Reference chart

Hydraulic fitting thread chart

Nominal male thread diameter and thread count for the fitting series a shop meets, plus the tap drill for the matching female port where the thread form allows it to be computed. To go the other way, from a measurement to a name, use the thread identifier.

NPT and NPTF tapered pipe (ANSI B1.20.1 and B1.20.3)

Size	Male thread OD	Pitch
1/16	0.313 in / 7.94 mm	27 TPI
1/8	0.405 in / 10.29 mm	27 TPI
1/4	0.540 in / 13.72 mm	18 TPI
3/8	0.675 in / 17.14 mm	18 TPI
1/2	0.840 in / 21.34 mm	14 TPI
3/4	1.050 in / 26.67 mm	14 TPI
1	1.315 in / 33.40 mm	11.5 TPI
1-1/4	1.660 in / 42.16 mm	11.5 TPI
1-1/2	1.900 in / 48.26 mm	11.5 TPI
2	2.375 in / 60.32 mm	11.5 TPI

Both forms are TAPERED and seal on the thread itself. NPTF shares every diameter and thread count with NPT and differs in the thread form: its crests and roots interfere, so a dryseal joint can seal without sealant. Tapered pipe threads have no single tap drill because the port is drilled and then taper reamed.

BSPP and BSPT, G and R (ISO 228 and ISO 7)

Size	Male thread OD	Pitch
1/8	0.383 in / 9.73 mm	28 TPI
1/4	0.518 in / 13.16 mm	19 TPI
3/8	0.656 in / 16.66 mm	19 TPI
1/2	0.825 in / 20.95 mm	14 TPI
5/8	0.902 in / 22.91 mm	14 TPI
3/4	1.041 in / 26.44 mm	14 TPI
1	1.309 in / 33.25 mm	11 TPI
1-1/4	1.650 in / 41.91 mm	11 TPI
1-1/2	1.882 in / 47.80 mm	11 TPI
2	2.347 in / 59.61 mm	11 TPI

BSPP is PARALLEL and BSPT is the TAPERED form of the same series. BSPT shares these diameters and thread counts and is the tapered form of the same series; BSPP is parallel and seals on a bonded washer or an O-ring instead. Both are 55 degree Whitworth threads, which is why the 60 degree tap drill is not computed for them.

JIC 37 degree flare and SAE O-ring boss (SAE J514 and J1926)

Size	Male thread OD	Pitch	Tap drill (75 pct)
-2 (5/16-24 UNF)	0.313 in / 7.94 mm	24 TPI	0.2719 in
-3 (3/8-24 UNF)	0.375 in / 9.52 mm	24 TPI	0.3344 in
-4 (7/16-20 UNF)	0.438 in / 11.11 mm	20 TPI	0.3888 in
-5 (1/2-20 UNF)	0.500 in / 12.70 mm	20 TPI	0.4513 in
-6 (9/16-18 UNF)	0.563 in / 14.29 mm	18 TPI	0.5084 in
-8 (3/4-16 UNF)	0.750 in / 19.05 mm	16 TPI	0.6891 in
-10 (7/8-14 UNF)	0.875 in / 22.22 mm	14 TPI	0.8054 in
-12 (1-1/16-12 UN)	1.063 in / 26.99 mm	12 TPI	0.9813 in
-14 (1-3/16-12 UN)	1.188 in / 30.16 mm	12 TPI	1.1063 in
-16 (1-5/16-12 UN)	1.313 in / 33.34 mm	12 TPI	1.2313 in
-20 (1-5/8-12 UN)	1.625 in / 41.27 mm	12 TPI	1.5438 in
-24 (1-7/8-12 UN)	1.875 in / 47.63 mm	12 TPI	1.7938 in

Parallel thread; the thread only clamps. The male 37 degree cone seats in a matching female flare. SAE O-ring boss uses the same thread list and seals on an O-ring at the port face instead.

