Mark out the area for your wall with stakes and string.
Set down the blocks for your wall 12 inches out from the dirt hill you are using the wall to retain.
Measure the length of the wall.
Measure the width of the wall, measuring from the back of the blocks to the dirt wall.
Determine the height of the wall.
Multiply the length, width and depth of the wall. Divide the number by 27 to get the cubic yards of gravel required for your wall.