Inch threads – size chart and designations

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What is an inch thread?

The inch thread is a type of thread that originated from the inch unit of measurement instead of the metric unit. As we use the metric system at our latitude, this type of thread was not popular in Europe until recently.

For a long time, of all the inch varieties, only the G pipe thread (also known as BSPP) was used, which found its way into construction hydraulic connections. However, other Anglo-Saxon threads - both British and American - have arrived over time.

As a result, you currently have at least a dozen standards of these installation parts to choose from.

What are the designations of the inch thread?

There are many designations for inch threads. They vary depending on which market we are looking at - British or American.

What are the designations of the US inch threads?

The American designations for inch threads include:

  • UNC - unified inch thread, regular; 
  • UNF - unified inch thread, fine thread, having a triangular outline and a 60 degree angle; 
  • UNEF - Unified inch threads, fine pitch, used for short threads and thin pipe wall; 
  • UN - Unified threads with preference pitch; 
  • UNS - Unified threads with special diameters and thread pitch; 
  • ACME - American trapezoidal symmetrical thread of general application; 
  • NPT - Briggs pipe thread, tapered, having a taper of 1:16; NPS — pipe thread, cylindrical.
  • NPS - pipe thread, cylindrical.
American inch threads

What are the designations of the British inch threads?

The British designations for inch threads include:

  • BSW - Whitworth inch thread, regular with a triangular cylindrical outline and an angle of 55 degrees; 
  • BSF - Whitworth inch thread, fine pitch; 
  • G (or BSPP) - cylindrical pipe thread, has a triangular outline of 55 degrees; 
  • Rp - pipe thread, cylindrical, female; 
  • R (or BSPT) - pipe thread, tapered, male; 
  • Rc - pipe thread, tapered, female.
British inch threads

What are the other inch threads designations?

Below you will find some other inch thread standards for specialised applications:

  • E - Edison thread, electro-technical; 
  • NC - special electro-technical thread; 
  • Pg - special installation thread, armoured; 
  • Rd - circular thread; 
  • RW, FG - bicycle thread; 
  • W - tapered thread for gas valves.

What are the dimensions of inch threads? Inch threads chart

You can check the dimensions of the various variants of inch threads in the chart below. It will allow you to convert inch threads into mm.

