You will need two materials. One for drainage, one for soil media. 24" = 2ft. You are going to fill the mix up to 23", leaving a lip at the top of the planter. 2ft. x 1.91ft x 13ft = 49.66 cubic feet of material or just about 1.8cu. yds. of material. Ideally, I would fill 15 inches deep with crush stone or coarse sand. This is 1.17cu.yd. This, added to the 1" of sand gives you a base of 16". .63 Cu.yds. of potting soil, ideally a Miracle Gro potting mix, will be added to the planter. What remains is 16" of base, 7" of potting soil, and a 1" gap to the top of the planter. For purchasing: 1 cu. yd. equals 27 cubic feet. You can round if needed, but typically, the potting soil comes in 3cu.ft. bags so you'll need 6+/-. The sand or crushed stone come in 1cu.ft. bags. You'll need 27. Hope that helps.