Planting
Raised Bed Soil Calculator
Enter the bed size and pick a mix. This splits the total into the components you actually have to buy.
Fill depth, not the height of the frame — leave 1-2 in below the rim.
Quantity estimate for planning and buying. Rates vary by variety, region and product — check the label on what you actually buy.
How much soil for a 4x8 raised bed?
A 4 x 8 ft raised bed 12 inches deep needs 32 cubic feet of soil — that is 1.19 cubic yards, or 32 bags at 1 cubic foot.
At 6 inches deep it is 16 cubic feet (0.59 cubic yards), and at 18 inches it is 48 cubic feet (1.78 cubic yards).
Do not fill it with topsoil alone — it compacts into a dense block that drains badly. In a standard mix those 32 cubic feet break down to roughly 19 ft³ topsoil, 10 ft³ compost and 3 ft³ perlite. Bulk delivery beats bags from about one cubic yard up, which is a single bed at 10 inches.
Do not fill a raised bed with topsoil alone
Bagged or bulk topsoil on its own compacts into a dense block in a raised bed. It has no structure to hold air, drains badly, and roots struggle. A raised bed needs a mix, and the reason is that a bed is a container: nothing about it is self-correcting the way ground soil is.
The standard proportions are roughly 60% topsoil or garden soil, 30% compost for nutrition and structure, and 10% perlite or coarse vermiculite for aeration. The square-foot-gardening recipe goes further and uses no soil at all — equal parts peat or coir, compost and coarse vermiculite — which is lighter, drains beautifully, and needs feeding sooner because there is no mineral reserve.
How deep
Deeper is better and also expensive, because volume grows linearly with depth. A 4 × 8 ft bed at 12 inches takes 32 ft³, which is 1.19 cubic yards or 32 one-cubic-foot bags. At 18 inches it is 48 ft³.
| Growing | Depth | Fill for 4 x 8 ft | Notes |
|---|---|---|---|
| Shallow herbs, lettuce | 6 in | 16.0 ft³ (0.59 yd³) | Minimum workable depth |
| General vegetables | 10-12 in | 26.7 ft³ (0.99 yd³) | The usual choice |
| Root crops: carrots, parsnips | 12-18 in | 32.0 ft³ (1.19 yd³) | Needs loose depth to form straight roots |
| Over a hard surface (patio, concrete) | 12-18 in | 32.0 ft³ (1.19 yd³) | No native soil to root into |
| Over lawn or soil | 6-10 in | 16.0 ft³ (0.59 yd³) | Roots can reach the ground beneath |
Filling the bottom, and the hugelkultur question
If the bed sits on soil or lawn, roots will grow down into the ground beneath, so 6-10 inches of good mix over loosened native soil works fine. If it sits on concrete or decking, the mix is all the plants will ever have and you want the full 12-18 inches.
Filling the bottom third with logs, branches and coarse organic matter — hugelkultur — genuinely reduces how much mix you buy, and the material breaks down into good soil over several years. The catch is that it also sinks, sometimes several inches in the first season, and decomposing wood ties up nitrogen as it goes. If you do it, expect to top the bed up annually and feed a little more in the first year or two.
Bulk delivery beats bags from about one cubic yard up, which is a single 4 × 8 ft bed at 10 inches. Below that, bags win on convenience.
Common questions
How much soil for a 4x8 raised bed?
At 12 inches deep, 32 cubic feet — 1.19 cubic yards, or 32 bags at 1 ft³. In a standard mix that is about 19 ft³ topsoil, 10 ft³ compost and 3 ft³ perlite.
Can I fill a raised bed with just topsoil?
You can, but it compacts badly and drains poorly. A raised bed is a container, so it needs a mix with compost for structure and an aeration component. Roughly 60/30/10 topsoil, compost and perlite.
How deep should a raised bed be?
Six inches is the workable minimum for herbs and salad, 10-12 inches suits most vegetables, and 12-18 inches is right for root crops or any bed over a hard surface.
Should I put anything in the bottom of a raised bed?
Over soil, loosen the ground and fill with mix. Over concrete or decking, use full-depth mix. Filling the lower third with logs and branches cuts cost but the bed will sink noticeably in the first season and needs topping up.