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Raised Bed Soil Calculator Guide
How much fill a raised bed takes, split into the three components of a decent mix.
Cubic feet = length × width × depth (all in feet)
An 8×4ft bed filled 12in deep = 32 cubic feet = 1.19 cubic yards = 22 bags at 1.5 cu ft.
Beds Take More Than People Expect
A single 8×4 bed at 12in deep swallows 32 cubic feet. That's 22 bags of soil for one bed — and most people build three or four at once.
This is the moment the project stops being cheap. At that volume you're well past the point where bagged soil makes sense, and a bulk delivery is a fraction of the price. Check the bulk figure before you load a trolley.
Don't Fill It With Topsoil Alone
Straight topsoil in a raised bed compacts into a dense brick within a season. It has nowhere to go — no worms working up from below, no freeze-thaw cycle at depth — so it just consolidates.
The mix exists to stop that. Topsoil provides mineral content and body, compost provides nutrition and structure, and the aeration component (perlite, coarse sand, bark fines) keeps air in the profile so roots can breathe. Skip the third and you'll be digging it out in two years.
It Will Drop, and That's Normal
Expect the level to fall noticeably in the first year as the compost breaks down and everything settles. That's not a failure — the organic fraction is doing its job by decomposing.
Top up annually with compost rather than refilling with soil. The bed gets better each year if you feed the organic content and worse if you keep adding mineral soil.
The Bottom Third Can Be Cheaper
On a deep bed, roots rarely reach the very bottom. Filling the lower portion with coarse organic material — logs, branches, leaves — displaces expensive mix and slowly rots into something useful.
The trade-off is honest: it will settle more, and you'll be topping up for a couple of years while it breaks down. On an 18in bed that's usually a good deal. On a 6in bed there's no room for it.
Width Matters More Than Length
4ft is the standard width for a reason — it's about as far as most people can reach into the middle from either side. Go wider and you'll end up standing on the bed, compacting exactly the soil you just paid to keep loose.
Related: topsoil calculator, mulch calculator, fertilizer calculator.
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