ISO metric screw thread

The ISO metric screw thread is the most commonly used type of general-purpose screw thread worldwide. They were one of the first international standards agreed when the International Organization for Standardization (ISO) was set up in 1947.
The "M" designation for metric screws indicates the nominal outer diameter of the screw thread, in millimetres. This is also referred to as "major" diameter in the information below. It indicates the diameter of smooth-walled hole that the a male thread (eg on a bolt) will pass through easily to create a well-located connection to an internally threaded component (eg a nut) on the other side. That is, an M6 screw has a nominal outer diameter of 6 millimetres and will therefore be a well-located, co-axial fit in a hole drilled to 6mm diameter.
Basic profile.
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" ("D"maj in the diagram), 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 thread depth is 0.54125 × pitch. The outermost and the innermost of the height "H" of the V-shape are cut off from the profile.
The relationship between the height "H" and the pitch "P" is found using the following equation where "θ" is half the included angle of the thread, in this case 30°:
or
In an external (male) thread (e.g., on a bolt), the major diameter "D"maj and the minor diameter "D"min define "maximum" dimensions of the thread. This means that the external thread must end flat at "D"maj, but can be rounded out below the minor diameter "D"min. Conversely, in an internal (female) thread (e.g., in a nut), the major and minor diameters are "minimum" dimensions; therefore the thread profile must end flat at "D"min but may be rounded out beyond "D"maj.
The minor diameter "D"min and effective pitch diameter "D"p are derived from the major diameter and pitch as
Designation.
A metric ISO screw thread is designated by the letter M followed by the value of the nominal diameter "D" (the maximum thread diameter) and the pitch "P", both expressed in millimetres and separated by the hyphen sign, "-" (e.g., M8-1.25). If the pitch is the normally used "coarse" pitch listed in ISO 261 or ISO 262, it can be omitted (e.g., M8).
The length of a machine screw or bolt is indicated by a following "x" and the length expressed in millimetres (e.g., M8-1.25x30 or M8x30).
A common error or colloquialism is to use the 'x' or '/' when describing the pitch, so "M8x1.0" or "M8/1" might be seen where "M8-1.0" would be the clearer text to use. Similarly a 30mm bolt with this pitch might be written as "M8x1x30", where "M8-1.0x30 could be used.
Tolerance classes defined in ISO 965-1 can be appended to these designations, if required (e.g., M500– 6g in external threads). External threads are designated by lowercase letter, g or h. Internal threads are designated by upper case letters, G or H.
Preferred sizes.
ISO 261 specifies a detailed list of preferred combinations of outer diameter "D" and pitch "P" for ISO metric screw threads. ISO 262 specifies a shorter list of thread dimensions – a subset of ISO 261.
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.
In addition to coarse and fine threads, there is another division of extra fine, or "superfine" threads, with a very fine pitch thread. 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. Fine and superfine threads also have a greater minor diameter than coarse threads, which means the bolt or stud has a greater cross-sectional area (and therefore greater load-carrying capability) for the same nominal diameter.
Spanner (wrench) sizes.
Below are some common spanner (wrench) sizes for metric screw threads. Hex head widths (width across flats, spanner size) are for DIN 934 hex nuts and hex head bolts. Other (usually smaller) sizes may occur for reasons of weight and cost reduction.