Enter what you have measured. Anything you have not checked can be left as Not checked - the result narrows as you add checks, and says plainly when two fittings cannot be told apart by measurement at all.
Enter a measurement and select Identify.
The flow
How to take the four measurements
Step 1 Measure the thread diameter
Put a caliper across the thread crests of a male fitting, or inside the crests of a female port. Take it at the first full thread from the open end and square to the axis, not over the seat and not over the hex. Read millimetres if you can - most of these standards are half a millimetre apart, and that is easier to see in metric than in thousandths.
A reading that lands between two sizes usually means the caliper is sitting on a chamfer or on a worn crest. Move back one thread and take it again.
Step 2 Count the threads
Seat a thread pitch gauge against the crests until a blade sits flat with no rocking, or count crests across one inch with a rule. On a fine thread, count over half an inch and double it. A gauge blade of the wrong form rocks instead of seating, which is itself a useful result.
One thread per inch is a real difference, not a rounding error: a -12 JIC is 12 TPI and a 3/4 in SAE 45 flare on the same 1 1/16 in diameter is 14. Nothing else separates those two.
Step 3 Decide parallel or tapered
Measure across the crests at the first full thread, then again at the last full thread. A parallel thread reads the same at both ends. A tapered thread reads smaller at the entering end - a 1 in 16 taper loses 1.59 mm of diameter over every 25.4 mm of length. Laying a straight edge along the crests shows it too.
This is the check that separates NPT from NPSM and BSPT from BSPP, and it is the one people skip. A tapered male in a parallel port will feel tight and will still leak.
Step 4 Look at the seat
Look straight into the end of the fitting and decide which of the shapes below you are holding: a cone, a flat face with an O-ring groove, an O-ring on a plain shoulder behind the thread, a flat shoulder for a bonded washer, or no sealing surface at all. The seat is the most reliable feature on a used fitting, because it is machined once and never re-cut, while thread crests wear and get plated.
Seat angle is the only thing separating a JIC 37 degree from an SAE 45 degree at six of their eight shared sizes, and the only thing separating a JIC from an SAE O-ring boss at every size.
Step 4, in pictures
The eight sealing surfaces
Every fitting in the database seals in one of these eight ways. Match the shape at the end of the fitting to a drawing, then set the Seat field above - it is the single check that resolves the most ambiguity, because it separates fittings that share a thread exactly.
37 degree flare cone
Schematic cross-section of the male half, upper side only, with the centre line dashed and the open end on the left. Cone angles are drawn at their true angle from the axis; nothing else is to scale and no dimension should be measured off it.
A male nose machined to a shallow cone, or a female fitting with a coned recess. Measured off the fitting axis the cone half-angle is 37 degrees, so the included angle across the cone is 74 degrees.
Where it seals: Metal to metal, cone against the flared tube or the mating cone. No O-ring, no sealant.
Schematic cross-section of the male half, upper side only, with the centre line dashed and the open end on the left. Cone angles are drawn at their true angle from the axis; nothing else is to scale and no dimension should be measured off it.
The same shape as a 37 degree flare but visibly blunter - a wider, shorter cone. Side by side the difference is obvious; alone it is not.
Where it seals: Metal to metal, cone against the flared tube. No O-ring, no sealant.
Schematic cross-section of the male half, upper side only, with the centre line dashed and the open end on the left. Cone angles are drawn at their true angle from the axis; nothing else is to scale and no dimension should be measured off it.
A deep, narrow cone - noticeably more pointed than a 37 degree flare. Common on Japanese equipment, usually with a loose female swivel nut.
Where it seals: Metal to metal, a 30 degree female seat against a 30 degree male cone.
Schematic cross-section of the male half, upper side only, with the centre line dashed and the open end on the left. Cone angles are drawn at their true angle from the axis; nothing else is to scale and no dimension should be measured off it.
A shallow internal cone at the bottom of a straight metric port, with a cutting ring or a captive soft seal sitting in it.
