What defines the efficiency of a building in terms of usable space versus total space?

The efficiency of a building in terms of usable space versus total space is best defined by the efficiency ratio. This term specifically quantifies how much of the total building area is actually usable for its intended purpose. In architectural and space planning contexts, the efficiency ratio is crucial because it directly impacts occupancy costs, functionality, and the overall design effectiveness of the building.

While other terms like utilization factor, space optimization, and load factor might relate to aspects of design efficiency, they don't directly measure the specific relationship between usable and total space in the way that efficiency ratio does. The utilization factor often pertains to how effectively a space is used, space optimization refers to arranging or planning space for maximum effectiveness, and load factor typically deals with the allocation of areas based on structural and mechanical loads rather than spatial efficiency. Thus, the efficiency ratio uniquely addresses the balance between usable and total areas, making it the correct choice for this question.