I once did all math in my head, but at 70 the body still good but the mind not so.?

Building decorative wall containment for plant bed. Math is; the wall drainage rock that goes between backside of wall and planter soil fill cubic space is ; 14.5 feet long X 1 foot dept X 1.33 feet high, my mind said 19.285 cubic feet, but that doesn't sound right to me. How many cubic feet of drain rock do I need??