outer diameterdimensions [mm] inner diameterdimensions [mm] number of coils per inch thread pitch[mm] inch pipeBSPBSPT inchUNF UN UNS(JIC, ORFS) inch pipeNPTNPTF
9.3 ÷ 9.7 8.5 ÷ 8.9 28 (0.91) 1/8    
9.3 ÷ 9.7 8.5 ÷ 8.9 27 (0.95)     1/8”
10.9 ÷ 11.1 9.7 ÷ 10 20 (1.27)   7/16”-20  
12.4 ÷ 12.7 11.3 ÷ 11.6 20 (1.27)   1/2”-20  
12.9 ÷ 13.1 11.4 ÷ 11.9 19 (1.34) 1/4    
12.9 ÷ 13.1 11.4 ÷ 11.9 18 (1.41)     1/4”
14 ÷ 14.3 12.7 ÷ 13 18 (1.41)   9/16”-18  
15.5 ÷ 15.8 14.4 ÷ 14.7 18 (1.41)   5/8”-18  
16.3 ÷ 16.6 14.9 ÷ 15.4 19 (1.34) 3/8    
16.3 ÷ 16.6 14.9 ÷ 15.4 18 (1.41)     3/8”
17.1 ÷ 17.4 15.8 ÷ 16.1 16 (1.59)   11/16”-16  
18.7 ÷ 19 17.3 ÷ 17.6 16 (1.59)   3/4”-16  
20.3 ÷ 20.6 18.9 ÷ 19.3 16 (1.59)   13/16”-16  
20.5 ÷ 20.9 18.6 ÷ 19 14 (1.81) 1/2    
20.7 ÷ 21.1 18.3 ÷ 18.7 14 (1.81)     1/2”
22 ÷ 22.2 20.2 ÷ 20.5 14 (1.81)   7/8”-14  
22.6 ÷ 22.9 20.6 ÷ 21 14 (1.81) 5/8    
25.1 ÷ 25.4 23.4 ÷ 23.8 14 (1.81)   1”-14  
26.3 ÷ 26.7 23.7 ÷ 24.1 14 (1.81) 3/4   3/4”
26.6 ÷ 26.9 24.3 ÷ -24.7 12 2.12   1.1/16”-12  
29.8 ÷ 30.1 27.6 ÷ 27.9 12 (2.12)   1.3/16”-12  
29.8 ÷ 30.2 27.8 ÷ 28.1 14 (1.81) 7/8    
33 ÷ 33.2 30.3 ÷ 30.8 11 (2.31) 1    
33 ÷ 33.3 30.8 ÷ 31.2 12 (2.12)   1.5/16”-12  
32.9 ÷ 33.4 30.3 ÷ 30.8 11.5 (2.21)     1”
36.2 ÷ 36.5 34.3 ÷ 34.7 12 (2.12)   1.7/16”-12  
37.6 ÷ 37.9 34.8 ÷ 35.1 11 (2.31) 1.1/8    
40.9 ÷ 41.2 38.7 ÷ 39.1 12 (2.12)   1.5/8”-12  
41.5 ÷ 41.9 39 ÷ 39.5 11 (2.31) 1.1/4    
41.4 ÷ 42 39.2 ÷ 39.6 11.5 (2.21)     1. 1/4”
42.5 ÷ 42.8 40.6 ÷ 41   (2,12)   1.11/16”-12  
47.3 ÷ 47.6 45.1 ÷ 45.5 12 (2.12)   1.7/8”-12  
47.4 ÷ 47.8 44.8 ÷ 45.3 11 (2.31) 1.1/2    
47.3 ÷ 47.9 45.1 ÷ 45.5 11.5 (2.21)     1. 1/2”
50.5 ÷ 50.8 48.6 ÷ 49 12 (2.12)   2”-12  
59.4 ÷ 59.8 56.5 ÷ 56.8 11 (2.31) 2    
59.9 ÷ 60.2 56.4 ÷ 56.7 11.5 (2.21)     2”
63.3 ÷ 63.6 61.3 ÷ 61.8 12 (2.12)   2. 1/2”-12  
65.4 ÷ 65.7 62.7 ÷ 63 11 (2.31) 2.1/4    
72.7 ÷ 73 68.8 ÷ 69.1 8 (3.175)     2. 1/2”
74.9 ÷ 75.2 72.2 ÷ 72.5 11 (2.31) 2.1/2    
87.5 ÷ 87.9 84.9 ÷ 85.3 11 (2.31) 3    
88.5 ÷ 88.9 84.7 ÷ 85.1 8 (3.175)     3”
112.6 ÷ 113 110.1 ÷ 110.5 11 (2.31) 4    
113.9 ÷ 114.3 110.2 ÷ 110.6 8 (3.175)     4”
138 ÷ 138.4 135.5 ÷ 135.9 11 (2.31) 5    
139.4 ÷ 139.7 127.5 ÷ 127.9   9.7   5. 1/2”- (DIN 11)  
139.8 ÷ 141.3   8 (3.175)     5”
163.4 ÷ 163.8 160.9 ÷ 161.4 11 (2.31) 6    
167.8 ÷ 168.3   8 (3.175)     6”
218.5 ÷ 219   8 (3.175)     8”

How to measure an inch thread with a calliper?

You will measure an inch thread with a calliper as follows:

  • grasp the thread at its widest point so that the jaws of the calliper rest on top of the coils; 
  • read the result on the scale.

This is how you will find out what the outer diameter is. On the other hand, if you need the inner diameter for an inch thread, you do the same, only you use the small jaws of the calliper, which you insert inside the thread.

As for measuring the pitch of the thread, it is as follows:

  • rest the jaws of the calliper in parallel on the thread so that they touch the top of two separate crests; 
  • read the distance; 
  • divide it by the number of roots between the calliper jaws.

The result will be the pitch of the thread in millimetres.

Photo of Bartosz Kułakowski - CEO of Hosetech sp. z o. o.

Articles author

Bartosz Kułakowski

CEO of Hosetech Sp. z o.o.

Bartosz Kulakowski is an industrial hoses and couplings specialist with over 10 years of experience. Bartosz has been present in the technical industry since 2013. He gained experience as a technical and commercial advisor in the sector of plastic conveyor belts, steel structures, industrial hoses, and connectors. Since 2016, he has specialized exclusively in hoses and connectors. In 2019 he opened his own business under the HOSETECH Bartosz Kulakowski brand and since July 2022 he has been the CEO of the capital company HOSETECH Sp z o. o. (LLC).

SPIS TREŚCI

What is an inch thread?

What are the designations of the inch thread?

What are the designations of the US inch threads?

What are the designations of the British inch threads?

What are the other inch threads designations?

What are the dimensions of inch threads? Inch threads chart

How to measure an inch thread with a calliper?

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