One cubic yard covers about 108 ft² at 3 inches deep, or 324 ft² at 1 inch.
This view answers the reverse question: how far does a yard of topsoil actually spread? At 3 inches deep a cubic yard covers about 108 ft², so it's easy to under-order for a big lawn top-dress.
Why order extra? The headline number is the exact material your project needs. When you actually order, add about 5–10% on top — the “with 5% waste” figure in the result is a safe starting point. That margin covers what you lose to compaction, cuts around edges and corners, and ordinary spillage during handling. It is cheaper to have a little left over than to stop mid-job and wait on a second delivery.