Measure the outside diameter of the thread with calipers and count its pitch with a thread gauge, and this lists the standard threads that fit, with their seal type and the look-alikes they are commonly mistaken for. Or look up any listed thread by size. NPT, BSPP and BSPT, and JIC and O-ring boss, share nominal sizes and sometimes exact dimensions, so the tool tells you when a diameter and pitch cannot settle it.
Reference tables
NPT / NPTF — American National Pipe Taper (ANSI/ASME B1.20.1)
Tapered, 60 degree thread. Seals on thread interference, normally with sealant or tape. NPTF (dryseal) has the same dimensions but crushes crests and roots for a dry seal.
Thread
Diameter (mm)
Diameter (in)
Threads per inch
Pitch (mm)
Tube OD
1/16 NPT
7.94
0.3125
27
0.94
—
1/8 NPT
10.29
0.405
27
0.94
—
1/4 NPT
13.72
0.54
18
1.41
—
3/8 NPT
17.15
0.675
18
1.41
—
1/2 NPT
21.34
0.84
14
1.81
—
3/4 NPT
26.67
1.05
14
1.81
—
1 NPT
33.4
1.315
11.5
2.21
—
1-1/4 NPT
42.16
1.66
11.5
2.21
—
1-1/2 NPT
48.26
1.9
11.5
2.21
—
2 NPT
60.33
2.375
11.5
2.21
—
2-1/2 NPT
73.03
2.875
8
3.18
—
3 NPT
88.9
3.5
8
3.18
—
4 NPT
114.3
4.5
8
3.18
—
BSPP (G) — British Standard Pipe Parallel, ISO 228
Parallel, 55 degree thread. The thread does not seal. It seals on a bonded washer or O-ring at the fitting face.
Thread
Diameter (mm)
Diameter (in)
Threads per inch
Pitch (mm)
Tube OD
G 1/8
9.73
0.383
28
0.91
—
G 1/4
13.16
0.518
19
1.34
—
G 3/8
16.66
0.656
19
1.34
—
G 1/2
20.96
0.825
14
1.81
—
G 5/8
22.91
0.902
14
1.81
—
G 3/4
26.44
1.041
14
1.81
—
G 7/8
30.2
1.189
14
1.81
—
G 1
33.25
1.309
11
2.31
—
G 1-1/4
41.91
1.65
11
2.31
—
G 1-1/2
47.8
1.882
11
2.31
—
G 2
59.61
2.347
11
2.31
—
BSPT (R / Rc) — British Standard Pipe Taper, ISO 7 / EN 10226
Tapered, 55 degree thread. Seals on thread interference, like NPT, with sealant or tape.
Thread
Diameter (mm)
Diameter (in)
Threads per inch
Pitch (mm)
Tube OD
R 1/8
9.73
0.383
28
0.91
—
R 1/4
13.16
0.518
19
1.34
—
R 3/8
16.66
0.656
19
1.34
—
R 1/2
20.96
0.825
14
1.81
—
R 3/4
26.44
1.041
14
1.81
—
R 1
33.25
1.309
11
2.31
—
R 1-1/4
41.91
1.65
11
2.31
—
R 1-1/2
47.8
1.882
11
2.31
—
R 2
59.61
2.347
11
2.31
—
JIC 37 degree flare — SAE J514 / ISO 8434-2
Parallel, 60 degree UN thread, 37 degree flared seat. Metal to metal on the 37 degree cone. The straight thread only pulls the seats together.
Thread
Diameter (mm)
Diameter (in)
Threads per inch
Pitch (mm)
Tube OD
JIC -3 (3/8-24 UNF)
9.53
0.375
24
1.06
0.1875 in
JIC -4 (7/16-20 UNF)
11.11
0.4375
20
1.27
0.25 in
JIC -5 (1/2-20 UNF)
12.7
0.5
20
1.27
0.3125 in
JIC -6 (9/16-18 UNF)
14.29
0.5625
18
1.41
0.375 in
JIC -8 (3/4-16 UNF)
19.05
0.75
16
1.59
0.5 in
JIC -10 (7/8-14 UNF)
22.23
0.875
14
1.81
0.625 in
JIC -12 (1-1/16-12 UN)
26.99
1.0625
12
2.12
0.75 in
JIC -14 (1-3/16-12 UN)
30.16
1.1875
12
2.12
0.875 in
JIC -16 (1-5/16-12 UN)
33.34
1.3125
12
2.12
1 in
JIC -20 (1-5/8-12 UN)
41.28
1.625
12
2.12
1.25 in
JIC -24 (1-7/8-12 UN)
47.63
1.875
12
2.12
1.5 in
JIC -32 (2-1/2-12 UN)
63.5
2.5
12
2.12
2 in
SAE O-ring boss (ORB) — SAE J1926 / ISO 11926 straight thread
Parallel, 60 degree UN thread, O-ring on a flat spotface. A captured O-ring compressed on a machined spotface. The thread does not seal.
