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You build the boxes. You put the dirt you have into them. If you need more dirt you buy more dirt, if you don't need more dirt then you don't buy more dirt. Tada.

As others have said 1 cubic yard of dirt is 27 cubic feet of dirt. If the boxes were 1' x 1' x 9' internal dimensions then to completely fill the boxes would take 27 cubic feet of dirt. If you don't completely fill them (most people leave an inch or two space) then you need slightly less. If the 1' x 1' x 9' is the external dimensions of the boxes then you need slightly less dirt as well. Also if the cubic yard of dirt is compacted and you aerate the soil in the boxes (don't tamp it down hard) then you will also not use all of the cubic yard either. If reverse is true (aerated cubic yard, tamped boxes) then you would need slightly more.