The National Pipe Straight (NPS) thread is part of the American National Pipe Thread (NPT) family, but unlike NPT (which is tapered), NPS threads are parallel (straight). This means NPS threads do not provide a pressure-tight seal on their own and require sealing washers, O-rings, or gaskets to ensure a leak-proof connection.
NPS is commonly used in low-pressure fluid and gas applications, fire protection systems, and flanges, couplings, and pipe fittings where a non-leak-tight but secure mechanical connection is needed.
Key Features of NPS Threads:
✔ Thread Angle: 60°, like NPT, but without the taper.
✔ Straight (Parallel) Threads: Do not create a mechanical seal like tapered threads.
✔ Requires sealing elements (gaskets, O-rings, or flanges for a leak-proof connection).
✔ Used in mechanical and structural applications, not for sealing under pressure.
✔ Standardization: Defined in ANSI B1.20.1 and ASME B1.20.1.
NPT vs. NPS – Key Differences
| Feature | NPT (Tapered) | NPS (Straight) |
|---|---|---|
| Thread Shape | Tapered | Parallel (Straight) |
| Sealing Method | Self-sealing through thread interference | Requires sealing washer/gasket |
| Common Use | Gas lines, high-pressure applications | Flanges, couplings, fire protection systems |
| Standard | ANSI B1.20.1, ASME B1.20.1 | ANSI B1.20.1, ASME B1.20.1 |
| Thread Angle | 60° | 60° |
Common Applications of NPS Threads:
- Flanges & Couplings (Commonly used in non-pressure mechanical connections.)
- Structural Pipe Systems (Fire protection, water distribution, and industrial piping.)
- Low-Pressure Fluid Transport (Water pipes and fluid drainage systems.)
- Mechanical Fastening (Used in light fixtures, brackets, and non-sealed connections.)
- Plumbing & Gas Lines (Only when used with a sealing element.)
NPS Standard Thread Sizes & Dimensions
The following tables provide both inch and metric equivalents for major, pitch, and minor diameters, as well as recommended tap drill sizes for NPS threads.
NPS (Straight) Thread Dimensions (Inches)
| Nominal Size | Threads Per Inch (TPI) | Major Diameter (in) | Pitch Diameter (in) | Minor Diameter (in) | Tap Drill Size (in) |
|---|---|---|---|---|---|
| 1/16″ | 27 | 0.3125 | 0.2773 | 0.2560 | 0.2500 |
| 1/8″ | 27 | 0.4050 | 0.3663 | 0.3400 | 0.3320 |
| 1/4″ | 18 | 0.5400 | 0.4875 | 0.4500 | 0.4380 |
| 3/8″ | 18 | 0.6750 | 0.6176 | 0.5800 | 0.5780 |
| 1/2″ | 14 | 0.8400 | 0.7727 | 0.7250 | 0.7190 |
| 3/4″ | 14 | 1.0500 | 0.9786 | 0.9220 | 0.9060 |
| 1″ | 11.5 | 1.3150 | 1.2336 | 1.1500 | 1.1410 |
NPS (Straight) Thread Dimensions (Metric – mm)
| Nominal Size | Major Diameter (mm) | Pitch Diameter (mm) | Minor Diameter (mm) | Tap Drill Size (mm) |
|---|---|---|---|---|
| 1/16″ | 7.94 | 7.04 | 6.50 | 6.35 |
| 1/8″ | 10.29 | 9.31 | 8.64 | 8.43 |
| 1/4″ | 13.72 | 12.39 | 11.43 | 11.13 |
| 3/8″ | 17.15 | 15.68 | 14.73 | 14.68 |
| 1/2″ | 21.34 | 19.63 | 18.42 | 18.26 |
| 3/4″ | 26.67 | 24.86 | 23.42 | 23.01 |
| 1″ | 33.40 | 31.34 | 29.21 | 28.99 |
Related Pipe Thread Standards
- NPT – National Pipe Tapered (Common in pressure-tight sealing applications.)
- BSP – British Standard Pipe (Used in Europe and many global industries.)
- ISO G Thread (Parallel pipe thread standardized under ISO 228-1.)
- ISO R Thread (Tapered pipe thread standardized under ISO 7-1.)
Important Notice on Standards Validity
This information is for reference purposes only. Users should consult official sources and updated standards to ensure compliance with current regulations. The validity of standards may change over time, so it is recommended to verify them through recognized engineering and standardization organizations (ANSI, ASME, ISO, etc.).
Check with relevant standardization bodies in your region to confirm whether a specific standard is still valid and applicable. Regulations and industry requirements may vary based on region and application.