NPT Threads
To speak about industrial and residential piping in North America is to speak of National Pipe Threads NPT threads in all their glory. These threads are specially designed for sealing against leaks in environments where they are under pressure. The threads conform to the ANSI/ASME B1.20.1 standard and are important in plumbing, hydraulics, and gas. This guide will walk you through the details of the NPT thread chart from tapered pipe thread dimensions to the productive application of the NPT size chart.
What Are NPT Threads?
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NPT threads are those that use a 60-degree thread angle and taper 1 inch per 16 inches (or 3/4 inch per foot). This means when these threads are engaged, wedging tightly, they make seals that resist leakage under pressures of hundreds to thousands of pounds per square inch. Unlike straight threads (NPS, for example), taper relies on sealants like Teflon tape to fill microscopic gaps in case NPT taper alone cannot form a sufficient seal.
Decoding the NPT Size Chart
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The NPT size chart organizes threads by Nominal Pipe Size (NPS), a historical term reflecting approximate inner diameters. Key metrics include:
- Nominal Pipe Size (NPS): Labeled in fractions (e.g., 1/8″, 1/2″).
- Threads Per Inch (TPI): Higher TPI = finer threads.
- Outside Diameter (OD): Measured at the thread’s largest end.
- Pitch Diameter: Determines thread engagement.
Example NPT Sizes
Small Sizes (1/16″ to 1/2″)
- 1/8″ NPT: 27 TPI, OD ≈ 0.405″ (common in gas lines).
- 1/2″ NPT: 14 TPI, OD ≈ 0.840″ (residential plumbing).
Medium to Large Sizes (3/4″ to 3″)
- 2″ NPT: 11.5 TPI, OD ≈ 2.375″ (industrial pipelines).
National Pipe Threads
Chart
| Outside Diameter D | Nominal Pipe Size | Threads per inch (Pitch) | Thread Pitch Length, P | Pitch Diameter at External Thread Start E0 | Hand Tight Thread Engagement Length, L1 | Diameter E1 | Maximum Thread Engagement L2 | Diameter E2 |
| 0.3125 | 1/16 | 27 | 0.03704 | 0.27118 | 0.160 | 0.28118 | 0.2611 | 0.28750 |
| 0.405 | 1/8 | 27 | 0.03704 | 0.36351 | 0.1615 | 0.37360 | 0.2639 | 0.38000 |
| 0.540 | 1/4 | 18 | 0.05556 | 0.47739 | 0.2278 | 0.49163 | 0.4018 | 0.50250 |
| 0.675 | 3/8 | 18 | 0.05556 | 0.61201 | 0.240 | 0.62701 | 0.4078 | 0.63750 |
| 0.840 | 1/2 | 14 | 0.07143 | 0.75843 | 0.320 | 0.77843 | 0.5337 | 0.79179 |
| 1.050 | 3/4 | 14 | 0.07143 | 0.96768 | 0.339 | 0.98887 | 0.5457 | 1.00179 |
| 1.315 | 1 | 11 1/2 | 0.8696 | 1.21363 | 0.400 | 1.23863 | 0.6828 | 1.25630 |
| 1.660 | 1 1/4 | 11 1/2 | 0.8696 | 1.55713 | 0.420 | 1.58338 | 0.7068 | 1.60130 |
| 1.900 | 1 1/2 | 11 1/2 | 0.8696 | 1.79609 | 0.420 | 1.82234 | 0.7235 | 1.84130 |
| 2.375 | 2 | 11 1/2 | 0.8696 | 2.26902 | 0.436 | 2.29627 | 0.7565 | 2.31630 |
| 2.875 | 2 1/2 | 8 | 0.12500 | 2.71953 | 0.682 | 2.76216 | 1.1375 | 2.79062 |
| 3.500 | 3 | 8 | 0.12500 | 3.4062 | 0.766 | 3.38850 | 1.2000 | 3.41562 |
| 4.000 | 3 1/2 | 8 | 0.12500 | 3.83750 | 0.821 | 3.88881 | 1.2500 | 3.91562 |
| 4.500 | 4 | 8 | 0.12500 | 4.33438 | 0.844 | 4.38712 | 1.300 | 4.41562 |
| 5.563 | 5 | 8 | 0.12500 | 5.39073 | 0.937 | 5.44929 | 1.4063 | 5.47862 |
| 6.625 | 6 | 8 | 0.12500 | 6.44609 | 0.958 | 6.50597 | 1.5125 | 6.54062 |
| 8.625 | 8 | 8 | 0.12500 | 8.43359 | 1.063 | 8.50003 | 1.7125 | 8.54062 |
| 10.750 | 10 | 8 | 0.12500 | 10.54531 | 1.210 | 10.62094 | 1.9250 | 10.66562 |
| 12.750 | 12 | 8 | 0.12500 | 12.53281 | 1.360 | 12.61781 | 2.1250 | 12.66562 |
| 14.000 | 14 | 8 | 0.12500 | 13.77500 | 1.562 | 13.87262 | 2.2500 | 13.91562 |
| 16.000 | 16 | 8 | 0.12500 | 15.76250 | 1.812 | 15.87575 | 2.4500 | 15.91562 |
| 18.000 | 18 | 8 | 0.12500 | 17.75000 | 2.000 | 17.87500 | 2.6500 | 17.91562 |
| 20.000 | 20 | 8 | 0.12500 | 19.73750 | 2.125 | 19.87031 | 2.8500 | 19.91562 |
| 24.000 | 24 | 8 | 0.12500 | 23.71250 | 2.375 | 23.86094 | 3.2500 | 23.91562 |
How to Use the NPT Thread Chart
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- Match NPS: Align the nominal size with the pipe or fitting.
- Verify TPI: Ensure compatibility between components.
- Measure OD: Use calipers to confirm dimensions.
- Gauge Engagement: Use L1/L2 gauges for proper thread depth.
Applications of NPT Threads
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NPT threads are ideal for:
- High-pressure hydraulic systems.
- Residential plumbing installations.
- Gas and oil pipeline connections.
NPT vs. Other Thread Standards
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- BSP (British Standard Pipe): Uses parallel or tapered threads (BSPP/BSPT).
- NPTF (Dryseal): Modifies thread crests for sealant-free use.
- NPS (Straight): Requires gaskets for sealing.
Installation Tips for NPT Fittings
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- Clean Threads: Remove debris to avoid cross-threading.
- Apply Sealant: Use Teflon tape or pipe dope.
- Tighten Gradually: Hand-tighten first, then add 2–3 wrench turns.
Common Mistakes to Avoid
- Mixing NPT and BSP threads.
- Skipping sealant or applying it incorrectly.
- Over-tightening, which cracks fittings.
Conclusion
Mastering the NPT size chart and tapered pipe thread specifications ensures reliable, leak-free connections in fluid and gas systems. Whether you’re working with hydraulic fittings or household plumbing, this guide equips you with the knowledge to navigate NPT thread standards confidently.
