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Get to Know Metric Bolt Sizes

Nuts and bolts are essential fastenings, critical in the safe assembly of machinery, equipment, furniture and much more. In most circumstances, it’s absolutely vital that the right size of fastening is used. In this brief guide we provide an introduction to standard ISO metric sizes for nuts and bolts which will hopefully help dispel any confusion.

BS3643 Parts 1&2

BS3643 defines the limits and tolerances for ISO screw thread gauges. The standard is divided into two parts. Part 1 is based on ISO 965/1 and ISO 965/3 providing the principles and basic data for specifications of ISO metric screw threads. 

And BS3643 part 2 provides tabulated diameters and tolerances for coarse and fine pitch threads, based on ISO 965/2.

Metric Bolt Sizes

Metric nuts and bolts are commonly referenced using ‘M’ sizes, for example: M3, M8, M12. But the size of a metric fastener is more accurately specified using diameter, pitch and length dimensions, in millimeters. For nuts the size dimensions used are simply diameter and pitch.

Here’s an example metric bolt specification: M8-1.0 x 20

This bolt specification breaks down like this:

• The letter ‘M’ indicates that this bolt uses a metric thread.
• The number ‘8’ refers to the nominal diameter of the bolt shaft in millimeters.
• The number 1.0 is the thread pitch of the bolt, which is the distance between threads, in millimeters.
• And the final figure, 20, is the length in millimeters.

But it’s very common to see metric bolts specified like this: M12-50.

This abbreviated format omits the pitch definition which means that the bolt has a coarse thread. Whenever the pitch dimension is omitted from a metric bolt specification then the bolt is always coarse threaded. This example (M12-50) has a diameter of 12mm and a length of 50mm.

Metric Thread Pitch

As noted, the specification of metric bolts includes the definition of thread pitch, but if this is omitted then a coarse pitch is assumed. Metric fasteners are available with either coarse or fine thread pitches with some metric bolt sizes offering extra fine thread pitch options. It is worth noting that metric bolts with a coarse thread pitch have less threads per inch (they are more closely spaced) than comparable imperial bolts.

Metric Bolt Lengths

The length of a metric bolt is measured and defined in exactly the same way as imperial, inch-based bolts and fasteners. Socket head, pan head, button head and hex head bolts are measured from the underside of the bolt head to the end of the shaft. For flat head bolts, the length includes the bolt head height and for dome head bolts the length is measured from the thickest, highest point on the curved head.

Metric bolts and fasteners are manufactured in conformance with standards set by the International Organisation for Standardisation (ISO) and the German Institute for Standardisation (DIN). Here’s a list of the most common DIN and ISO numbers and what they describe.

Hole Sizes for Metric Bolts

The defined diameter of metric bolts is actually slightly larger than the actual diameter of the bolt shaft. So an M8 bolt would have a shaft diameter that’s slightly under 8mm which means that the bolt should fit through an 8mm hole. But it’s common practice to drill clearance holes that are slightly larger to allow for misalignment.

When it’s necessary to cut a thread then the hole size needs to be a smaller diameter to allow for the thread to be tapped. The necessary tapping hole size can be easily calculated by deducting the thread pitch from the metric bolt diameter. For example, an M8 bolt with a standard coarse thread pitch of 1.25mm would require a hole of 6.75mm diameter to be tapped. But if the M8 bolt had a fine pitch thread of 1mm then the diameter of the hole required for tapping would be 7mm.

The following table provides examples of clearance hole sizes, standard or coarse thread pitches along with fine thread pitches and their corresponding tapping hole sizes.

Standard Spanner and Allen Key Sizes

Using the correct tool for the job is always essential. It’s important to use the right size spanner or allen key in order to avoid slipping and potentially causing damage to the nut or bolt by rounding off the edges.

Here’s a short list of standard spanner and allen key sizes that apply to the most commonly used metric nuts and bolts.

If you have any questions or uncertainty regarding the fasteners you need then remember that we are here to help. Give us a call on 01273 475500 and we’ll provide you with free, expert advice and guidance.

This message was added on Friday 19th June 2020