechanical load required for the assembly.
🛠️ Common NPS Thread Applications
- • Mechanical & Structural Connections
- • Low-Pressure Plumbing & Gas Lines
- • Water & Sewage Manifold Systems
- • Hydraulic Reservoir Ports
- • Industrial Machinery & Tooling
- • Automotive Fluid Reservoirs
- • Manufacturing Alignment Jigs
- • Transportation Chassis Hardware
Engineering Note: NPS threads are generally not recommended for high-pressure environments. The absence of a taper means the joint relies entirely on the mechanical compression of a gasket or O-ring. Under extreme
2. What are the Applications of NPS pipe - National Standard Straight Pipe Threads
| Nominal Size (NPS) | TPI (Threads/In) | Major Diameter (in) | Pitch Diameter (in) | Tap Drill (in) |
|---|---|---|---|---|
| 1/8 | 27 | 0.4050 | 0.3635 | 21/64 |
| 1/4 | 18 | 0.5400 | 0.4774 | 7/16 |
| 3/8 | 18 | 0.6750 | 0.6120 | 37/64 |
| 1/2 (Common) | 14 | 0.8400 | 0.7584 | 23/32 |
| 3/4 | 14 | 1.0500 | 0.9677 | 59/64 |
| 1 (Common) | 11.5 | 1.3150 | 1.2136 | 1 5/32 |
| 1 1/4 | 11.5 | 1.6600 | 1.5571 | 1 1/2 |
| 1 1/2 | 11.5 | 1.9000 | 1.7960 | 1 47/64 |
| 2 (Common) | 11.5 | 2.3750 | 2.2690 | 2 7/32 |
| 2 1/2 | 8 | 2.8750 | 2.7195 | 2 5/8 |
| 3 | 8 | 3.5000 | 3.3406 | 3 1/4 |
| 3 1/2 | 8 | 4.0000 | 3.8375 | 3 3/4 |
| 4 (Standard) | 8 | 4.5000 | 4.3344 | 4 1/4 |
NPS threads adhere to precise dimensional constants essential for global compatibility. In every National Pipe Straight thread, 1 inch of thread length is equal to exactly 25.4 mm, ensuring seamless conversion between Imperial and Metric systems.
NPS threads feature a 60-degree included angle with truncated flat peaks and valleys (crests and roots). This geometry is identical to NPT threads; both standards share the same thread angle, profile shape, and TPI (Threads Per Inch) for any given size. Furthermore, NPS threads comply with the ASME B1.20.1 standard for General Purpose Pipe Threads.
As NPS threads are categorized as non-self-sealing threads, they are available in numerous sizes, requiring careful selection based on your project's specific mechanical tolerances. The comprehensive NPS Thread Chart provided below serves as a guided technical reference to help you identify the optimal thread specification, ensuring the hardware dimensions align perfectly with your application requirements.
3. What is the Size Chart of NPS pipe - American National Standard Straight Pipe Threads
4. Conclusion
NPS (National Pipe Straight) standards represent the most prevalent alternative to NPT (National Pipe Taper) across the United States and Canada. These straight threads are specifically engineered for connecting fittings and plumbing lines where the wedge-action of a tapered thread is unnecessary.
While NPS and NPT components are occasionally used in tandem, it is important to note that their sealing integrity can diminish over time. In such hybrid setups, the reliance on sealant tape or thread lubricants may lead to loosening under vibration or thermal cycling.
Consequently, their applications are highly situational: NPT threads remain the standard for high-pressure, leak-proof seals due to their tapered geometry, whereas straight pipe threads excel in the specific mechanical and structural roles detailed throughout this guide.
NPS Thread Technical FAQ
Technical Summary: NPS Thread Standards
Definition & Standard
NPS (National Pipe Straight) threads are American standard straight pipe threads designed for mechanical joints according to ASME B1.20.1. Unlike NPT, NPS threads do not have a taper (1:16), making them ideal for applications requiring free-fitting assembly without thread-crushing seals.
Size Range & TPI
This chart covers standard NPS sizes from 1/8" to 4". Common thread pitches include 27, 18, 14, 11.5, and 8 TPI. The Major Diameter ranges from 0.4050" (for 1/8 NPS) up to 4.5000" (for 4 NPS), ensuring compatibility with standard industrial piping components.
Application & Sealing
Commonly used in NPSM (Mechanical) and NPSL (Locknut) joints. For pressure-tight systems, NPS threads require a secondary sealing element such as a compressed gasket or O-ring. They are frequently found in lighting fixtures, electrical conduits, and decorative hardware.
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