4 cu. ft. Perlite The Home Depot
You don''t use JUST Perlite. You mix it 1/3 Perlite, 1/3 compost, and 1/3 peat moss. Just do the math buy this 4 cu ft of Peralit, 4 40lb bags of compost, and 4 cu ft of peat moss. Mix on a tarp. That amount makes 12 cu ft. If your garden is 10 inches deep (pretty typical), you will need 360 cu ft of material to fill an 18 ft x 24 ft garden.