When unit loads are stacked in multiple tiers, as the number of tiers increases, the space consumption per unit load decreases; at the same time, the number of relocations during retrieval operations increases. This study derives formulas fur estimating the expected number of relocations for retrieving a random unit load from various types of stacks with different combinations of the number of rows, the number of columns, and the number of tiers. Based on the derived formulas, the handling cost and the space cost of the different types of stacks are compared with each other.