What Is UNS Thread – UNS Thread Meaning, Standard, Fit Classes & Dimensions Chart

2025.6.26

The thread geometry is defined with different sizes in various systems. As a typical thread system, Unified Series includes UNC, UNF, UNR, and UNS. In this article, we’ll introduce the basics of UNS threads (designation meaning, reading, full form, angle, standard, and fit classes) and list the UNS thread chart for their dimensions (diameter, pitch, TPI, and tap drill sizes).

What Is UNS Thread? – UNS Thread Meaning and Full Form

The UNS Thread, with the full form being Unified Special Thread, is a designation from one of the major screw thread systems nowadays – The Unified (Inch) Screw Thread System. As the name replies, UNS threads are used for special purposes. The Unified series is mainly used in the United States and Canada. Even though both of them transfer to the ISO system, there are more than half of the people still use the Unified systems.

How to Read or Understand the UNS Thread Designation?

The designation consists of three parts:

For example, a thread designated as 1″ – 14 UNS means the thread has an outside diameter of 1 inch and it has 14 threads per inch. The pitch (distance between threads) is not one of the standard coarse, fine, or extra fine pitches but is a special pitch within a limited range.

UNS Thread Angle and Profile

The flank angle of a Unified Special Thread is 60°.

UNS Thread Standard

The UNS thread dimensions and specifications follow the standard ASME B1.1-2003.

UNC vs UNS Thread

UNC (Unified National Coarse) threads are the most commonly used and preferred for general purposes. UNC thread, which is characterized by coarse pitch, results in deeper threads that are easier to assemble and less prone to cross-threading. In contrast, UNS (Unified National Special) threads require the threads per inch (TPI) to be explicitly specified, as they offer a limited and non-standard range of pitches for a given diameter. While UNC threads have fixed, standardized coarse pitches for each diameter, UNS threads provide special pitch options that fall outside the typical coarse, fine, or extra fine categories to meet unique application needs.

Additionally, other Unified thread series include UN (Unified Constant Pitch), which covers a broad range of diameter-pitch combinations with TPI values like 4, 6, 8, 12, 16, 20, 28, and 32 for use cases where standard UNC, UNF (Unified National Fine), UNEF (Unified National Extra Fine), and UNS threads are not covered.

UNS (Unified) Thread Fit Classes

When it comes to the UNS thread types, they can be classified by the thread fit, which describes how tightly or loosely the male (external) and female (internal) threads fit together. It is controlled by the class of fit, which are tolerance/allowance combinations applied to both external and internal threads. Tolerances and allowances affect the maximum thickness of coatings that can be applied without interfering with thread engagement. Proper fit ensures the right balance between strength, ease of assembly, and sealing capability.

External threads are represented with A classes, including 1A (loose fit), 2A (medium fit), and 3A (tight fit), and internal threads can be divided into 1B (loose fit), 2B (medium fit), and 3B (tight fit) variations.

UNS Thread Dimensions Chart

Note:
1. The Unified Special Threads have TIPS specified. But the number of TPI for a specific diameter is limited, for example, within 1“ diameter size, the TPI allowed only includes 1“ – 10 UNS, 1“ – 14 UNS, 1“ – 18 UNS, 1“ – 24 UNS, and 1“ – 27 UNS.

2. The outside (major) diameter is often converted into millimeters for ease of use in metric systems, along with the pitch and tap drill sizes converted into millimeters.

