You are calculating cubic feet correctly! A raised bed of that size requires about 24 cubuc feet of material to fill it 9 inches deep. I believe the confusion is coming from the fact that in the soil recipe above, we call for 4 bags of topsoil and each bag contains 2 cubic feet (8 cubic feet of topsoil total). The same is true for the other ingredients: 2 pails of coir at 3 cubic feet each (6 cubic ft of coconut coir total) and 2 bags of compost at 2-3 cubic ft each (about 6 cubic ft of compost total). The leaves then equal about 5.5 cubic feet. Altogether, that makes about 25 cubic feet of material.
You are calculating cubic feet correctly! A raised bed of that size requires about 24 cubuc feet of material to fill it 9 inches deep. I believe the confusion is coming from the fact that in the soil recipe above, we call for 4 bags of topsoil and each bag contains 2 cubic feet (8 cubic feet of topsoil total). The same is true for the other ingredients: 2 pails of coir at 3 cubic feet each (6 cubic ft of coconut coir total) and 2 bags of compost at 2-3 cubic ft each (about 6 cubic ft of compost total). The leaves then equal about 5.5 cubic feet. Altogether, that makes about 25 cubic feet of material.