Raised Bed Soil Calculator
Fill a raised bed without the guesswork: soil volume for any bed size and depth, bags in the sizes stores sell, bulk yards, and the standard topsoil–compost blend split.
Filling a raised bed, counted properly
A raised bed is a box, and a box is a volume: length × width × the depth you actually fill. The surprises come from the sizes — a pair of 8 × 4 beds filled a foot deep swallows over 60 cubic feet of soil, which is why the bag count always shocks people at the store. The calculator counts every bed, adds the allowance, and answers in the three ways soil is sold: 1, 1.5 and 2 cu ft bags, or bulk cubic yards.
Volume per bed = length × width × depth · Bags = volume (with waste) ÷ bag size · Yards = volume ÷ 27
Example: two 8 × 4 ft beds filled 12 in deep hold 32 cu ft each — 64 cu ft in total. With the 10% settling allowance that is 70.4 cu ft to buy: 36 bags of 2 cu ft, or 2.61 cubic yards delivered.
The blend matters as much as the volume. The widely used raised-bed mix is roughly 60% topsoil, 30% compost, 10% aeration material (perlite, coarse sand or fine bark) — the calculator splits your total that way so each part can be ordered on its own. The other famous recipe, Mel's Mix for square-foot gardens, uses equal thirds of compost, peat moss or coir, and vermiculite. Either way, expect the fill to settle an inch or two in the first season as the compost fraction breaks down — the waste allowance is partly there for exactly that.
Frequently asked questions
How deep should a raised bed be filled?
10–12 in suits most vegetables; 6–8 in is enough for salads and herbs, and deep-rooted crops like carrots do best at 12 in or more. Fill to within an inch or two of the rim — soil settles, and a brim-full bed sheds water and mulch over the sides.
Should I put anything in the bottom of a deep bed?
Deep beds (18 in and up) are often part-filled with logs, branches and leaves — the hügelkultur approach — which saves soil and breaks down slowly into the bed. Keep the top 10–12 in as proper growing mix, and expect a little more settling in year one.
Bags or bulk delivery — which is cheaper?
Bulk wins on price per cubic foot almost every time past about a cubic yard — two mid-size beds already pass that line. Bags win on handling: no driveway pile, no wheelbarrow marathon, and the blend arrives pre-mixed. The calculator shows both so the trade-off is visible.
How much does the soil settle after filling?
Plan on 10–20% in the first season, mostly from the compost fraction decomposing and air pockets collapsing. Water a new bed deeply once before planting to settle it, then top up to the rim.
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