Nominal Diameter Major Diameter (Inch) Major Diameter (MM) Tapping Drill Size (MM) TPI Pitch (MM)
10 – 28 UNS 0.1900 4.826 3.95 28 0.907
10 – 36 UNS 0.1900 4.826 4.15 36 0.706
10 – 40 UNS 0.1900 4.826 4.20 40 0.635
10 – 48 UNS 0.1900 4.826 4.35 48 0.529
10 – 56 UNS 0.1900 4.826 4.39 56 0.454
12 – 36 UNS 0.2160 5.486 4.80 36 0.706
12 – 40 UNS 0.2160 5.486 4.90 40 0.635
12 – 48 UNS 0.2160 5.486 4.98 48 0.529
12 – 56 UNS 0.2160 5.486 5.05 56 0.454
1/4″ – 24 UNS 0.2500 6.350 5.35 24 1.058
1/4″ – 27 UNS 0.2500 6.350 5.45 27 0.941
1/4″ – 36 UNS 0.2500 6.350 5.70 36 0.706
1/4″ – 40 UNS 0.2500 6.350 5.75 40 0.635
1/4″ – 48 UNS 0.2500 6.350 5.85 48 0.529
1/4″ – 56 UNS 0.2500 6.350 5.95 56 0.454
5/16″ – 27 UNS 0.3125 7.938 7.10 27 0.941
5/16″ – 36 UNS 0.3125 7.938 7.25 36 0.706
5/16″ – 40 UNS 0.3125 7.938 7.40 40 0.635
5/16″ – 48 UNS 0.3125 7.938 7.50 48 0.529
3/8″ – 18 UNS 0.3750 9.525 8.20 18 1.411
3/8″ – 27 UNS 0.3750 9.525 8.70 27 0.941
3/8″ – 36 UNS 0.3750 9.525 8.90 36 0.706
3/8″ – 40 UNS 0.3750 9.525 9.00 40 0.635
0.390″ – 27 UNS 0.3900 9.906 9.00 27 0.941
7/16″ – 18 UNS 0.4375 11.112 9.75 18 1.411
7/16″ – 24 UNS 0.4375 11.112 10.10 24 1.058
7/16″ – 27 UNS 0.4375 11.112 10.20 27 0.941
1/2″ – 12 UNS 0.5000 12.700 10.75 12 2.117
1/2″ – 14 UNS 0.5000 12.700 11.00 14 1.814
1/2″ – 18 UNS 0.5000 12.700 11.40 18 1.411
1/2″ – 24 UNS 0.5000 12.700 11.70 24 1.058
1/2″ – 27 UNS 0.5000 12.700 11.85 27 0.941
9/16″ – 14 UNS 0.5625 14.288 12.60 14 1.814
9/16″ – 27 UNS 0.5625 14.288 13.40 27 0.941
5/8″ – 14 UNS 0.6250 15.875 14.20 14 1.814
5/8″ – 27 UNS 0.6250 15.875 15.00 27 0.941
3/4″ – 14 UNS 0.7500 19.050 17.40 14 1.814
3/4″ – 18 UNS 0.7500 19.050 17.75 18 1.411
3/4″ – 24 UNS 0.7500 19.050 18.10 24 1.058
3/4″ – 27 UNS 0.7500 19.050 18.20 27 0.941
7/8″ – 10 UNS 0.8750 22.225 19.75 10 2.540
7/8″ – 18 UNS 0.8750 22.225 21.00 18 1.411
7/8″ – 24 UNS 0.8750 22.225 21.20 24 1.058
7/8″ – 27 UNS 0.8750 22.225 21.40 27 0.941
1″ – 10 UNS 1.0000 25.400 23.00 10 2.540
1″ – 14 UNS 1.0000 25.400 23.75 14 1.814
1″ – 18 UNS 1.0000 25.400 24.25 18 1.411
1″ – 24 UNS 1.0000 25.400 24.50 24 1.058
1″ – 27 UNS 1.0000 25.400 24.50 27 0.941
1 1/8″ – 10 UNS 1.1250 28.575 26.25 10 2.540
1 1/8″ – 14 UNS 1.1250 28.575 27.00 14 1.814
1 1/8″ – 24 UNS 1.1250 28.575 27.75 24 1.058
1 1/4″ – 10 UNS 1.2500 31.750 29.50 10 2.540
1 1/4″ – 14 UNS 1.2500 31.750 30.00 14 1.814
1 1/4″ – 24 UNS 1.2500 31.750 30.75 24 1.058
1 3/8″ – 10 UNS 1.3750 34.925 32.60 10 2.540
1 3/8″ – 14 UNS 1.3750 34.925 33.30 14 1.814
1 1/2″ – 10 UNS 1.5000 38.100 35.70 10 2.540
1 1/2″ – 14 UNS 1.5000 38.100 36.50 14 1.814
