There's this website I know of that's pretty good for stuff like that. You can use it to find the specific weight of bags, as well as particle sizes of certain substrates. If you can find the volume of the desired substrate, you can measure your tank and get the volume of it. Then put the relevant information into the site and it can help you with calculations of how much you need.
I forget the name exactly but I remember it kinda looks like goggles.