Multiply the length, width, and depth (all in feet) to get cubic feet, then divide by 27 to convert to cubic yards. Fill dirt is sold by the cubic yard.
Fill dirt and topsoil serve completely different roles, and using the wrong one creates problems that surface months later. Fill dirt is subsoil excavated from below the topsoil layer. It contains little to no organic matter, compacts firmly, and holds its shape under load. It is the right material for raising grade, filling low spots, building pads, and backfilling around foundations. Topsoil is the dark, nutrient-rich surface layer used for planting. If you use it as structural fill, the organic matter breaks down over time and the grade sinks. A good approach is to use fill dirt to build to within 4 to 6 inches of the final grade, then finish with topsoil for any planting areas.
Clean fill dirt, meaning subsoil free of debris, rocks, and contaminants, costs more than general fill but is required near waterways and in many jurisdictions. Many excavation contractors offer clean fill dirt at low cost or even free since they need to dispose of it from other job sites. If your project also involves stone or aggregate for drainage or a sub-base, run a stone calculator estimate at the same time to get a complete materials order in one trip.
The volume you calculate is the theoretical void you need to fill. In practice, loose fill placed at finished depth will compact under its own weight and rainfall, settling 10 to 20% over the first few months. This means a 6-inch fill placed without compaction may end up at 5 inches after a season, leaving a low spot that reappears on a freshly graded lawn. To avoid this, either add 10-15% to your order upfront, or use mechanical compaction in 6-inch lifts during placement to eliminate air voids before the surface settles on its own.
For areas with variable fill depth, where one end needs 4 inches and the other needs 10, calculate each zone separately and add the totals rather than averaging across the whole footprint. Measure the fill depth needed at several points across the area, and use the average of those measurements as your depth input. Before verifying the total area, a square footage check of the project footprint ensures your length and width numbers are accurate before you commit to a delivery order.