NPS - American National Standard Straight Pipe Threads size chart

NPS, also known as National Pipe Straight, is a standard used for straight pipe threads that are widely used in the United States.

These threads are known as straight threads as they are not tapered along the length. Unlike tapered threads, they have a constant diameter along the pipe length.

This type of threads are usually used for the type of applications where there is no requirement of a sealed fit. For instance, like mechanical and structural connections as well as couplings and flanges. NPS or National Pipe Straight threads have a nominal size of 1/8 inch to 1 1 2 pipe thread dimensions. The threads that can be found on National Pipe Straight threads are designed in a way that are intended to create a tight seal when the pipes are joined together. However, these straight threads are not used in places where there is a use of high pressure applications.

NPS threads are usually used together along with NPT thread chart in the process of designing and assembling piping systems or other mechanical equipment. NPS is used in applications that are frequently disassembled and the integrity of the connection should not be compromised either. This is because the NPS thread simplifies the process of installation and removal due to its structure. They also come in various sizes and can be chosen based on their applications. NPS threads are used in a wide range of industries like plumbing, construction as well as manufacturing. The thread form of NPS threads are determined through ANSI/ASME B1.20.1.

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    1. What are the Characteristics of NPS threads

    NPT and NPS threads both have approximately the same features among each other. They have a similar thread angle, shape, and pitch. But the main difference among them is that the NPT threads are tapered while NPS threads are straight. npt chart


    The usage of NPS threads are also different as to NPT threads. As angled threads are important to be tightly joint preventing leaks, some kind of thread sealant or a sealant tape is used. However when using NPS or straight pipe threads, a gasket or an O-ring is used to tighten or create a tight seal. These threads are used when joining pipes together but however, they are not used in instances where high pressure application is required.


    Usually NPT or tapered pipe threads are used for sealing of pipes and fittings as they do not require any physical sealing or tightening tools to be secured, only some sealants or plumbing tape along with some hand strength would be enough to ensure a proper and tight seal. However, in sanitary fittings, NPS or straight pipe threads are used as tighteners. This is due to two reasons. The reasons being, using of physical tightening tools such as wrenches or pliers can cause damage or slight scratches to the fittings. Also, it would be easier for assembly and disassembly of the fittings and make it a quicker process.

    2. What are the Applications of NPS - National Standard Straight Pipe Threads

    Similar to NPT threads, NPS threads are also used in a wide variety of applications. However, it is NPT threads that are most commonly used as sealants and fittings in pipes and fluid carrying systems. This is because tapered threads can create an airtight seal and also prevent leaks. However it is also based on the specific requirement and the application as to whether the NPT or NPS threads should be used. This is determined according to the amount of sealant and leak prevention needed for the particular application.

    Given below are some common applications where NPS threads are used.

    1. Plumbing lines
    2. Gas lines
    • Water and sewage systems
    1. Hydraulic systems
    2. Mechanical connections
    3. Structural connections
    • Manufacturing processes
    • Industrial machinery and equipment
    1. Automotive equipment
    2. Transportation equipment

    Although NPS threads can be seen to be used in a variety of applications, they are not often used in places where high pressure is applied.

    This is because of their lack of taper which can result in a less tight seal and thereby not fulfill the requirement properly.

    3. What is the Size Chart of NPS - American National Standard Straight Pipe Threads

    NPS threads contain a specific dimension that can be seen commonly throughout all of them. The dimensions are as follows:

    In every National Pipe Straight thread, 1 inch of thread is equal to 25.4mm.

    NPS threads have a 60 degrees of included angle and contain flat peaks and valleys. This is a similarity that can be seen in NPT threads as well. Moreover, both NPT and NPS threads meet the same thread ingles, similar shape and an equal number of threads per inch (TPI). Also, NPS threads meet the B1.20.2 Standard on Pipe Threads that concern general purpose pipe threads.

    As NPS threads are also called as non-self sealing threads and come in various sizes, their applications may require careful choosing of the required specification. The chart given below will be able to take you through a guided path to help you choose the best NPS thread that can suit your specific application.the dimensions of the threads can be aligned with the dimensions of the application, thereby making sure that the requirements are duly met.

    Size & Pitch External Thread OD Internal Thread ID
    inch TPI Max Min Max Min
    1/8" 27 0.397 0.390 0.358 0.364
    1/4" 18 0.526 0.517 0.468 0.481
    3/8" 18 0.662 0.653 0.603 0.612
    1/2" 14 0.823 0.813 0.747 0.759
    3/4" 14 1.034 1.024 0.958 0.970
    1" 11.5 1.293 1.281 1.201 1.211
    1-1/4" 11.5 1.638 1.626 1.546 1.555
    1-1/2" 11.5 1.877 1.865 1.785 1.794
    2" 11.5 2.351 2.339 2.259 2.268
    2-1/2" 8 2.841 2.826 2.708 2.727
    3" 8 3.467 3.452 3.334 3.353
    3-1/2" 8 3.968 3.953 3.835 3.848
    4" 8 4.466 4.451 4.333 4.346

    4. Conclusion

    NPS or National Pipe Straight Standard Threads are the most common standard next to that of NPT or National Pipe Taper Thread Standard which is used in the United States and Canada. Straight threads are used in connecting fittings and plumbing lines where there is no necessity of using tapered threads. Although often NPT threads and NPS threads are used together, with time their tightening wears off due to the tape that NPT threads contain.

    Therefore, their application differs based on the circumstance they are used. Due to their taper, NPT threads are more often used for tight seals. However, Straight pipe threads also have their own applications as stated above in the article.

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