How much soil does a raised bed take?
The volume is simply length × width × fill depth, with every dimension converted to feet before you multiply. A 4 ft × 8 ft bed filled 10 inches deep is 4 × 8 × 0.833 = 26.7 cubic feet. Two things trip people up. First, measure the inside of the frame: 2× lumber steals roughly three inches off each dimension, and on a small bed that is a real chunk of volume. Second, fill depth is not frame height — leave an inch or two of freeboard at the top so water and mulch stay in the bed instead of washing over the sides. If you are buying bagged soil, divide the cubic feet by the bag size printed on the bag; if you are buying bulk, divide by 27 to get cubic yards, because one cubic yard is 27 cubic feet.
That 27-to-1 ratio is exactly why bags stop making sense quickly. A single cubic yard is the equivalent of roughly fourteen 2-cubic-foot bags, and bulk soil sold by the yard is typically a fraction of the per-cubic-foot price of bagged product even after a delivery fee. The usual crossover point is somewhere around one to two cubic yards — below that, bags win because you avoid the delivery charge and a pile on the driveway; above it, ring a landscape supplier and get a quote per yard delivered. Worth asking two questions when you do: what the minimum delivery is (often one or two yards), and whether they will dump where you can actually reach it with a wheelbarrow.
For what goes in the bed, a widely used starting point is roughly equal thirds by volume: about a third compost for fertility, a third topsoil or a bagged garden-soil blend for body and mineral content, and a third something that keeps the mix open and draining — coarse perlite, vermiculite, coir or fine bark. Straight compost alone slumps and goes airless within a season; straight bagged potting mix is expensive and dries out fast in a large volume. Whatever you use, order about 10% extra. Fresh mix settles noticeably after the first few waterings, and every bed needs topping up with a couple of inches of compost each spring anyway.
Estimates for planning only. Bag volumes are as-packed and mixes compress differently, so treat the bag count as a minimum. If you are growing food, check what the soil or compost was made from — some municipal composts, manures and reclaimed soils carry persistent herbicide residues or elevated metals. Ask the supplier for test results, and if you are filling a bed over an old structure or driveway, consider testing the underlying ground too.