How To Find The Circumference Of Any Fitting

  • IDM - Inside Diameter Male
  • ODM - Outside Diameter Male
  • TPI - Threads Per Inch
  • How To Connect Using NH / NST, NPSH, NPT and GHT Threads

    NH / NST Threads

    The most common thread type in use by United States Fire Departments today is National Hose (NH). It is also called National Standard Thread (NST) or National Hose Thread (NHT). It is often referred to as “fire hose thread”. NH, NST and NHT are different abbreviations for the exact same thread.

    The female and male side of a fire hose coupling with NH threads will only connect to fittings, nozzles or hoses with the same threads. For example, a fire hose with NH threaded couplings will require a nozzle with NH threads.

    National Hose (NH) is considered a “straight” thread which means the threads are consistent from tip to collar and the seal is made with the gasket inside the female coupling mating against the square face of the male coupling. The threads do not make the seal, they merely hold the two fittings tightly together. The size of an NH thread refers to the inside diameter of the waterway on a male coupling. For example 2.5" NH male fitting has a 2.5" waterway.

    Some fire departments in the United States have special threaded fire hoses and fire hydrants.

    NPSH Threads

    Fire hoses and adapters with National Pipe Straight Hose Thread (NPSH or NPS) has the same threads per inch (TPI) as National Pipe Thread (NPT) but the threads do not taper. This allows a female coupling to thread onto a NPT fitting without being a permanent connection. But it will NOT allow a male coupling or adapter to thread into a female NPT. For example, a female fire hose coupling with NPSH threads will only connect to a PVC pipe, metal pipe or couplings with NPSH or NPT threads. Fittings that are connected together by NPT by NPT require a sealing agent like pipe cement or teflon tape because the threads are making the seal and a permanent connection. When a female NPSH is threaded onto a male NPT fitting, it allows this connection to thread on and off frequently by using a gasket to seal.

    The male side with NPSH threads will only connect to fittings, nozzles or hoses with NPSH threads. It will NOT thread into a NPT female. For example, a male fire hose coupling with NPSH threads will require a nozzle with NPSH threads.

    NPT Threads

    National Pipe Tapered (NPT) is common in plumbing and some industrial uses. Sometimes referred to as “pipe thread” because pvc pipe and schedule 40 pipe are typically threaded in NPT. As its name implies, this is a tapered thread so that the outside diameter of the male (ODM) gets smaller towards the end of the fitting and the inside diameter of the female narrows with depth. As the two fittings are threaded together the opposite tapers force thread friction and makes the seal without the use of a gasket. The taper is 1/16" or .0625" per 1" of thread length.

    Fire hoses with NPT threads are not practical for a number of reasons. One being the female coupling on a hose needs to swivel to make a connection without twisting the hose. NPT fittings cannot swivel which makes NPSH a great alternative.

    NPTF threads are used when the application is such that pipe sealing compounds may fail due to higher heat or pressure than normal NPT threads can withstand. The threads are designed to seal mechanically by slightly, but sufficiently, crushing the threads when tightened with a wrench. This allows for joining the pipe and fitting without sealants.

    The NPT and NPTF threads can be interchanged if sealants such as PTFE tape or suitable pipe joint compounds are used. Female NPT threads can be designated as FPT, FNPT, NPT(F) or FIP and male NPT threads can be designated as MPT, MNPT, NPT(M) or MIP.

    Garden Hose Threads (GHT)

    Garden Hose Threads (GHT), just like NST and NPSH, seals with a gasket in the female coupling. A typical garden hose may be 5/8" or 3/4" inside diameter but the fitting on the ends will be GHT. GHT is not compatible with NPT. For example, 3/4" GHT is not the same as 3/4" NPT.

    BSP Threads

    British Standard Pipe (BSP) can be most commonly found in Canada, UK, Europe, Asia and Australia. It is a finer thread than National Pipe Thread (NPT), and will not seal properly if used in conjunction with it. Two types of threads are used: British Standard Pipe Taper (BSPT) and British Standard Pipe Parallel (BSPP). British Standard Pipe Taper uses a tapered thread system, and sealant is provided to help secure the male and female connection. British Standard Pipe Parallel uses a parallel thread system, and a rubber gasket is often included to secure the male and female connection.


    Storz couplings are sometimes referred to as a sexless coupling, because rather than having a male and a female end connected by screw threads, either identical end can be joined to any other end of the same diameter. Storz hoses are widely employed in firefighting supply line applications in the United States.

    To couple a Storz connection, the two opposing couplings are pressed together such that the hooks of each one are inserted into the slots in the flange of the other. Then they are rotated in opposite directions until they are tight, or latches engage. This creates a water-tight connection between the internal packing gaskets.

    To uncouple them, the latches are released and the connectors are turned in the opposite directions from coupling, and then separated when the hooks and slots are aligned. Special storz wrenches are designed for assisting with use of Storz connectors. The NFPA requires locking mechanism on all storz coupling and fitting larger than three inches.

    1" (25mm) Storz Adapter

    • Inside Distance Between Lugs - 1 7/32" (31mm)
    • Number of Lugs - 2

    1.5" (38mm) Storz Adapter

    • Inside Distance Between Lugs - 2 1/16" (52mm)
    • Number of Lugs - 2

    2" (50mm) Storz Adapter

    • Inside Distance Between Lugs - 2 5/8" (66mm)
    • Number of Lugs - 2

    2.5" (65mm) Storz Adapter

    • Inside Distance Between Lugs - 3 3/16" (81mm)
    • Number of Lugs - 2

    3" (75mm) Storz Adapter

    • Inside Distance Between Lugs - 3 1/2" (89mm)
    • Number of Lugs - 2

    4" (100mm) Storz Adapter

    • Inside Distance Between Lugs - 4 1/2" (115mm)
    • Number of Lugs - 2

    5" (125mm) Storz Adapter

    • Inside Distance Between Lugs - 5 13/16" (148mm)
    • Number of Lugs - 2

    6" (150mm) Storz Adapter

    • Inside Distance Between Lugs - 6.29" (160mm)
    • Number of Lugs - 3