Unified Thread Standard

Unified Thread Standard

Bolt and nut

Figure 1: Bolt and nut

The Unified Thread Standard (UTS) is the main standard in the United States and Canada for the form and series of bolts, nuts, and other threaded fasteners. This standard, ASME B1.1-2019, is controlled by the American Society of Mechanical Engineers (ASME) and the American National Standards Institute (ANSI).

Table of contents

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UTS basic profile

Unified screw threads are defined by their major diameter (Figure 2 labeled H) and pitch (Figure 2 labeled A). These threads are symmetrical and have a 30° flank angle (Figure 2 labeled C), which is the angle from the crest (Figure 2 labeled D) to the groove (Figure 2 labeled E). Therefore, a UTS thread has a 60° profile similar to an ISO metric screw thread. The difference between UTS and ISO is that UTS measures pitch as an inch fraction, whereas ISO measures pitch in millimeters. In UTS, the pitch value is the reciprocal of the threads-per-inch (TPI). For example, if a bolt has 20 TPI, its pitch is 1/20” or 0.05”. Finally, UTS threads are single-start threads. Therefore, the pitch is equal to the thread’s lead, which is the distance a bolt travels when it rotates 360°. Read our thread design article for more details on each thread design parameter.

Thread dimensions: pitch (A), depth (B), flank angle (C), crest (D), groove (E), minor diameter (F), pitch diameter (G), and major diameter (H)

Figure 2: Thread dimensions: pitch (A), depth (B), flank angle (C), crest (D), groove (E), minor diameter (F), pitch diameter (G), and major diameter (H)

UTS designation

A standard UTS designation has a number that indicates the major diameter followed by a number that indicates the TPI. Following these two numbers is an indication of the thread’s coarseness: Unified Coarse Thread (UNC), Unified Fine Thread (UNF), or Unified Extra Fine Thread (UNEF). UTS designates threads with a ¼” major diameter and above using the diameter in the designation. For threads with smaller major diameters, UTS assigns a number. For example, a #1-64 UNC bolt has a major diameter of 0.0073” and a TPI of 64.

Unified Coarse Thread

UNC bolts are for mass production and applications that require rapid installation. The extra space between the threads allows for more variation in manufacturing and application without failure. A 1-inch UNC bolt has 8 TPI so is designated as 1-8 UNC.

Unified Fine Thread

UNF threads are for applications that require tight and fine threads, such as aerospace and automotive applications. A 1-inch UNF bolt has 12 TPI so is designated as 1-12 UNF.

Unified Extra Fine Thread

UNEF threads are for tapped holes in hard material, thin walls, and thin material. A 1-inch UNEF bolt has 20 TPI so is designated as 1-20 UNEF.

UTS thread tolerance

A number indicated the UTS thread tolerance may follow the fastener’s designation. UTS uses the following tolerance classes:

  • Class 1: These threads fit loosely. They are suitable for easy assembly and dirty work environments.
  • Class 2: These threads are free fit, meaning they have greater tolerance. They are the most common and maximize strength.
  • Class 3: These threads are common and have a closer tolerance for high-quality work.
  • Class 4: This class is more theoretical than practical. It was meant for a fit even closer than Class 3. It is now obsolete.
  • Class 5: Class 5 threads are interference threads. This means they require a wrench to screw and are meant for semi-permanent to permanent installations.

UTS charts

The following charts give information on coarse, fine, and extra fine Unified Threads. Click the button below to download a PDF copy of the charts.

