Formula to determine weight needed for a certain size

I'm as new to this as one can be. My goal is to make myself a bookmark using various gold pieces I have. My question: Is there a formula to determine accurately the amount, by weight, of a metal, (in this case gold), needed to fill a predetermined space? Basically, I want to know how much gold I should melt in order to fill a small thin rectangle sized casting. I've probably used the wrong terminology, but I hope the idea is clear enough. And I'm not yet sure that casting is feasible but am thinking such a formula might be useful regardless. Thanks.