How to measure NH, NST, and NPT threading
Posted by Firepenny on Jul 10th 2024
Measuring NH (National Hose) and NST (National Standard) Threads
NH (National Hose) threads, also referred to as NST (National Standard Thread), are the most common connections found on firefighting equipment such as hoses, nozzles, and hydrants. These threads are known for their durability, standardization, and reliability in high-pressure use.
Here’s how to measure NH threads accurately:
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Thread Pitch – Use a thread pitch gauge (or a ruler for quick checks) to measure the distance between the crests of adjacent threads. NH threads follow a standard pitch based on their nominal size.
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Thread Diameter – Measure the outside diameter (OD) of the male thread and the inside diameter (ID) of the female thread using calipers or a measuring tape. This determines the correct nominal size for compatibility.
Measuring NPT (National Pipe Thread) Threads
NPT (National Pipe Thread) is different from NH/NST. NPT threads are tapered, which allows them to form a tight seal—commonly used in plumbing, piping, and sometimes in fire protection systems for threaded fittings.
Here’s how to measure NPT threads correctly:
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Thread Taper – NPT threads have a 1 in 16 taper on the diameter, meaning they gradually narrow. Precision tools help confirm this taper for proper fit.
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Thread Pitch – Count the number of threads per inch (TPI) along the threaded section.
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Thread Diameter – Measure the OD of the male thread and the ID of the female thread. This identifies the nominal size and ensures fittings seal properly.
NH and NST Thread Reference
| Nominal Size (inches) | OD (inches) | TPI (Threads per Inch) |
|---|---|---|
| 1/2 | 0.75 | 8 |
| 1 | 1.00 | 11.5 |
| 1 1/2 | 1.25 | 9 |
| 2 | 1.50 | 8 |
| 2 1/2 | 1.875 | 7.5 |
| 3 | 2.125 | 7 |
| 3 1/2 | 2.375 | 6 |
| 4 | 2.625 | 6 |
| 4 1/2 | 2.875 | 6 |
| 5 | 3.125 | 6 |
| 5 1/2 | 3.375 | 6 |
| 6 | 3.625 | 6 |
NPT Thread Reference
| Nominal Size (inches) | OD (inches) | TPI (Threads per Inch) |
|---|---|---|
| 1/8 | 0.405 | 27 |
| 1/4 | 0.540 | 18 |
| 3/8 | 0.675 | 18 |
| 1/2 | 0.840 | 14 |
| 3/4 | 1.050 | 14 |
| 1 | 1.315 | 11.5 |
| 1 1/4 | 1.660 | 11.5 |
| 1 1/2 | 1.900 | 11.5 |
| 2 | 2.375 | 11.5 |
| 2 1/2 | 2.875 | 8 |
| 3 | 3.500 | 8 |
| 3 1/2 | 4.000 | 8 |
| 4 | 4.500 | 8 |
| 5 | 5.563 | 8 |
| 6 | 6.625 | 8 |
Example Measurements
NH (National Hose) Thread Example
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Nominal Size: 2 inches
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Thread Pitch: 8 TPI
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Thread Diameter:
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Male Thread OD: 1.50 inches
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Female Thread ID: 1.875 inches
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NPT (National Pipe Thread) Example
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Nominal Size: 1/2 inch
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Thread Taper: 1 in 16
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Thread Pitch: 14 TPI
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Thread Diameter:
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Male Thread OD: 0.840 inches
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Female Thread ID: 0.710 inches
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Get the right fit every time—shop Firepenny’s fire hose adapters, nozzles, and fittings today and keep your crew mission-ready.
Understanding the difference between NH/NST fire hose threads and NPT pipe threads is critical when ordering new nozzles, adapters, or couplings. Choosing the wrong thread type can lead to dangerous leaks, wasted time on the fireground, and costly returns. At Firepenny.com, we stock a full selection of fire hose adapters, fire hose nozzles, and wildland fittings to ensure your department always has the right connection. Whether you need fire hoses, PPE, or specialty wildland gear, Firepenny provides the trusted equipment that keeps firefighters mission-ready—always at some of the lowest prices in the business.
