given only the volume of the project...
if you mean cement sand AND gravel then the answer could be anything if you mean cement sand or gravel then i guese you would take the volume and devide it by how much one bage covers in volume but if you don't know how much volume one bag covers than you can't solve this problem with only knowing the volume of the project
There are a couple of rules of thumb for determining quantities required for a project. The old rule is a ratio of 1:3:4 Cement to Fine Aggregate (sand) to Coarse Aggregate. But this describes the mix proportions and since the cement and fine aggregate will disappear into the voids in the coarse aggregate another ratio is recommended. A common approach is by the bag mix ratio. A 6 or 7 bag mix will be quoted. These are ratios of numbers of regular bags of cement per cubic yard of mix. A 6 bag mix will normally require 6 bags of cement 15 cubic feet of fine aggregate and 20 cubic feet of coarse aggregate. A 7 bag mix will require 15 cubic feet of fine and 21 cubic feet of coarse aggregate. Why don't they add up to 27? Welcome to the wonderful world of concrete.
Too long for simple answer, see websites below:
A 12 X 8 is pretty big size, 96 square feet. If you want to use the bags, then on the bag it will tell you how many square feet it would cover. You would need at least 4 boards to build your frame, and then either re-bar or wire panels. The panels would be about $10.00 each. If it were me, I would build the frame and then call redy-mix company to come and pour the cement.
