This article will go through the steps of how to identify thread sizes to help you determine the correct size thread you need for your application.

Measure the diameter of the thread

The first step to identify thread sizes of your components is measuring the diameter. You will need a caliper to do this. If you are measuring the male half of the coupling (external thread) make sure you use the bottom clamps of the caliper, and if you are measuring the female half of the coupling (internal thread) use the top clamps of the caliper to get the right measure.

These measurements will be part of the data you need to match with the information in the selection charts, but keep in mind that your measurement may not give you the exact number appearing on the chart. This can because of manufacturing tolerance or wear, but do not worry, usually the difference is minimal and you just have to choose the closest number from the selection chart.

Popular threads found in the most common fittings

National Pipe Thread (NPT): the special characteristic of this style of thread that is going to help you identify it is the presence of an inverted 30° seat in the female half and an 30° internal chamfer in the male half.

Inch size	Dash size	Threads per Inch	Male Thread O.D. (in)		Female thread I.D (in)
1⁄8	-2	27	13⁄32	0.41	3⁄8	0.38
1⁄4	-4	18	17⁄32	0.54	1⁄2	0.49
3⁄8	-6	14	11⁄16	0.68	5⁄8	0.63
1⁄2	-8	14	27⁄32	0.84	25⁄32	0.77
3⁄4	-12	14	1 1⁄16	1.05	1	0.98
1	-16	11 1⁄2	1 5⁄16	1.32	1 1⁄4	1.24
1 1⁄4	-20	11 1⁄2	1 21⁄32	1.66	1 19⁄32	1.58
1 1⁄2	-24	11 1⁄2	1 29⁄32	1.90	1 13⁄16	1.82
2	-32	11 1⁄2	2 3⁄8	2.38	2 5⁄16	2.30

SAE Straight Thread O-Ring (ORB): these are the SAE J1926-1 and ISO 11296-1 styles which also conform to MS16142, UN/UNF threads which are recommended for medium and high pressure hydraulic systems. Here, the characteristics to identify are the presence of an o-ring on the male half and a small chamfer to accept the o-ring on the female half.

Inch size	Dash size	Thread Size	Male Thread O.D. (in)		Female thread I.D (in)
1⁄8	-2	5⁄16 – 24	5⁄16	0.31	9⁄32	0.27
3⁄16	-3	3⁄8 – 24	3⁄8	0.38	11⁄32	0.34
1⁄4	-4	7⁄16 – 20	7⁄16	0.44	13⁄32	0.39
5⁄16	-5	1⁄2 – 20	1⁄2	0.50	15⁄32	0.45
3⁄8	-6	9⁄16 – 18	9⁄16	0.56	17⁄32	0.51
1⁄2	-8	3⁄4 – 16	3⁄4	0.75	11⁄16	0.69
5⁄8	-10	7⁄8 – 14	7⁄8	0.88	13⁄16	0.81
3⁄4	-12	1 1⁄16 – 12	1 1⁄16	1.06	1	0.98
7⁄8	-14	1 3⁄16 – 12	1 3⁄16	1.19	1 1⁄8	1.10
1	-16	1 5⁄16 – 12	1 5⁄16	1.31	1 1⁄4	1.23
1 1⁄4	-20	1 5⁄8 – 12	1 5⁄8	1.63	1 9⁄16	1.54
1 1⁄2	-24	1 7⁄8 – 12	1 7⁄8	1.88	1 13⁄16	1.79
2	-32	2 1⁄2 – 12	2 1⁄2	2.50	2 7⁄16	2.42

JIC 37° Flare or SAE J514: this is a style commonly found in fluid power systems. The special characteristic to identify it is the presence of a 37° flared seat in both the male and female halves.

Inch size	Dash size	Thread Size	Male Thread O.D. (in)		Female thread I.D (in)
1⁄8	-2	5⁄16 – 24	5⁄16	0.31	9⁄32	0.27
3⁄16	-3	3⁄8 – 24	3⁄8	0.38	11⁄32	0.34
1⁄4	-4	7⁄16 – 20	7⁄16	0.44	13⁄32	0.39
5⁄16	-5	1⁄2 – 20	1⁄2	0.50	15⁄32	0.45
3⁄8	-6	9⁄16 – 18	9⁄16	0.56	17⁄32	0.51
1⁄2	-8	3⁄4 – 16	3⁄4	0.75	11⁄16	0.69
5⁄8	-10	7⁄8 – 14	7⁄8	0.88	13⁄16	0.81
3⁄4	-12	1 1⁄16 – 12	1 1⁄16	1.06	1	0.98
7⁄8	-14	1 3⁄16 – 12	1 3⁄16	1.19	1 1⁄8	1.10
1	-16	1 5⁄16-12	1 5⁄16	1.31	1 1⁄4	1.23
1 1⁄4	-20	1 5⁄8 – 12	1 5⁄8	1.63	1 9⁄16	1.54
1 1⁄2	-24	1 7⁄8 – 12	1 7⁄8	1.88	1 13⁄16	1.79
2	-32	2 1⁄2 – 12	2 1⁄2	2.50	2 7⁄16	2.42

O-Ring Face Seal (ORFS) or SAE J1453: the male connector of this style presents an o-ring located in its face. On the other hand, the female half has a machined flat face where the o-ring is compressed to create the seal.

Inch size	Dash size	Thread Size	Male Thread O.D. (in)		Female thread I.D (in)
1⁄4	-4	9⁄16 – 18	9⁄16	0.56	17⁄32	0.51
3⁄8	-6	11⁄16 – 16	11⁄16	0.69	5⁄8	0.63
1⁄2	-8	13⁄16 – 16	13⁄16	0.82	3⁄4	0.75
5⁄8	-10	1 – 14	1	1.00	15⁄16	0.93
3⁄4	-12	1 3⁄16 – 12	1 3⁄16	1.19	1 1⁄8	1.11
1	-16	1 7⁄16 – 12	1 7⁄16	1.44	1 3⁄4	1.36
1 1⁄4	-20	1 11⁄16 – 12	1 11⁄16	1.69	15⁄8	1.61
1 1⁄2	-24	2-12	2	2.00	1 15⁄16	1.92

British Standard Pipe (BSP): Also referred to as Whitworth threads, the BSP thread type fittings seal using metal to metal angled surfaces or a combination of metal to metal and an O-ring. The angle of the sealing surfaces is 60° for both forms.
There are two popular thread forms, British Standard Pipe Parallel (BSPP) and British Standard Pipe Tapered (BSPT). Identification is made by measuring the outside diameter of the thread and the number of threads per inch (25.4 mm)

Dash Size	Size (inches)	TPI	Male O/D (mm)	Female I/D (mm)	Hex
-2	1⁄8	28	9.7	8.9	14
-4	1⁄4	19	13.2	11.9	19
-6	3⁄8	19	16.5	15.2	22
-8	1⁄2	14	20.8	19.1	27
-10	5⁄8	14	22.4	20.3	30
-12	3⁄4	14	26.4	24.6	32
-16	1	11	33.0	31.0	41
-20	1 1⁄4	11	41.9	39.6	50
-24	1 1⁄2	11	47.8	45.5	55
-32	2	11	59.7	57.4	70

Final considerations

There are other numbers that can be found on the selection charts such as dash numbers and the nominal size. If you already know the style of your fitting and one of this number it is enough to select the one you need quickly and easily. However, as the name itself says, these are only nominal (name) expressions to represent the fitting. If you are not sure, the safest way to proceed is following the steps presented to you in this article.