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ISO Metric (MM Threads)

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ISO Metric Thread

The ISO metric screw threads are the worldwide most commonly used type of general-purpose screw thread. They were one of the first international standards agreed upon when the International Organization for Standardization (ISO) was set up in 1947.

The design principles of ISO general-purpose metric screw threads ("M" series threads) are defined in international standard ISO 68-1. Each thread is characterized by its major diameter and its pitch. ISO metric threads consist of symmetric V-shaped thread. In the plane of the thread axis, the flanks of the V have an angle of 60° to each other.

  ISO Metric Thread Prefered Size

ISO 261 specifies a detailed list of preferred combinations of outer diameter and pitch for ISO metric screw threads. The 1st choice column follows the R10 series from ISO 3, while the 2nd choice column is rounded off values from the R20 series from ISO 3.

ISO 262 specifies a shorter list of thread dimensions - a subset of ISO 261.

Superfine pitch metric threads are occasionally used in automotive components, such as suspension struts, and are commonly used in the aviation manufacturing industry. This is because extra fine threads are more resistant to coming loose from vibrations.

  Thread Count (TPI)

TPI stands for Threads Per Inch, which means the total number of threads per inch measured along the length of a fastener. TPI is used only with American fasteners. Metric Fasteners use a thread Pitch. In general, smaller fasteners have finer threads, so the thread count is higher.

  Thread Pitch

Metric fasteners are specified with a thread pitch, instead of a thread count. The thread pitch is the distance between threads expressed in millimeters. In general, smaller fasteners have finer threads, so they have a lower thread pitch.