1 1/2″ – 24 UNS 1.5000 38.100 37.30 24 1.058
1 5/8″ – 10 UNS 1.6250 41.275 39.00 10 2.540
1 5/8″ – 14 UNS 1.6250 41.275 39.50 14 1.814
1 5/8″ – 24 UNS 1.6250 41.275 40.35 24 1.058
1 3/4″ – 10 UNS 1.7500 44.450 42.00 10 2.540
1 3/4″ – 14 UNS 1.7500 44.450 42.85 14 1.814
1 3/4″ – 18 UNS 1.7500 44.450 43.25 18 1.411
1 7/8″ – 10 UNS 1.8750 47.625 45.30 10 2.540
1 7/8″ – 14 UNS 1.8750 47.625 46.00 14 1.814
1 7/8″ – 18 UNS 1.8750 47.625 46.35 18 1.411
2″ – 10 UNS 2.0000 50.800 48.40 10 2.540
2″ – 14 UNS 2.0000 50.800 49.25 14 1.814
2″ – 18 UNS 2.0000 50.800 49.50 18 1.411
2 1/16″ – 16 UNS 2.0625 52.387 51.00 16 1.587
2 3/16″ – 16 UNS 2.1875 55.563 54.10 16 1.587
2 1/4″ – 10 UNS 2.2500 57.150 54.75 10 2.540
2 1/4″ – 14 UNS 2.2500 57.150 55.50 14 1.814
2 1/4″ – 18 UNS 2.2500 57.150 55.75 18 1.411
2 5/16″ – 16 UNS 2.3125 58.737 57.30 16 1.587
2 7/16″ – 16 UNS 2.4375 61.912 60.50 16 1.587
2 1/2″ – 10 UNS 2.5000 63.500 61.20 10 2.540
2 1/2″ – 14 UNS 2.5000 63.500 61.75 14 1.814
2 1/2″ – 18 UNS 2.5000 63.500 62.30 18 1.411
2 3/4″ – 10 UNS 2.7500 69.850 67.50 10 2.540
2 3/4″ – 14 UNS 2.7500 69.850 68.25 14 1.814
2 3/4″ – 18 UNS 2.7500 69.850 68.50 18 1.411
3″ – 10 UNS 3.0000 76.200 73.80 10 2.540
3″ – 14 UNS 3.0000 76.200 74.50 14 1.814
3″ – 18 UNS 3.0000 76.200 75.00 18 1.411
3 1/4″ – 10 UNS 3.2500 82.550 80.25 10 2.540
3 1/4″ – 14 UNS 3.2500 82.550 81.00 14 1.814
3 1/4″ – 18 UNS 3.2500 82.550 81.25 18 1.411
3 1/2″ – 10 UNS 3.5000 88.900 86.55 10 2.540
3 1/2″ – 14 UNS 3.5000 88.900 87.25 14 1.814
3 1/2″ – 18 UNS 3.5000 88.900 87.60 18 1.411
3 3/4″ – 10 UNS 3.7500 95.250 93.00 10 2.540
3 3/4″ – 14 UNS 3.7500 95.250 93.60 14 1.814
3 3/4″ – 18 UNS 3.7500 95.250 94.00 18 1.411
4″ – 10 UNS 4.0000 101.600 99.30 10 2.540
4″ – 14 UNS 4.0000 101.600 100.00 14 1.814
4 1/4″ – 10 UNS 4.2500 107.950 105.60 10 2.540
4 1/4″ – 14 UNS 4.2500 107.950 106.30 14 1.814
4 1/2″ – 10 UNS 4.5000 114.300 112.00 10 2.540
4 1/2″ – 14 UNS 4.5000 114.300 112.65 14 1.814
4 3/4″ – 10 UNS 4.7500 120.650 118.35 10 2.540
4 3/4″ – 14 UNS 4.7500 120.650 119.00 14 1.814
5″ – 10 UNS 5.0000 127.000 124.70 10 2.540
5″ – 14 UNS 5.0000 127.000 125.35 14 1.814
5 1/4″ – 10 UNS 5.2500 133.350 131.00 10 2.540
5 1/4″ – 14 UNS 5.2500 133.350 131.70 14 1.814
5 1/2″ – 10 UNS 5.5000 139.700 137.40 10 2.540
5 1/2″ – 14 UNS 5.5000 139.700 138.00 14 1.814
5 3/4″ – 10 UNS 5.7500 146.050 143.75 10 2.540
5 3/4″ – 14 UNS 5.7500 146.050 144.40 14 1.814
6″ – 10 UNS 6.0000 152.400 150.00 10 2.540
6″ – 14 UNS 6.0000 152.400 150.75 14 1.814

 

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