ISO (The International Organization for Standardization)

The design principles of ISO general-purpose metric screw threads ("M" series threads) are defined in international standard ISO 68-1.[2] Each thread is characterized by its major diameter, D (Dmaj 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 1⁄8 and the innermost 1⁄4 of the height H of the V-shape are cut off from the profile.


1 ISO metric fastener threads come in coarse, fine, and extra fine series, just as the U.S. inch system threads always have. The usage of coarse to fine threads of ISO threads is about the same as in the inch threads. The overall usage is in the vicinity of 90 percent coarse, 9 percent fine, and 1 percent extra fine. OMFS had no impact on usage in the long run.

Thread Chart

Thread Standards - A sound understanding of the information below will be useful to thoroughly understand and select appropriate thread types and sizes. Generally, there are two thread series, namely The ISO Metric Screw Thread (in millimetres) and The Unified Screw Thread (in inches). For both these series, there are two types of threads – fine thread and coarse thread. Between these two types of threads, fine threads of the same hardness are generally stronger as they are able to withstand larger forces under tension and in shear. Fine threads can be easily tapped into materials and are therefore ideal for thin sections of walls.

In materials of low tensile strength, the use of coarse threads provides more resistance from stripping when compared with fine or extra fine threads. Coarse threads have more resistance to failure via fatigue and allows for easier and quicker assembly of parts without the risk of cross threading.

The ISO metric screw thread system is the most widely used thread series. It is quite simple to identify and comprehend thread sizes that use the ISO metric thread designation. The sizes start with an “M” denoting that it is measured using metric thread designation. The “M” is followed by the first number, which is the major diameter of the external thread in millimetres. The second number corresponds to the pitch, which is the distance between adjacent threads. Finally, the third number represents the thread length.