Hydraulic and pneumatic reference
Hydraulic and Pneumatic Fitting Thread Identifier
A fitting with no markings is identified by two measurements and nothing else: the outside diameter across the male thread, and the number of threads in an inch or the pitch in millimetres. Enter those and this returns every standard whose nominal size lands inside the measured band, closest first, together with where each one actually seals. Candidates are ranked rather than reduced to one answer, because some pairs genuinely need a third observation.
Measure the outside diameter across the male thread to start
| Thread | Nominal OD | Pitch | Form | Seals on |
|---|
How it works
Start with the shape rather than the numbers. Look along the male thread: if it visibly narrows towards the end it is a pipe thread that seals on the thread itself, which means NPT, NPTF or BSPT and a sealant. If the flanks run parallel like a bolt, the thread is only a clamp and the seal happens somewhere else, which puts you in JIC, ORFS, SAE ORB, BSPP or metric. Noting taper before measuring removes half the table.
Then measure. Calipers across the crests of the male thread give the outside diameter, and a pitch gauge gives threads per inch for the American and British series or pitch in millimetres for metric. The pair that proves why both readings are needed is quarter inch: NPT is 0.540 inches at 18 threads per inch, BSPP is 0.518 inches at 19. That is twenty two thousandths and one thread apart, close enough that either measurement alone will let you thread a BSPP male into an NPT female far enough to feel right and never seal. Finally look at the seat: a 37 degree cone is JIC, a flat face with an O-ring groove is ORFS, an O-ring on a shoulder at the base is SAE ORB, and a bonded washer under a parallel thread is BSPP.
Worked example
A male thread measuring 0.540 in at 18 TPI is 1/4 NPT. Measure 0.518 in at 19 TPI instead and it is 1/4 BSPP. That is 22 thousandths of an inch and one thread apart, which is close enough to start in the wrong port and never seal.
Frequently asked questions
How do I tell NPT from BSPP?
Count the threads and measure the diameter, because at small sizes the two look alike. Quarter inch NPT is 0.540 inches across the male thread at 18 threads per inch, while quarter inch BSPP is 0.518 inches at 19 threads per inch. NPT also tapers along its length and BSPP is parallel, so a thread gauge laid along the flank separates them even before you count.
Why is the nominal size nothing like the measured diameter on pipe threads?
Pipe sizes are named after an old nominal bore, not the thread. A quarter inch NPT male measures about 0.540 inches over the thread because the number refers to the approximate inside diameter of the pipe it was designed for. Flare and O-ring fittings do not work that way: a dash size refers to the tube in sixteenths of an inch, and the thread is a separate Unified size.
What is a dash size?
Dash sizes number the tube or hose in sixteenths of an inch, so a -8 is eight sixteenths, meaning half inch tube. The thread that goes with it is a separate fact: a -8 JIC uses a 3/4-16 UNF thread. That is why two fittings with the same dash number in different families do not share a thread.
Can I seal a BSPP male into an NPT female?
No. The thread forms differ in angle and the BSPP thread is parallel while the NPT female is tapered, so the two only engage on a few crests. It will often start and feel tight, which is exactly what makes the mistake common, and it will either leak or split the port under pressure. Identify before assembling.
Why does it show more than one answer?
Because diameter and pitch cannot always separate a pair. NPT and NPTF share both, differing only in the thread form that lets NPTF seal without sealant, and JIC and SAE ORB share a thread list and differ only in the seat. When the measurements are genuinely ambiguous the tool says so rather than picking one for you, and the seat description is what resolves it.
Does the outside diameter have to be exact?
No, and it will not be. A used male thread measures a few thousandths under nominal because the crest is truncated and the part is worn, so matching runs on a tolerance band rather than an equality. Anything within about thirty thousandths of a nominal size is offered as a candidate and the list is ordered by how close it sits.
Related calculators
Coverage
Implements ANSI B1.20.1 (NPT, NPTF), ISO 228 (BSPP), ISO 7 (BSPT), SAE J514 (JIC 37 deg), SAE J1453 (ORFS), SAE J1926 (SAE ORB), DIN 2353 / ISO 8434-1 (metric L and S): Nominal male thread outside diameter and thread count for every listed series, Ranked candidates from a measured diameter plus a counted pitch, with a tolerance band for wear, Taper filter, and the sealing surface for each candidate, JIC, ORFS and SAE ORB rows cross-checked against the site's own Unified thread series. Deliberately not offered yet:
- Pressure ratings, torque figures and assembly specifications - these depend on the material, the manufacturer and the assembly method; a wrong number here would be a safety issue rather than an inconvenience
- Komatsu, Flareless and JIS B 2351 metric variants - not covered rather than partially covered; their thread lists overlap the metric series and would produce false candidates
- Full thread flank angle and truncation dimensions - tabulated in the standards themselves, which are sold by SAE and ISO
- Independent verification of the NPT, BSP and metric DIN 2353 rows - the JIC, ORB and ORFS inch rows are cross-checked against this site's own verified Unified thread series, and ORFS -10 was confirmed against a supplier chart; the pipe and metric rows rest on concurring supplier tables only, which is concordance rather than proof. Measure your own fitting before committing to a port
Sources
- Thread identification for hydraulic and pneumatic threads (Apex Fluid Power)
- Hydraulic fittings guide: BSP, JIC, ORFS and how to identify them (AIMS Industrial)
- How to identify NPT thread and other hydraulic fitting types (Rubber and Specialties)
- ORFS thread identification and torque chart, PDF (Surplus Center)
Every formula on this page is shown and sourced. See how we verify.
These calculators are for planning and as a starting point. Recommended speeds and feeds are published starting values that vary with your specific tool, coating, machine rigidity, workholding and coolant. Always start conservative, listen to the cut, and follow your tool maker data sheet.