O-ring face seal (SAE J1453)

Size	Male thread OD	Pitch	Tap drill (75 pct)
-4 (9/16-18 UNF)	0.563 in / 14.29 mm	18 TPI	0.5084 in
-6 (11/16-16 UN)	0.688 in / 17.46 mm	16 TPI	0.6266 in
-8 (13/16-16 UN)	0.813 in / 20.64 mm	16 TPI	0.7516 in
-10 (1-14 UNS)	1.000 in / 25.40 mm	14 TPI	0.9304 in
-12 (1-3/16-12 UN)	1.188 in / 30.16 mm	12 TPI	1.1063 in
-16 (1-7/16-12 UN)	1.438 in / 36.51 mm	12 TPI	1.3563 in
-20 (1-11/16-12 UN)	1.688 in / 42.86 mm	12 TPI	1.6063 in
-24 (2-12 UN)	2.000 in / 50.80 mm	12 TPI	1.9188 in

Parallel thread; the thread only clamps. A flat face with an O-ring groove seals against the mating flat face. Note this is its OWN thread list, not the JIC one.

Metric light and heavy series (DIN 2353 / ISO 8434-1)

Size	Male thread OD	Pitch	Tap drill (75 pct)
6L	0.472 in / 12.00 mm	1.5 mm	10.54 mm
8L	0.551 in / 14.00 mm	1.5 mm	12.54 mm
10L	0.630 in / 16.00 mm	1.5 mm	14.54 mm
12L	0.709 in / 18.00 mm	1.5 mm	16.54 mm
15L	0.866 in / 22.00 mm	1.5 mm	20.54 mm
18L	1.024 in / 26.00 mm	1.5 mm	24.54 mm
22L	1.181 in / 30.00 mm	2 mm	28.05 mm
28L	1.417 in / 36.00 mm	2 mm	34.05 mm
35L	1.772 in / 45.00 mm	2 mm	43.05 mm
42L	2.047 in / 52.00 mm	2 mm	50.05 mm
6S	0.551 in / 14.00 mm	1.5 mm	12.54 mm
8S	0.630 in / 16.00 mm	1.5 mm	14.54 mm
10S	0.709 in / 18.00 mm	1.5 mm	16.54 mm
12S	0.787 in / 20.00 mm	1.5 mm	18.54 mm
14S	0.866 in / 22.00 mm	1.5 mm	20.54 mm
16S	0.945 in / 24.00 mm	1.5 mm	22.54 mm
20S	1.181 in / 30.00 mm	2 mm	28.05 mm
25S	1.417 in / 36.00 mm	2 mm	34.05 mm
30S	1.654 in / 42.00 mm	2 mm	40.05 mm
38S	2.047 in / 52.00 mm	2 mm	50.05 mm

Parallel thread. The heavy series carries a larger thread at the same tube size for higher pressure. Both are parallel and seal on a 24 degree cone or a bonded washer.

Fittings that can be mistaken for each other

Nearly every leak that gets blamed on a bad seal is a fitting from the wrong standard. The table below is computed from the dimensions above: pairs from different standards whose outside diameters sit within 30 thousandths of an inch. At that distance a fitting will start by hand and feel entirely normal.

Two whole families are left out of it, because each is one fact rather than a list of rows. G and R of the same size are dimensionally identical across all 10 sizes here: same outside diameter, same thread count, differing only in that R is tapered and G is parallel. A G male threads into an R port and seals on nothing, which is the most common of all these mistakes, and the tables above merge the two for the same reason. JIC and SAE ORB likewise share one SAE straight thread across all 12 dash sizes - a -6 of either screws into the other perfectly, and then one seals on a 37 degree cone while the other wanted an O-ring against the port face. And the metric light and heavy series share one thread in 7 sizes: an 8L and a 6S are both M14x1.5, so they assemble, and the difference is the tube they are rated for rather than anything you will feel on the wrench.

