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How do I measure an unknown thread to identify the fitting?
Three measurements, the diameter, the pitch or threads per inch, and whether it tapers, will identify almost any hose or tap thread.
Male or female, and the diameter
First note whether the thread is male (on the outside) or female (inside a bore). For a male thread, measure the outer diameter across the crests with a caliper. For a female thread, measure across the inside of the thread valleys. This is the single most useful number.
Remember that pipe-style sizes are nominal, not literal. A 3/4 inch BSP thread measures around 26.4mm across, not 19mm, because the inch figure refers to a nominal bore rather than the thread diameter.
Pitch or threads per inch
Metric threads are described by pitch, the distance between adjacent crests in millimetres, such as the 1.5mm pitch of M22 x 1.5. Imperial threads are described by threads per inch, the count of crests in one inch, such as the 11.5 of GHT.
Use a thread pitch gauge, which is a set of toothed blades you rest against the thread until one matches. If you have no gauge, lay the thread against a ruler and count crests over a known length, then work out the count per inch or the spacing in millimetres.
Check for taper
Decide whether the thread is straight or tapered, because it changes how the joint seals. Sight along the crests or run a straightedge over them. A straight thread such as GHT, NPSH or parallel BSPP keeps one diameter and seals on a washer. A tapered thread such as NPT or BSPT narrows toward the tip and seals on the thread.
Taper is easy to miss on a short thread, so measure the diameter at both ends if you are unsure.
Match to a standard
With diameter, pitch and taper in hand, compare against the common families. Around 26.4mm outer at a matching pitch with a washer seat points to 3/4 inch BSP parallel. A 3/4 inch-11.5 straight thread points to GHT. A 22mm metric thread at 1.5mm pitch points to M22. Identify the standard before you buy, so the adapter names both ends correctly.
Get the right fitting
Open the HoseFitFinder toolIdentify a thread from measurementsFind thread pitch gauge metric imperial on eBayaffiliateRelated
Sources: ISO 228 (BSP) thread standard ยท Sheldon Brown plumbing reference