How much soil does a raised bed take?
A raised bed is the one soil order small enough to work out exactly, and the one people underestimate most reliably. Here is the arithmetic, what actually goes in the box, and the honest answer to whether you can fill the bottom half with something cheaper.
How much soil will your raised bed take?
Working out a whole yard rather than one bed? Sketch it to scale in the planner and every area is measured, costed and totalled together.
The formula
A bed is a box, so its volume is just its three sides multiplied together. Work in cubic feet rather than cubic yards, because a single bed is a fraction of a yard and bags are sold by the cubic foot:
cubic feet = length × width × depth, all in feet
Depth is the part to get right: 12 inches is one foot, 6 inches is 0.5, 18 inches is 1.5. A 4 by 8 foot bed filled 12 inches deep is 4 × 8 × 1 = 32 cubic feet. If your supplier quotes in yards, divide by 27, which makes that bed 1.2 cubic yards.
Worth knowing before you order: one cubic yard fills a 4 by 8 bed to about 10 inches. Almost every bed takes more than a yard, and almost nobody expects that.
Common bed sizes
| Bed | 6 in deep | 12 in deep | 18 in deep |
|---|---|---|---|
| 4 × 4 ft | 8 cu ft | 16 cu ft | 24 cu ft |
| 4 × 8 ft | 16 cu ft | 32 cu ft | 48 cu ft |
| 4 × 12 ft | 24 cu ft | 48 cu ft | 72 cu ft |
| 2 × 8 ft | 8 cu ft | 16 cu ft | 24 cu ft |
In bags, divide by the bag size. Garden soil is usually 1.5 cubic feet a bag, so that 4 by 8 bed at 12 inches is 22 bags. Stacked on a trolley that is a surprising amount of shopping, which is the moment most people start pricing bulk.
How deep
| What you are growing | Soil depth | Notes |
|---|---|---|
| Salad, herbs, greens | 6 in | Shallow roots, quick turnover |
| Most vegetables | 10 – 12 in | Beans, peppers, squash, brassicas |
| Tomatoes, root crops | 18 in | Carrots and parsnips fork in shallow soil |
| Bed on concrete or a deck | 18 in minimum | Roots cannot go down, so it all has to be in the box |
A bed sitting on soil is deeper than it looks, because roots carry on into the ground underneath once they reach it. Loosen that ground with a fork before you fill, and a 6 inch bed on open soil grows more than a 12 inch bed on a patio.
What to fill it with
Not pure compost, and not the stuff dug out of the lawn. A bed wants a blend with the bulk of it holding structure and a serious minority of it holding nutrition:
- About half to sixty per cent topsoil or garden soil. This is the body of the mix, and the reason the bed still has structure in year three.
- About a third compost. The nutrition and the water holding. Going much past half compost gives you a bed that is rich, airy and sinks by a third the first winter.
- A tenth or so of something coarse, like perlite, coarse sand or fine bark, if your soil is heavy. Optional on a light mix, worth it on a sticky one.
Buy the topsoil as screened topsoil rather than fill dirt, and buy the compost separately rather than trusting a bagged blend to be honest about its ratio. A bag labelled "garden soil" is often mostly bark fines.
Can you fill the bottom with something cheaper?
For a deep bed, yes, with one exception. Logs, branches, leaves and turf stacked in the bottom third of an 18 inch bed rot down into decent soil and save you real money on the fill. It settles hard in the first year and it borrows nitrogen from the layer above while it breaks down, so top it up and feed it, but the principle is sound.
The exception is gravel, which does not belong in the bottom of a raised bed. The idea that a coarse layer improves drainage is exactly backwards: water will not cross from fine material into coarse until the fine layer above is saturated, so a gravel bottom raises the wet zone into the root zone rather than draining it. If the bed sits on soil, it drains through the soil. If it sits on a hard surface, drill the base.
Settling, and the second season
Fresh mix settles ten to twenty per cent as it is watered in, and a bed with woody material underneath settles more. Fill it proud of the boards and expect to top it up the following spring. That top-up is compost, not another full fill, and it is why the second year of a bed costs almost nothing.
One more number worth sitting with: that 4 by 8 bed at 12 inches holds about 1.2 cubic yards, which is somewhere around 2,600 pounds of wet soil. Decide where the bed goes before you fill it, not after.
Ordering
- Work out cubic feet first, then convert. Bags are sold by the cubic foot and bulk by the yard, and mixing the two up is how people end up with half a bed.
- One bed is a bag job. Most yards will not deliver under two or three cubic yards, so bulk only wins once you are filling three or four beds at once.
- Order the compost and the topsoil separately and mix them on a tarp. It is cheaper than a pre-blended bag and you know what is in it.
- Line the bottom with cardboard if the bed is going over lawn. It smothers the grass and is gone by the end of the season.
Lay your beds out in the planner
Draw the beds where they are actually going, at the size they are actually going to be, and the planner totals the soil for all of them together. Four beds is usually the point where a delivery beats a car full of bags, and seeing the total is how you find out.