Sand is usually measured/sold by the cubic yard, or by the ton.
here's a easy way for you to calculate how much sand you will need.
1 cubic yard of sand will cover:
80 Square feet @ 4" deep
90 Square feet @ 3" deep
100 Square feet @ 2" deep
110 Square feet @ 1" deep
btw- if you are going to go buy some, you will probably get a better price if you call a concrete company and ask to buy it from them, rather than getting bags at a home or garden store. ( I know this because I own a concrete company ).
Also, if you look in your yellow pages under concrete, and look up "Short-load" or "Mobile-mix" or "Mix on site" you can probably go to their yard and they will put a scoop in your truck with their loader.
But one thing to remember is that 1 cubic yard of sand weighs approx. 3500 lbs., so dont think you are going to go get a whole cubic yard in the back of your little Toyota or Nissan pick-up truck. Use a full sized 3/4 or 1 ton pick-up truck.
hope this helps,
Ron