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What thread does fire hose use, and does it match garden hose?

Fire hose runs on NST, the National Standard Thread, in 1-1/2 inch and 2-1/2 inch sizes, which is a different animal from garden hose despite sharing the NH family name.

What NST is

NST, the National Standard Thread, also written NH, is the common coupling thread on fire hose and hydrant outlets in the United States. The two everyday sizes are 1-1/2 inch and 2-1/2 inch, referring to the hose bore rather than the thread diameter.

Like garden hose thread, NST is a straight thread that seals on a gasket at the face, which is why fire couplings carry a rubber washer in the female half.

Not the same as garden hose

Garden hose thread is also part of the NH family, but the sizes and diameters are different, so a fire hose coupling does not thread onto a garden tap and a garden hose does not thread onto a hydrant outlet. The shared NH label refers to the family of straight hose threads, not to a single interchangeable size.

If you need to draw water from a hydrant into smaller kit, you go through the proper reducing adapters rather than trying to force a size that will not seat.

Sizes and adapters

The 1-1/2 inch NST is common on attack lines and standpipe outlets, while 2-1/2 inch NST appears on larger supply lines and many hydrant outlets. Reducers step 2-1/2 inch down to 1-1/2 inch, and further adapters bridge NST to other threads where local rules allow.

Because fire connections vary by region and some areas use their own threads, confirm the local standard before assuming NST.

Sealing and handling

NST couplings seal on the gasket in the female swivel, so keep that gasket in good condition and replace it if it is cracked or missing. Hand-tighten the swivel onto the male thread and check the gasket has seated, rather than reaching for tape, because the seal is the gasket at the face, not the thread.

Related

Sources: NFPA 1963 fire hose connection standard