How much soil does a raised bed take?
Volume is length × width × depth. An 8 × 4 ft bed that is 12 in deep holds 32 cubic feet, or about 1.2 cubic yards — sixteen 2-cubic-foot bags. Bulk soil is sold by the cubic yard (27 cubic feet), which is usually much cheaper than bags once you need more than about one yard.
Settling
Fresh, fluffy mixes with lots of compost settle 10–20% in the first season, so order a little extra or plan to top up with compost each spring.
What to fill a raised bed with
A common, reliable mix is about 60% screened topsoil or loam, 30% finished compost and 10% something coarse for drainage, such as pine bark fines, perlite or pumice. Mixes that are mostly compost hold a lot of water and shrink quickly. For very deep beds you can fill the bottom with logs, branches or leaves (sometimes called hugelkultur) and use the “bottom filled” field to subtract that depth. Avoid treated wood scraps and anything with weed seeds.
Estimates only; check product labels for bag volumes, and ask your supplier how their bulk soil is measured.