  ISO Metric Thread Chart | Metric Thread Dimension

ISO Metric profile External Thread Internal Thread
Thread Major Diameter Pitch Diameter Minor Diameter Minor Diameter Pitch Diameter Major Diameter Tap
Size x Pitch max min max min max min min max min max min max Drill
M 2 x 0.4 1.981 1.886 1.721 1.654 1.548 1.408 1.567 1.679 1.74 1.83 2 2.148 1.6
M 2.2 x 0.45 2.18 2.08 1.888 1.817 1.693 1.54 1.713 1.838 1.908 2.003 2.2 2.36 1.75
M 2.3 x 0.45 2.28 2.18 1.988 1.917 1.793 1.64 1.813 1.938 2.008 2.103 2.3 2.46 1.85
M 2.5 x 0.45 2.48 2.38 2.188 2.117 1.993 1.84 2.013 2.138 2.208 2.303 2.5 2.66 2.05
M 2.6 x 0.45 2.58 2.48 2.288 2.217 2.093 1.94 2.113 2.238 2.308 2.393 2.6 2.75 2.15
M 3 x 0.5 2.98 2.874 2.655 2.58 2.439 2.272 2.459 2.599 2.675 2.775 3 3.172 2.5
M 3.5 x 0.6 3.479 3.354 3.089 3.004 2.829 2.635 2.85 3.01 3.11 3.222 3.5 3.699 2.9
M 4 x 0.7 3.978 3.838 3.523 3.433 3.22 3.002 3.242 3.422 3.545 3.663 4 4.219 3.3
M 4.5 x 0.75 4.478 4.338 3.991 3.901 3.666 3.439 3.688 3.878 4.013 4.131 4.5 4.726 3.75
M 5 x 0.8 4.976 4.826 4.456 4.361 4.11 3.869 4.134 4.334 4.48 4.605 5 5.24 4.2
M 5.5 x 0.5 5.48 5.374 5.155 5.065 4.939 4.757 4.959 5.099 5.175 5.295 5.5 5.692 5
M 6 x 1 5.974 5.794 5.324 5.212 4.891 4.596 4.917 5.153 5.35 5.5 6 6.294 5
M 7 x 1 6.974 6.794 6.324 6.212 5.891 5.596 5.917 6.153 6.35 6.5 7 7.294 6
M 8 x 1.25 7.972 7.76 7.16 7.042 6.619 6.272 6.647 6.912 7.188 7.348 8 8.34 6.75
M 9 x 1.25 8.972 8.76 8.16 8.042 7.619 7.272 7.647 7.912 8.188 8.348 9 9.34 7.75
M 10 x 1.5 9.968 9.732 8.994 8.862 8.344 7.938 8.376 8.676 9.026 9.206 10 10.396 8.5
M 11 x 1.5 10.97 10.73 9.994 9.862 9.344 8.938 9.376 9.676 10.026 10.196 11 11.387 9.5
M 12 x 1.75 11.97 11.7 10.83 10.68 10.072 9.601 10.106 10.441 10.863 11.063 12 12.453 10.25
M 14 x 2 13.96 13.68 12.66 12.5 11.797 11.271 11.835 12.21 12.701 12.913 14 14.501 12
M 15 x 1.5 14.97 14.73 13.99 13.85 13.344 12.93 13.376 13.676 14.026 14.216 15 15.407 13.5
M 16 x 2 15.96 15.68 14.66 14.5 13.797 13.271 13.835 14.21 14.701 14.913 16 16.501 14
M 17 x 1.5 16.97 16.73 15.99 15.85 15.344 14.93 15.376 15.676 16.026 16.216 17 17.407 15.5
M 18 x 2.5 17.96 17.62 16.33 16.16 15.252 14.624 15.294 15.744 16.376 16.601 18 18.585 15.5
M 20 x 2.5 19.96 19.62 18.33 18.16 17.252 16.624 17.294 17.744 18.376 18.6 20 20.585 17.5
M 22 x 3 21.95 21.58 20 19.82 18.704 17.97 18.752 19.252 20.051 20.296 22 22.677 19
M 24 x 3 23.95 23.58 22 21.8 20.704 19.955 20.752 21.252 22.051 22.316 24 24.698 21
M 25 x 2 24.96 24.68 23.66 23.49 22.797 22.261 22.835 23.21 23.701 23.925 25 25.513 23
M 26 x 1.5 25.97 25.73 24.99 24.84 24.344 23.92 24.376 24.676 25.026 25.226 26 26.417 24.5
M 27 x 3 26.95 26.58 25 24.8 23.704 22.955 23.752 24.252 25.051 25.316 27 27.698 24
M 28 x 2 27.96 27.68 26.66 26.49 25.797 25.261 25.835 26.21 26.701 26.925 28 28.513 26
M 30 x 3.5 29.95 29.52 27.67 27.46 26.158 25.306 26.211 26.771 27.727 28.007 30 30.785 26.5
M 32 x 2 31.96 31.68 30.66 30.49 29.797 29.261 29.835 30.21 30.701 30.925 32 32.513 30
M 33 x 3.5 32.97 32.54 30.7 30.48 29.179 28.327 29.211 29.771 30.727 31.007 33 33.785 29.5
M 35 x 1.5 34.97 34.73 33.99 33.84 33.344 32.92 33.376 33.676 34.026 34.226 35 35.416 33.5
