There is a general rule in fillet weld that “the leg should be equal to the thickness of metals.” Why?

Let’s take an example of 6mm thick plates to illustrate the rule. In case 12mm leg is adopted in the fillet weld, the weld volume would be 3-4 times more than required. It would result in waste of weld metal and welder’s time. Worse still, over-welding may weaken the structure and result in distortion owing to the formation of residue stress. As such, the resulting weld could support less stress than fillet weld with “the leg equal to the thickness of metals.”

On the other hand, for welding the same 6mm thick plates, if 3mm leg is used instead, it is under-welded. The resulting weld may break through the leg of the weld.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.