in my 48x18 90, I ended up with about 3"-4" of sand. To be honest, I've not done a ton of reading on sand beds, and I know alot of people believe that my sand is either too deep, or not deep enough, but I like the look of that much sand. It provides enough depth to set rocks down in deep, gives sand dwelling animals enough to be happy, etc.
I think I had about 120 pounds of sand to get my dimensions deep, but I was using someone elses old sand, and then two new bags of Caribsea "Ocean Direct Caribbean Live Sand". I only used the live sand because it was cheaper than dry sand at the time, but I like the grain size of it for the most part. It stays down, unless I get agressive with a glass scrubber.
It's reported to contain 77% or more at a grain size of less than 1mm.