If you search on the Premium Aquatics forum, there is a figure you use to plug into the sandbed calculator, and it tells you what you need. I want to say it was 84, or .84 or somehting like that. Basically, you input your tank footprint, that calculation factor, and the desired thickness, and it tells you what you need. In my case, the 90lbs should give me 1.25-1.5" but probably more because I will be placing rock directly on the bottom. Its pretty simple if you search for it. There is actually a large Reef Flakes thread tha has those figures.