Pipe threads are named, not measured
A pipe thread size is nominal: G 1/2 measures about 21 mm across the crests, not half an inch. A caliper reading near 21 mm therefore points to a 1/2 pipe thread.
Reference tool — not inspection
Identify the thread before it goes in an RFQ.
Thread measurement
Enter the diameter and the thread count.
A point or a comma both work as the decimal separator; the result updates as you type. Further down, an offered thread designation can be checked against the reading: pitch, diameter, form and flank angle.
Diameter across the crests
Threads per inch or pitch
Examples:
Replacement check
Against your reading and the thread tables alone: the sealing method and the mating part decide.
All 130 engineering tools
Gauges, sensors, plugs, valves and hose ends are ordered by thread, and the threads that look alike are the ones that get confused: a 1/2 BSP and a 1/2 NPT both have 14 threads per inch. Measure the diameter across the crests and count the threads; the tool lists the standard threads that fit, from ISO 228 and ISO 7 pipe threads to NPT, ISO metric and Unified.
A measurement never matches a table exactly, and the tool does not pretend it does: each candidate shows how far it is from your reading. It narrows the search; a thread gauge, the drawing or the part number confirms it.
Why threads get confused
The name, the flank angle and where it seals.
55° or 60°
BSP threads to ISO 228 and ISO 7 use the 55° Whitworth form; NPT, ISO metric and Unified threads use 60°. Swagelok’s pitch gauges label the forms W (Whitworth, 55°), U (Unified) and M (metric).
Where the joint seals
A G thread to ISO 228-1 is for joints that are not pressure-tight on the threads: a gasket, O-ring or metal-to-metal contact seals them. An R thread to ISO 7-1 and an NPT seal on the thread itself, and, as Swagelok’s guide notes, a tapered pipe thread always needs a sealant.
| Size | BSP, threads per inch | BSP, mm | NPT, threads per inch | NPT, mm |
|---|---|---|---|---|
| 1/8 | 28 | 9.728 | 27 | 10.19 |
| 1/4 | 19 | 13.157 | 18 | 13.54 |
| 3/8 | 19 | 16.662 | 18 | 16.97 |
| 1/2 | 14 | 20.955 | 14 | 21.13 |
| 3/4 | 14 | 26.441 | 14 | 26.49 |
| 1 | 11 | 33.249 | 11.5 | 33.15 |
| 1 1/4 | 11 | 41.910 | 11.5 | 41.88 |
| 1 1/2 | 11 | 47.803 | 11.5 | 47.96 |
| 2 | 11 | 59.614 | 11.5 | 59.99 |
Highlighted rows have the same thread count. BSP diameters follow ISO 228-1; NPT diameters are the nominal values in Swagelok’s thread identification guide.
PT, PF and PS on Japanese drawings
Drawings and parts lists from Japanese makers may give a pipe thread as PT, PF or PS. These are the conventional JIS names, kept in an appendix of the current JIS B 0203 and B 0202: PT corresponds to R, PF to G, and PS to the parallel female thread that mates with a PT male. The tool shows these names next to the candidates.
Sources
Pipe thread diameters and thread counts: ISO 228-1 and ISO 7-1 as tabulated in Swagelok’s Thread and End Connection Identification Guide (MS-13-77); NPT from the same guide (ASME B1.20.1); metric pitches from ISO 261; Unified threads from ASME B1.1; JIS names from Kuroda Precision’s note on JIS pipe threads; the end connections that use each thread family follow the thread-to-end-connection matrix of the same Swagelok guide. The diameter of a parallel internal thread is calculated from the basic profile. PG conduit threads (DIN 40430): diameters and pitches from Norres’ thread table, matching Heyco’s and Lutze’s; the internal diameter as the tapping drill size in gewinde-normen.de’s table. The metric M × 1.5 cable-gland sizes of EN 60423 and their PG counterparts follow the PG-to-metric tables of Wikipedia and Baer. The 24° cone fittings behind a metric thread (ISO 8434-1, DIN 2353, preferred L and S series) follow the ISO 8434-1 table reproduced by globalfastener.com, checked against DICSA’s stainless 24° catalogue and Parker’s EO part listings. Hydraulic dash sizes behind a Unified thread (JIC 37° flare SAE J514, O-ring boss SAE J1926, O-ring face seal SAE J1453) follow the thread charts of Engine Oil Journal and Hose and Fittings.
Before you send the thread
Questions about threads in an RFQ
Is a 1/2 BSP thread the same as a 1/2 NPT?
No. Both have 14 threads per inch, but BSP uses a 55° flank angle and NPT 60°, and their diameters differ by a few tenths of a millimetre. Swagelok’s identification guide notes that at 1/2 and 3/4 the two are so close that positive identification may need an optical comparator, so the request should say which one the equipment uses.
What do PT, PF and PS mean on a Japanese drawing?
They are the conventional JIS names for pipe threads, kept in an appendix of the current JIS. PT is the taper thread now written R (JIS B 0203), PF the parallel thread now written G (JIS B 0202), and PS the parallel female thread that mates with a PT male.
Why does my measurement not match the table exactly?
Every thread has tolerances, threads wear, and a taper thread reads differently along its length. The tool shows how far each candidate is from your reading instead of hiding the difference; a gauge, the drawing or the part number settles it.
Is a PG thread metric?
No. PG (DIN 40430) is a conduit thread with an 80° flank angle and its own diameters: PG 13.5 is 20.4 mm with 18 threads per inch. It does not fit an M20 × 1.5 or M25 × 1.5 cable gland; adapters exist, and new installations use M × 1.5 glands to EN 62444.
What if nothing matches?
Check the measurement across the crests and the thread count, then send a photograph of the thread next to a ruler with both figures. Hydraulic-specific, old British (BSW, BSF), trapezoidal and other special threads are outside this tool.
Thread identified, photo taken?
Send the measurement with the photograph.
An identified thread is a cross-check on your own measurement, not proof. Attach a photograph of the thread next to a ruler, give both figures and the equipment it sits on, and Vessel Core checks them against each other before an offer goes back.
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