How deep should topsoil be?
For a new lawn from seed, plan on 4–6 inches of quality topsoil — grass roots mostly live in the top 4 inches. Raised vegetable beds want 8–12 inches of soil to be genuinely productive. Filling sunken spots in an existing lawn is usually a 1–2 inch topdressing job, done in thin layers so the grass grows through.
Not all delivered 'topsoil' is topsoil
Screened topsoil should pass through a half-inch screen and be free of construction rubble. Cheap fill dirt is exactly that — fill. When you call the supplier, ask what the soil is screened to and whether it is blended with compost; for garden beds, a 60/40 topsoil-compost blend is worth the upcharge.
Order 10% extra
Topsoil settles as organic matter breaks down and rain compacts it, and every wheelbarrow run loses a little. Like everything on this site, the calculator gives the exact mathematical volume — the real world eats about a tenth of it.