Measure the empty space inside the bed
Wall thickness occupies part of the outside footprint. Measure between the inner walls, then measure from the existing fill to your intended soil level. If you plan to leave space below the rim, exclude that space from the fill depth.
For an 8 × 4 ft bed with 12 inches of space to fill, the volume is 8 × 4 × 1 = 32 cubic feet. Divide by 27 to get about 1.19 cubic yards. In metric, inside length × inside width × fill depth, all in meters, gives cubic meters.
Account for soil that is already there
If the same bed only needs another 6 inches, its additional fill is 16 cubic feet rather than 32. Measure existing fill at several points, especially when it is uneven. Use separate sections when one average depth would hide large differences.
The calculation describes geometric space. Settling depends on the blend and how it is handled; there is no universal settling percentage. Ask the supplier about the specific product and choose any extra allowance explicitly.
Turn volume into a purchase
With no allowance, 32 cubic feet requires 32 one-cubic-foot bags, 16 two-cubic-foot bags, or 22 one-and-a-half-cubic-foot bags. The last option purchases 33 cubic feet because bags must be rounded up.
With 10% extra, the target becomes 35.2 cubic feet. That is 36 one-cubic-foot bags, 18 two-cubic-foot bags, or 24 one-and-a-half-cubic-foot bags. Compare the complete bag totals with an actual bulk quote, including its minimum order and delivery charge.
Split a chosen blend without inventing a recipe
The raised-bed mix calculator lets you divide the fill into a compost component and a remaining blend. If you deliberately choose 30% compost for a 32-cubic-foot total, the arithmetic is 9.6 cubic feet of compost plus 22.4 cubic feet of the remaining blend. This example is not a recommendation to use that recipe for every crop.
Choose the growing medium separately using suitable local gardening guidance and the product description. Enter prices for both nonzero components to see a combined material cost. If you are buying one premixed product, the topsoil calculator is a simpler volume-and-bag tool.
Measure the unfilled inside volume first; choose a suitable growing medium separately from the quantity calculation.
Worked examples and sample prices are illustrative, not current supplier quotes. These articles explain quantity arithmetic, not structural or horticultural design. See methodology and limits.