There is a easy calculation for this.

Take the total amount of gallons you need to fill and multiply it by   0.13370 to see how many cubic feet of soil you will need.

For example, if you have 25 x 45 gallon Smart Pots you would have 1,125 total gallons you need to fill.

So if you take 1,125 x 0.13370 that would equal 150.41 cubic feet of soil that you need to fill your pots.

Then it's simply choosing the soil you want( let's use Fox Farm Ocean Forest, which is 1.5 cubic feet in a bag) you would divide 150.41 by 1.5 which equals 100.27 bags of Ocean Forest you would need to fill your pots.