Where it seals: The 24 degree cone against a cutting ring bitten into the tube, or against a soft seal captured in the cone on the sealing-cone version.
Schematic cross-section of the male half, upper side only, with the centre line dashed and the open end on the left. Cone angles are drawn at their true angle from the axis; nothing else is to scale and no dimension should be measured off it.
A completely flat male nose with a shallow O-ring groove machined into the face, and a flat female face with no groove at all.
Where it seals: The O-ring squashed between the two flat faces. The thread only holds the load.
Schematic cross-section of the male half, upper side only, with the centre line dashed and the open end on the left. Cone angles are drawn at their true angle from the axis; nothing else is to scale and no dimension should be measured off it.
A straight male thread with a plain unthreaded shoulder behind it carrying an O-ring, screwing into a port with a flat spotface at the top of the threads.
Where it seals: The O-ring squashed between the fitting shoulder and the machined spotface of the port. The thread only holds the load.
Schematic cross-section of the male half, upper side only, with the centre line dashed and the open end on the left. Cone angles are drawn at their true angle from the axis; nothing else is to scale and no dimension should be measured off it.
A straight thread with a flat shoulder or a chamfered face behind it, carrying a bonded seal washer or an O-ring.
Where it seals: A bonded seal washer or O-ring at the face. The parallel thread cannot seal on itself.
Schematic cross-section of the male half, upper side only, with the centre line dashed and the open end on the left. Cone angles are drawn at their true angle from the axis; nothing else is to scale and no dimension should be measured off it.
No cone, no groove, no flat sealing face - just a thread that visibly narrows towards the end of the fitting.
Where it seals: The thread flanks wedging together, normally with a thread sealant or PTFE tape. The seal is the thread.
Across the thread crests with a caliper, at the first full thread from the open end and square to the fitting axis. On a male fitting that is the outside diameter; on a female port it is the inside diameter across the crests. Do not measure over the seat, over the hex or over a chamfer - all three read high or low and all three are the usual reason an identification comes out wrong.
Why does the trade size not match what I measure?
Pipe threads are named after a bore that no longer exists on the fitting. A 1/4 in NPT measures about 13.7 mm across the threads and a G 1/4 BSPP about 13.2 mm - neither is a quarter of an inch anywhere. The rule of thumb is to subtract about 6 mm, a quarter inch, from a pipe-thread measurement to get its trade size. Dash-sized fittings are different again: a dash number is the tube outside diameter in sixteenths of an inch, so a -08 serves half-inch tube but its thread is 3/4-16.
Two fittings measure the same. Which one is it?
Often neither answer is available from measurement, and this site says so instead of guessing. Several of these standards share a thread outright: a -08 JIC, a -08 SAE O-ring boss and a 1/2 in tube SAE 45 flare are all 3/4-16 UNF. When that happens the identifier lists both candidates and tells you which single check separates them - normally the seat.
Can I tell JIC from BSPP with a caliper?
Not on diameter alone at every size, but the thread form gives it away. JIC is a 60 degree Unified thread with sharp, pointed crests; BSPP is a 55 degree Whitworth thread with visibly rounded crests and roots. A 55 degree pitch gauge blade rocks on a 60 degree thread and vice versa. The seat is different too: JIC has a 37 degree cone and a plain BSPP male has a flat shoulder for a bonded washer.
Is a thread identification enough to order a part?
It is enough to name the connection, not enough to specify the part. The body material, the pressure rating and the port form still come from the manufacturer's catalogue, and the pressure rating in particular varies by maker, material and whether the stud is straight or adjustable. Every rating published on this site prints the exact conditions it applies under, and where no two sources agreed, no number is published at all.
What this tool will not do
It will not rank two candidates on a difference smaller than a caliper can read. Where two standards sit inside one measurement band it lists both and tells you what to check next, because a confident wrong answer about a pressure connection is worse than an honest ambiguous one.
It also refuses impossible input rather than working from it. A diameter outside the range the database covers, or a thread count no covered standard uses, returns a message against that field and no result - never a number derived from a reading that was already rejected.