An approximation is 10 lbs will cover 1 square foot to a depth of 1″.
The pros and cons of a deep sand bed (DSB) are another of the many (and often frustrating) debates. Some say a DSB is just a “ticking time bomb” because of the toxins that can build-up. As a result, many go bare bottom, or an inch or so for the look. Consider the animals you want to keep for depth of the sand bed.
For a new tank with a complete sand and rock cycle, consider a dry “sand” (actually calcium carbonate) like Arag Dry shipped in dry bags.
For a quick switch from one mature tank to a new setup, where most everything from the old tank goes to the new one, EXCEPT the sand, consider starting with Arag Live which contains some beneficial bacteria already in the wet bag. Note that a 20 lb bag of Live gives about 18 lbs of sand because of the water content in the shipping bag.
The oolite grade (pic to the left) for sand sifters and sand diving animals (the spherical shaped sand “flows” like ball bearings), and standard grade (granules with some angles) with some rubble and shells for burrowing animals that dig caves that need support. The oolite would likely just cave-in, or flow in and around the rubble caves.