How Long is the Thread Length of a Metric Bolt?

You may not know this, but there is actually a reasonably accurate rule of thumb that you can use to determine the length of the part of a metric bolt that is threaded.

Here’s how it works;

First, you need to know that equation differs depending on the overall size of the bolt, so make sure you use the right calculation from the three options below;

For Bolts up to 125mm in length

Multiply the diameter by 2 and add 6mm – this is the minimum length that the thread is likely to be.

FOR BOLT UP TO 125MM LONG

So for example, if this was an M12 bolt 50mm long, the minimum that the thread length is likely to be is 30mm (2 x 12 +6).

Remember though that this is a guide and not an absolute equation.

For Bolts between 125mm and 200mm in length

Multiply the diameter by 2 and add 12mm – this is the minimum length that the thread is likely to be.

FOR BOLTS BETWEEN 125MM & 200MM LONG

FOR BOLTS BETWEEN 125MM AND 200MM LONG

So for example, if this was an M12 bolt 130mm long, the minimum that the thread length is likely to be would be 36mm (2 x 12 +12).

For Bolts over 200mm in length

Multiply the diameter by 2 and add 25mm – this is the minimum length that the thread is likely to be.

FOR BOLTS OVER 200MM LONG

So for example, if this was an M12 bolt 220mm long, the minimum that the thread length is likely to be would be 49mm (2 x 12 +25).

All thread lengths will vary by manufacturer, but this is a rule of thumb that has proved to be fairly accurate in our experience.