Fitting	OD	TPI	Can be started into	OD	TPI	What actually happens
-6 JIC 37 deg	0.563	18	-4 ORFS	0.563	18	Threads fully and holds torque, then leaks: the threads match but the seal does not (37 degree cone against flat face O-ring).
-14 JIC 37 deg	1.188	12	-12 ORFS	1.188	12	Threads fully and holds torque, then leaks: the threads match but the seal does not (37 degree cone against flat face O-ring).
-6 SAE ORB	0.563	18	-4 ORFS	0.563	18	Threads fully and holds torque, then leaks: the threads match but the seal does not (parallel, seals at the face against flat face O-ring).
-14 SAE ORB	1.188	12	-12 ORFS	1.188	12	Threads fully and holds torque, then leaks: the threads match but the seal does not (parallel, seals at the face against flat face O-ring).
3/4 NPT	1.050	14	G 3/4 (BSPP)	1.041	14	Threads fully and holds torque, then leaks: the threads match but the seal does not (tapered thread against parallel, seals at the face).
3/4 NPT	1.050	14	R 3/4 (BSPT)	1.041	14	Threads fully and holds torque, then leaks: the threads match but the seal does not (tapered thread against tapered thread).
1/2 NPT	0.840	14	G 1/2 (BSPP)	0.825	14	Threads fully and holds torque, then leaks: the threads match but the seal does not (tapered thread against parallel, seals at the face).
1/2 NPT	0.840	14	R 1/2 (BSPT)	0.825	14	Threads fully and holds torque, then leaks: the threads match but the seal does not (tapered thread against tapered thread).
1/4 NPT	0.540	18	-6 JIC 37 deg	0.563	18	Threads fully and holds torque, then leaks: the threads match but the seal does not (tapered thread against 37 degree cone).
1/4 NPT	0.540	18	-6 SAE ORB	0.563	18	Threads fully and holds torque, then leaks: the threads match but the seal does not (tapered thread against parallel, seals at the face).

The worst case is the one with matching threads and a different seal. It assembles properly, torques up, passes a dry check, and weeps once there is pressure behind it, because a tapered thread is sealing on its flanks while the port was cut for a washer or an O-ring at the face.

Telling them apart before you fit them

Three measurements settle almost every case, and none of them needs a specialist tool:

Is the thread tapered or parallel? Run a caliper over the first and last full thread. A visible difference means NPT, NPTF or BSPT; no difference means BSPP, ORB, JIC, ORFS or metric. This single check splits the field in half.
Count the threads. A pitch gauge on the straight portion separates the pairs that survived the first test - and it is the check that catches the 60 degree American series against the 55 degree British one, which look identical to the eye.
Look at the seat. A cone at roughly 37 degrees is JIC. A flat face with an O-ring groove is ORFS. A plain flat with an O-ring at the base of the thread is ORB. A parallel thread with nothing but a chamfer wants a bonded washer.

If the identification still will not resolve, the hydraulic thread identifier takes a measured diameter and thread count and returns the standards that match, ranked, rather than asking you to scan the table by eye.

Frequently asked questions

Why is there no tap drill for the NPT and BSPP tables?

Because it cannot be computed honestly from 60 degree geometry. NPT is tapered, so the port is drilled and then taper reamed rather than drilled to one size. BSPP and BSPT are 55 degree Whitworth threads with a different thread depth constant. The tap drills shown on the other tables are computed by our own 60 degree engine, and printing one of those numbers against a 55 degree or tapered thread would be quietly wrong.

Is the nominal OD what I will measure?

Close, but expect to read a few thousandths under. The listed diameter is the nominal major diameter of the thread, and a real male thread has a truncated crest and some wear, so calipers come in low. That is why the identifier matches on a tolerance band rather than looking for an exact number.

Do JIC and SAE ORB share threads?

Yes. The two share the same thread list dash size for dash size, which is why the identifier returns both when you give it a diameter and a thread count. They differ in where the seal happens: JIC seats a 37 degree cone against a matching flare, while ORB seals with an O-ring on the male boss against the port face.

Where does the dash number come from?

It is the tube or hose size in sixteenths of an inch, so a -8 is half inch tube. The thread that goes with it is a separate Unified size, in that case 3/4-16 UNF. Two families can share a dash number and use different threads, which is exactly why measuring beats assuming.

This is reference and identification data, not a procedure. Confirm it against the manufacturer data sheet and the specification that applies to your work, and verify the part in your hand, before relying on it for anything that carries load or holds pressure.