Thread
Diameter (mm)
Diameter (in)
Threads per inch
Pitch (mm)
Tube OD
ORB -3 (3/8-24 UNF)
9.53
0.375
24
1.06
0.1875 in
ORB -4 (7/16-20 UNF)
11.11
0.4375
20
1.27
0.25 in
ORB -5 (1/2-20 UNF)
12.7
0.5
20
1.27
0.3125 in
ORB -6 (9/16-18 UNF)
14.29
0.5625
18
1.41
0.375 in
ORB -8 (3/4-16 UNF)
19.05
0.75
16
1.59
0.5 in
ORB -10 (7/8-14 UNF)
22.23
0.875
14
1.81
0.625 in
ORB -12 (1-1/16-12 UN)
26.99
1.0625
12
2.12
0.75 in
ORB -14 (1-3/16-12 UN)
30.16
1.1875
12
2.12
0.875 in
ORB -16 (1-5/16-12 UN)
33.34
1.3125
12
2.12
1 in
ORB -20 (1-5/8-12 UN)
41.28
1.625
12
2.12
1.25 in
ORB -24 (1-7/8-12 UN)
47.63
1.875
12
2.12
1.5 in
ORB -32 (2-1/2-12 UN)
63.5
2.5
12
2.12
2 in
O-ring face seal (ORFS) — SAE J1453 / ISO 8434-3
Parallel, 60 degree UN thread, O-ring in a flat face. An O-ring in the face of the fitting seals against a flat mating face. The thread only clamps.
Thread
Diameter (mm)
Diameter (in)
Threads per inch
Pitch (mm)
Tube OD
ORFS -4 (9/16-18 UN)
14.29
0.5625
18
1.41
0.25 in
ORFS -6 (11/16-16 UN)
17.46
0.6875
16
1.59
0.375 in
ORFS -8 (13/16-16 UN)
20.64
0.8125
16
1.59
0.5 in
ORFS -10 (1-14 UNS)
25.4
1
14
1.81
0.625 in
ORFS -12 (1-3/16-12 UN)
30.16
1.1875
12
2.12
0.75 in
ORFS -16 (1-7/16-12 UN)
36.51
1.4375
12
2.12
1 in
ORFS -20 (1-11/16-12 UN)
42.86
1.6875
12
2.12
1.25 in
ORFS -24 (2-12 UN)
50.8
2
12
2.12
1.5 in
Metric fine (M x pitch) — ISO 261 metric fine thread as used for port and fitting threads (DIN 3852, ISO 8434-1)
Parallel, 60 degree ISO metric thread. Depends on the port style: an O-ring, a bonded washer or a cutting ring. The thread itself does not seal.
Thread
Diameter (mm)
Diameter (in)
Threads per inch
Pitch (mm)
Tube OD
M8 x 1
8
0.315
25.4
1
—
M10 x 1
10
0.3937
25.4
1
—
M12 x 1.5
12
0.4724
16.9
1.5
—
M14 x 1.5
14
0.5512
16.9
1.5
—
M16 x 1.5
16
0.6299
16.9
1.5
—
M18 x 1.5
18
0.7087
16.9
1.5
—
M20 x 1.5
20
0.7874
16.9
1.5
—
M22 x 1.5
22
0.8661
16.9
1.5
—
M26 x 1.5
26
1.0236
16.9
1.5
—
M27 x 2
27
1.063
12.7
2
—
M30 x 2
30
1.1811
12.7
2
—
M33 x 2
33
1.2992
12.7
2
—
M36 x 2
36
1.4173
12.7
2
—
M42 x 2
42
1.6535
12.7
2
—
M45 x 2
45
1.7717
12.7
2
—
M52 x 2
52
2.0472
12.7
2
—
Diameters are the nominal major diameter (pipe outside diameter for NPT, gauge-plane major diameter for BSP). Real threads sit inside their tolerance bands. Dash sizes are for JIC, O-ring boss and O-ring face seal.
Frequently asked questions
How do I identify a thread?
Measure the outside diameter of a male thread across the crests with calipers, count the threads per inch (or measure the pitch in millimetres) with a thread pitch gauge, and look at the shape. A tapered thread visibly narrows toward the tip, and a straight thread does not. Then check how it seals: a cone or flared seat, an O-ring, a flat face, or the thread itself. Diameter and pitch alone narrow it to a few candidates, and the seal style and thread angle settle the rest.
What is the difference between NPT and BSP threads?
NPT is the American standard with a 60 degree thread form, and it is always tapered. BSP is the British standard with a 55 degree thread form, in a parallel version (BSPP, also written G) and a tapered version (BSPT, written R or Rc). Both use the same nominal sizes, and at 1/2 and 3/4 inch both are 14 threads per inch, so they are close enough to start and not close enough to seal. A caliper and a pitch gauge often cannot separate them, so check a thread gauge or the datasheet.
Are JIC and O-ring boss fittings interchangeable?
No, even though the threads are identical. A -8 JIC and a -8 SAE O-ring boss are both 3/4-16 UNF and will screw together, but JIC seals metal to metal on a 37 degree cone and an O-ring boss seals on an O-ring against a flat spotface. Neither has what the other needs, so the joint tightens and does not seal. Identify the seal, not just the thread.
What does a dash size like -8 mean?
On JIC, O-ring boss and O-ring face seal fittings the dash number is the tube outside diameter in sixteenths of an inch, so -8 is 8/16 inch, or 1/2 inch tube. The thread size is a separate table entry, for example 3/4-16 UNF for a -8 JIC, and cannot be worked out from the dash number.
What is NPTF and how does it differ from NPT?
NPTF is the dryseal version of NPT. It has the same nominal dimensions, but the crests and roots are controlled so they crush against each other and seal without tape or sealant. It is used where sealant could contaminate a system, such as hydraulics. An NPT fitting will usually thread into an NPTF port, but the joint may then need sealant to hold pressure.
Why does the tool sometimes show several matches?
Because several standards genuinely share a diameter and pitch band. The tool lists every standard thread within a small tolerance of your measurement, closest first, and flags the pairs that a diameter and pitch cannot separate. It does not pick one for you. Tapered threads are measured with a wider tolerance because the diameter changes along the thread.
Can KKM source a fitting that is not listed here?
Yes. Email the thread type and size, the seal style, the material and the pressure rating to orderdesk@kkmsolutions.com. We source per order across our supplier network, not only what is indexed on the site.
