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M12 Bolt Dimensions

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A bolt is a fastener used to attach parts together. Its external structure is threaded for a firm grip. Every bolt is produced according to standards and specifications. The size of each structure on a bolt is proportional to each other with a constant ratio. For example, the head size of a bolt is proportional to its diameter. If the head size is known, calculation of the diameter can be done to find out the strength of the bolt.

The dimensions of an M12 bolt can be described by the various properties listed below.


An M12 bolt has a major diameter of 12mm, which is measured from the thread. The bolt should be matched with an M12 nut and a washer with a 12mm-diameter hole.

Head size

The head size of a bolt is defined as the width across the hexagonal top. Different standards set different head sizes for bolts of all sizes.  According to the standards by Deutsches Institut für Normung (DIN), a bolt’s head-to-diameter ratio is 1.6:1. Therefore, an M12 bolt would have a head size 1.6 times its diameter, which is 19mm.

Thread length

The standard thread length of an M12 bolt longer than 125mm is 36mm while for bolts with the length of 125mm or shorter, the thread length is 30mm.

Thread pitch

Thread pitch refers to the distance between each thread on a bolt. For an M12 bolt, the size of the thread pitch is 1.75mm, 1.5mm, and 1.25mm for standard thread, fine thread and super fine thread respectively.

Mechanical properties

A mechanical property of a bolt comprises the proof load, minimum yield strength and minimum tensile strength. An M12 bolt of class 8.8 would have the load bearing properties of a 580MPa-proof load, a 640MPa-minimum yield strength and a 800MPa-minimum tensile strength. The bolt would fail its function when given loads beyond these values.

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