For bolts longer than 200mm - multiply the diameter by 2 and add 25mm – this is the minimum length that the thread is likely to be. For bolts up to 125mm long - multiply the diameter by 2 and add 6mm – this is the minimum length that the thread is likely to be. There are generally three dimensions that you need to know when sourcing bolts – diameter, length and thread length. Spanner size is also useful. For bolts between 125mm and 200mm long - multiply the diameter by 2 and add 12mm – this is the minimum length that the thread is likely to be.

On countersunk bolts the length is measured from the top of the head to the tip of the thread. Thread length

Details

On any bolt that isn’t countersunk, the length is measured from the underside of the head to the tip of the thread. These dimensions are highlighted below. At the end of this article is a table showing standard metric dimensions and spanner sizes. METRIC BOLTS - KEY DIMENSIONS