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

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 On any bolt that isn’t countersunk, the length is measured from the underside of the head to the tip of the thread. 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.