Unified Coarse Thread

Nominal Diameter Major Diameter (in) Major Diameter (mm) TPI Pitch (in) Pitch (mm) Tap drill size
#0 0.06 1.524 - - - -
#1 0.073 1.854 64 0.016 0.397 #53
#2 0.086 2.184 56 0.018 0.454 #51
#3 0.099 2.515 48 0.021 0.529 5/64”
#4 0.112 2.845 40 0.025 0.635 #43
#5 0.125 3.175 40 0.025 0.635 #38
#6 0.138 3.505 32 0.031 0.794 #36
#8 0.164 4.166 32 0.031 0.794 #29
#10 0.19 4.826 24 0.042 1.058 #25
#12 0.216 5.486 24 0.042 1.058 #17
1⁄4″ 0.25 6.350 20 0.050 1.270 #7
5⁄16″ 0.3125 7.938 18 0.056 1.411 F
3⁄8″ 0.375 9.525 16 0.063 1.588 5/16”
7⁄16″ 0.4375 11.113 14 0.071 1.814 U
1⁄2″ 0.5 12.700 13 0.077 1.954 27/64”
9⁄16″ 0.5625 14.288 12 0.083 2.117 31/64”
5⁄8″ 0.625 15.875 11 0.091 2.309 17/32”
3⁄4″ 0.75 19.050 10 0.100 2.540 21/32”
7⁄8″ 0.875 22.225 9 0.111 2.822 49/64”
1″ 1 25.400 8 0.125 3.175 7/8”
1 1⁄8″ 1.125 28.575 7 0.143 3.629 63/64”
1 1⁄4″ 1.25 31.750 7 0.143 3.629 1-7/64”
1 3⁄8″ 1.375 34.925 6 0.167 4.233 1-11/32”
1 1⁄2″ 1.5 38.100 6 0.167 4.233 1-35/64’
1 3⁄4″ 1.75 44.450 5 0.200 5.080 1-25/32”
2″ 2 50.800 4 1⁄2 0.222 5.644 -
2 1⁄4″ 2.25 57.150 4 1⁄2 0.222 5.644 -
2 1⁄2″ 2.5 63.500 4 0.250 6.350 -
2 3⁄4″ 2.75 69.850 4 0.250 6.350 -
3″ 3 76.200 4 0.250 6.350 -
3 1⁄4″ 3.25 82.550 4 0.250 6.350 -
3 1⁄2″ 3.5 88.900 4 0.250 6.350 -
3 3⁄4″ 3.75 95.250 4 0.250 6.350 -
4″ 4 101.600 4 0.250 6.350 -

Unified Fine Thread

Nominal Diameter Major Diameter (in) Major Diameter (mm) TPI Pitch (in) Pitch (mm) Tap drill size
#0 0.06 1.524 80 0.013 0.318 3/64”
#1 0.073 1.854 72 0.014 0.353 #53
#2 0.086 2.184 64 0.016 0.397 #50
#3 0.099 2.515 56 0.018 0.454 #46
#4 0.112 2.845 48 0.021 0.529 #42
#5 0.125 3.175 44 0.023 0.577 #37
#6 0.138 3.505 40 0.025 0.635 #33
#8 0.164 4.166 36 0.028 0.706 #29
#10 0.19 4.826 32 0.031 0.794 #21
#12 0.216 5.486 28 0.036 0.907 #15
1⁄4″ 0.25 6.350 28 0.036 0.907 #3
5⁄16″ 0.3125 7.938 24 0.042 1.058 I
3⁄8″ 0.375 9.525 24 0.042 1.058 Q
7⁄16″ 0.4375 11.113 20 0.050 1.270 W
1⁄2″ 0.5 12.700 20 0.050 1.270 29/64”
9⁄16″ 0.5625 14.288 18 0.056 1.411 33/64”
5⁄8″ 0.625 15.875 18 0.056 1.411 37/64”
3⁄4″ 0.75 19.050 16 0.063 1.588 11/16”
7⁄8″ 0.875 22.225 14 0.071 1.814 13/16”
1″ 1 25.400 12 0.083 2.117 15/16”
1 1⁄8″ 1.125 28.575 12 0.083 2.117 1-3/64”
1 1⁄4″ 1.25 31.750 12 0.083 2.117 1-11/64”
1 3⁄8″ 1.375 34.925 12 0.083 2.117 -
1 1⁄2″ 1.5 38.100 12 0.083 2.117 1-59/64

Unified extra fine thread

Nominal Diameter Major Diameter (in) Major Diameter (mm) TPI Pitch (in) Pitch (mm) Tap drill size
#12 0.216 5.486 32 0.031 0.794 #16
1⁄4″ 0.25 6.350 32 0.031 0.794 #7
5⁄16″ 0.3125 7.938 32 0.031 0.794 F
3⁄8″ 0.375 9.525 32 0.031 0.794 5⁄16”
7⁄16″ 0.4375 11.113 28 0.036 0.907 U
1⁄2″ 0.5 12.700 28 0.036 0.907 27/64”
9⁄16″ 0.5625 14.288 24 0.042 1.058 31/64”
5⁄8″ 0.625 15.875 24 0.042 1.058 17/32”
3⁄4″ 0.75 19.050 20 0.050 1.270 21/32”
7⁄8″ 0.875 22.225 20 0.050 1.270 49/64”
1″ 1 25.400 20 0.050 1.270 7/8”

Read our articles on NPT and BSP standards for more information on the features of each standard.

FAQs

What is UNF thread standard?

Unified Fine Thread (UNF) standard is part of the Unified Thread Standard (UTS). It describes the major diameter and pitch of fine threaded fasteners used primarily in the United States and Canada.

What is UNC thread standard?

Unified Coarse Thread (UNC) standard is part of the Unified Thread Standard (UTS). It describes the major diameter and pitch of coarse threaded fasteners used primarily in the United States and Canada.

Are metric and UTS standards the same?

The ISO metric standard and UTS have the same 60° profile. However, metric measures in millimeters and UTS measures in inch fractions.

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