Raised Bed Soil Calculator
Volume and the mix, because straight topsoil sets like a brick.
Work out Raised Bed Soil. Volume and the mix, because straight topsoil sets like a brick. Roll, pallet and box sizes are your supplier's, not an assumption.
6–8 for salads, 12+ for roots.
Zero for a bulk order.
Perlite, coarse sand or bark fines.
Soil needed
32.0 cu ft
1.19 cubic yards · 22 bags
Filling with straight topsoil is the standard mistake — in a container there are no worms, roots or freeze-thaw cycles to keep it open, so it compacts into a brick within a season. 22 bags is also a lot more bagged soil than most people expect, and usually the point at which bulk delivery becomes cheaper.
How the Raised Bed Soil Calculator works
Filling a raised bed with straight topsoil is the standard mistake. In a container there are no worms, roots or freeze-thaw cycles to keep it open, so it compacts into a brick within a season. The usual answer is roughly half topsoil, a third compost and the rest something for aeration — and the ratios are inputs here because gardeners disagree about them with some justification.
Also known as: how much soil for a raised bed · garden bed soil calculator · raised garden bed soil mix · raised bed volume calculator
The calculation itself
Length times width times depth, times the number of beds. The only trap is depth in inches against dimensions in feet, which is why the conversion is done here rather than left to the reader.
The mix is then that volume split by ratio. The shares do not need to sum to a hundred — they are normalised — so entering equal parts as 1, 1, 1 works exactly as well.
In practice
A 4 by 8 foot bed filled twelve inches deep is 32 cubic feet, or about 1.2 cubic yards.
In 1.5 cubic foot bags that is 22 bags for one bed. Which is the number that surprises people — a single raised bed is a car full of soil, and three beds is a bulk delivery.
At a 50/30/20 mix that splits as 16 cubic feet of topsoil, 9.6 of compost and 6.4 of aeration material.
Why the mix matters more than the volume
Straight topsoil compacts in a container. In open ground it is kept loose by worms, roots and freeze-thaw cycles; in a raised bed none of those act the same way, and within a season it sets hard and drains badly.
Compost supplies structure and nutrition but breaks down, which is part of why beds need topping up annually. Perlite, coarse sand or bark fines hold the structure open for longer.
The exact ratio is genuinely contested and depends on what you are growing and what your base soil is like — which is why the shares are inputs rather than a recommendation dressed as arithmetic.
Acting on it
Work out the bag count and the bulk yardage both, then compare delivered prices. Two beds is usually where bulk wins.
Expect to top up in year one. Compost decomposes and any woody fill in the base settles noticeably.
Fill to a couple of inches below the rim rather than level, or watering washes soil over the edge.
Frequently asked questions
How much soil do I need for a raised bed?
Length times width times depth. A 4 by 8 foot bed 12 inches deep is 32 cubic feet, or about 1.2 cubic yards — which is a lot more bagged soil than most people expect.
What is the best soil mix for a raised bed?
A common starting point is about 50% topsoil, 30% compost and 20% something for aeration such as perlite, coarse sand or bark fines. The proportions matter less than including all three.
Why not just use topsoil?
Because it compacts. Garden soil is kept open by worms, roots and weather, none of which act on a container the same way, so straight topsoil in a bed sets hard and drains badly.
How deep should a raised bed be?
Six to eight inches works for salads and herbs; twelve or more for root crops and anything long-lived. Deeper beds also dry out more slowly, which matters in hot weather.
Can I fill the bottom with something cheaper?
Partly — logs, branches and leaves in the base will decompose and reduce the soil needed. They also settle noticeably in the first year, so expect to top the bed up.
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