M 36 x 4 35.94 35.47 33.34 33.12 31.61 30.654 31.67 32.27 33.402 33.702 36 36.877 32
M 38 x 1.5 37.97 37.73 36.99 36.84 36.344 35.92 36.376 36.676 37.026 37.226 38 38.417 36.5
M 39 x 4 38.94 38.47 36.34 36.12 34.61 33.654 34.67 35.27 36.402 36.702 39 39.877 35
M 40 x 3 39.95 39.58 38 37.8 36.704 35.955 36.752 37.252 38.051 38.316 40 40.698 37
M 42 x 4.5 41.94 41.44 39.01 38.78 37.066 36.006 37.129 37.799 39.077 39.392 42 42.965 37.5
M 45 x 4.5 44.94 44.44 42.01 41.78 40.066 39.006 40.129 40.799 42.077 42.392 45 45.965 40.5
M 48 x 5 47.93 47.4 44.68 44.43 42.516 41.351 42.587 43.297 44.752 45.087 48 49.057 43
M 50 x 4 49.94 49.47 47.34 47.11 45.61 44.642 45.67 46.27 47.402 47.717 50 50.892 46
M 52 x 5 51.93 51.4 48.68 48.45 46.516 45.365 46.587 47.297 48.752 49.067 52 53.037 47
M 55 x 4 54.94 54.47 52.34 52.11 50.61 49.642 50.67 51.27 52.402 52.717 55 55.892 51
M 56 x 5.5 55.93 55.37 52.35 52.09 49.971 48.7 50.046 50.796 52.428 52.783 56 57.149 50.5
M 58 x 4 57.94 57.47 55.34 55.11 53.61 52.642 53.67 54.27 55.402 55.717 58 58.892 54
M 60 x 5.5 59.93 59.37 56.35 56.09 53.971 52.7 54.046 54.796 56.428 56.783 60 61.149 54.5
M 62 x 4 61.94 61.47 59.34 59.11 57.61 56.642 57.67 58.27 59.402 59.717 62 62.892 58
M 63 x 1.5 62.97 62.73 61.99 61.83 61.344 60.91 61.376 61.676 62.026 62.238 63 63.429 61.5
M 64 x 6 63.92 63.32 60.02 59.74 57.425 56.047 57.505 58.305 60.103 60.478 64 65.241 58
M 65 x 4 64.94 64.47 62.34 62.11 60.61 59.642 60.67 61.27 62.402 62.717 65 65.892 61
M 68 x 6 67.92 67.32 64.02 63.74 61.425 60.047 61.505 62.305 64.103 64.478 68 69.241 62
M 70 x 6 69.92 69.32 66.02 65.74 63.425 62.047 63.505 64.305 66.103 66.478 70 71.241 64
M 72 x 6 71.92 71.32 68.02 67.74 65.425 64.047 65.505 66.305 68.103 68.478 72 73.241 66
M 75 x 6 74.92 74.32 71.02 70.74 68.425 67.047 68.505 69.305 71.103 71.478 75 76.241 69
M 76 x 6 75.92 75.32 72.02 71.74 69.425 68.047 69.505 70.305 72.103 72.478 76 77.241 70
M 78 x 2 77.96 77.68 76.66 76.48 75.797 75.251 75.835 76.21 76.701 76.937 78 78.525 76
M 80 x 6 79.92 79.32 76.02 75.74 73.425 72.047 73.505 74.305 76.103 76.478 80 81.241 74
M 82 x 2 81.96 81.68 80.66 80.48 79.797 79.251 79.835 80.21 80.701 80.937 82 82.525 80
M 85 x 6 84.92 84.32 81.02 80.74 78.425 77.047 78.505 79.305 81.103 81.478 85 86.241 79
M 90 x 6 89.92 89.32 86.02 85.74 83.425 82.047 83.505 84.305 86.103 86.478 90 91.241 84
M 95 x 6 94.92 94.32 91.02 90.72 88.425 87.027 88.505 89.305 91.103 91.503 95 96.266 89
M 100 x 6 99.92 99.32 96.02 95.72 93.425 92.027 93.505 94.305 96.103 96.503 100 101.27 94

  ISO Metric Thread Chart | ISO 262 selected sizes for screws, bolts and nuts

1st choice 2nd choice Coarse Fine
1 - 0.25 0.2
1.2 - 0.25 0.2
- 1.4 0.3 0.2
1.6 - 0.35 0.2
- 1.8 0.35 0.2
2 - 0.4 0.25
2.5 - 0.45 0.35
3 - 0.5 0.35
- 3.5 0.6 0.35
4 - 0.7 0.5
5 - 0.8 0.5
6 - 1 0.75
- 7 1 0.75
8 - 1.25 1 | 0.75
10 - 1.5 1.25 | 1
12 - 1.75 1.5 | 1.25
- 14 2 1.5
16 - 2 1.5
- 18 2.5 2 | 1.5
20 - 2.5 2 | 1.5
- 22 2.5 2 | 1.5
24 - 3 2
- 27 3 2
30 - 3.5 2
- 33 3.5 2
36 - 4 3
- 39 4 3
42 - 4.5 3
- 45 4.5 3
48 - 5 3
- 52 5 4
56 - 5.5 4
- 60 5.5 4
64 - 6 4

ISO Metric (MM Threads) Charts