Making heads and tails of bolt sizes: What size head is a M12 bolt?

Finding the right fastening for the job can be tricky. With hardware stores stocking every different variety of screw and bolt under the sun, it's easy enough to get confused.

And nobody wants to spend all that time and effort at the shop only to get back to the worksite to discover that whatever they've purchased isn't suitable for the task at hand.

You have to be particularly careful with screws and nails, as the slightest mismeasurement can throw your whole project out of line and cause delays as you work to source a new product. That’s not to mention the wasted expense.

Luckily, nuts and bolts have standardised naming conventions to indicate their particular features. In this article, we’re going to explore one of the less often thought-of dimensions of the bolt, the head size. First, let’s take a quick look at the M12 bolt.

What does M12 mean?

The M12 in the name refers to 12 millimetres. That’s not the diameter of the bolt’s head, though. It’s the diameter of the bolt’s thread and thus what you should be keeping in mind when measuring your hole. Other M-labelled bolts all follow the same system – an M16 refers to a bolt with a 16-millimetre diameter thread.

Note that this length can increase slightly near the head of the bolt, where there is no threading. On an M12 bolt, the diameter of the unthreaded bolt below the head can reach 12.7 millimetres.

What about head size?

The dimensions of the bolt’s head, however, are not clearly stated in the bolt’s name. That’s because the size of a bolt’s head can vary from bolt to bolt, even between bolts with the same diameter thread. That’s to cater to the wide range of different applications you might need your bolts for.

One bolt, for example, might have a round head with a diameter of 18 millimetres and a height of 9 millimetres. Another might be fitted with a hex head and have a width of between 20 and 21 millimetres, and a height between 7 and 8 millimetres.

It all depends on the exact specifications of the job you are working on and the type of tool you will be using to install the fastening. You always want to measure the working space surrounding your hole to ensure that the bolt is going to fit where you want it to go.

Torque

The other crucial factor that will determine the size of the head of a bolt is torque. Torque is the amount of force the bolt must bear to hold whatever is being fastened together. Applying the correct amount of torque is crucial to properly securing your join.

If a bolt is not strong enough for the job at hand, it will deform over time and may even fail completely, which can lead to structural weakness and the risk of catastrophic collapse. Thus, choosing the right bolt is a matter not just of professional standards but of safety as well.

The larger the bolt head, the more torque you will be able to apply to it. Bolt head size also makes up one element of what determines the bolt’s torque capacity, along with its depth, shaft dimensions, and material. The breaking pressure of a bolt is represented by a bolt’s PSI, or pounds per square inch. Bolts with a wider diameter will have a higher PSI.

It’s important to understand all the different factors which go into the selection of the right bolt. Correct bolt selection is an often overlooked but nevertheless crucial part of ensuring the success of your project. M12 bolts, like other standard bolts, can come with a variety of different head sizes, and their suitability will depend on your individual requirements.

How to Ensure You Get the Right Bolt

Bolt head dimensions are not often included in standard product listings from companies, so you might have to lookup more detailed technical specifications on a manufacturer’s website. And, of course, always have your tape measure handy.

The best thing to do if you’re unsure of what bolt to buy is to get in touch with our qualified professionals at Jaybro and talk to the experts.

We’ll be happy to go through the specifics of your project with you and recommend the best product for the job.