They look similar coming off the truck. Their use cases don't overlap. Using the wrong one under a slab is a common and expensive mistake.
Topsoil is the upper layer of earth — dark, organic, nutrient-rich. Fill dirt is subsoil, typically marl or clay-based material from below the organic layer. Both are sold by the cubic yard. Their use cases do not overlap, and confusing them is a common site error.
Fill dirt goes in first. If you're raising grade, filling a void, or building up the base below a slab or structure, use fill dirt. It compacts well, holds load, and doesn't shrink or shift over time the way organic-heavy soil does. Never use topsoil as structural fill — it will settle and cause cracking.
Topsoil is a finish material. It goes on top: 4–6 inches for lawn establishment, deeper for garden beds and planting areas. Mix with compost for heavy planting. Keep it off anything that carries load. If you're not sure which material your job needs, call and describe what you're doing — we'll tell you what to order.