Preferred Metric Thread Sizes ISO 261, titled ISO general purpose metric screw threads—General plan, is an international standard that defines a detailed list of preferred combinations of outer diameter D and pitch P for ISO metric screw threads. ISO 262, titled ISO general purpose metric screw threads—Selected sizes for screws, bolts and nuts, defines a shorter list of thread dimensions – a subset of the combinations listed in ISO 261 – which is listed below: Nominal diameter D (mm) Pitch P (mm) 1st choice 2nd choice coarse fine 1   0.25   1.2   0.25     1.4 0.3   1.6   0.35     1.8 0.35   2   0.4   2.5   0.45   3   0.5     3.5 0.6   4   0.7   5   0.8   6   1     7 1   8   1.25 1 10   1.5 1.25 or 1 12   1.75 1.5 or 1.25   14 2 1.5 16   2 1.5   18 2.5 2 or 1.5 20   2.5 2 or 1.5   22 2.5 2 or 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 The "coarse" pitch is the commonly-used default pitch for a given diameter. In addition, one or two smaller "fine" pitches are defined, for use in applications where the height of the normal "coarse" pitch would be unsuitable (e.g., threads in thin-walled pipes). The terms "coarse" and "fine" have in this context no relation to the manufacturing quality of the thread.   Basic profile   Basic profile of all ISO metric screw threads 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 D and its pitch P. ISO metric threads consist of a symmetric V-shaped thread. In the plane of the thread axis, the flanks of the V have an angle of 60° to each other. The outermost 1/8 and the innermost 1/4 of the height H of the V-shape are cut off from the profile.