🌱 Garden
How Much Soil Does a Raised Bed Need?
Length × width × depth, with a top-up allowance for settling. Enter the bed and the depth you want, and the answer comes back in cubic metres, cubic yards, litres and bags.
How much soil does a raised bed need?
A 4 × 8 ft bed filled 12 inches deep needs 32 cubic feet of soil — 0.9 cubic metres, 906 litres, or about 23 bags of 40 litres. That is the figure most people are looking for, and it is simply length × width × depth: 4 × 8 × 1 = 32 cubic feet.
Two adjustments are worth making. First, beds are rarely filled to the brim; leave 2 inches of freeboard so mulch and water stay in, which takes a 12 inch bed down to 10 inches of soil and cuts the volume by 17 percent. Second, soil settles by 5 to 10 percent in the first season, so buying 10 percent extra avoids a top-up trip in spring.
Soil for common raised bed sizes
| Bed | Depth | Cubic feet | Litres | 40 L bags |
|---|---|---|---|---|
| 4 × 4 ft | 6 in | 8 | 227 | 6 |
| 4 × 4 ft | 12 in | 16 | 453 | 11 |
| 4 × 8 ft | 6 in | 16 | 453 | 11 |
| 4 × 8 ft | 12 in | 32 | 906 | 23 |
| 4 × 8 ft | 18 in | 48 | 1,359 | 34 |
| 4 × 12 ft | 12 in | 48 | 1,359 | 34 |
Bags or bulk?
Up to about 1 cubic metre, bags are usually cheaper and far easier to move. Beyond that, bulk delivery wins on price but arrives as a loose heap on the driveway, often 2 to 3 tonnes of it. A cubic metre of moist topsoil weighs roughly 1.3 tonnes, so the 4 × 8 ft bed above is over a tonne of material — about 50 trips with a 20 kg bag if you are carrying it yourself.
Check access before ordering bulk: a delivery lorry needs about 3 metres of width and a firm surface, and many suppliers will only tip on the road. Bagged soil can be carried through a side gate a few at a time, which is slower but usually the only option for a back garden.
What to actually put in the bed
Garden soil alone compacts and drains poorly in a raised bed. The usual mix is 60 percent topsoil, 30 percent compost and 10 percent aeration material such as coarse grit, perlite or pine bark fines. For a 32 cubic foot bed that is about 19 cubic feet of topsoil, 10 of compost and 3 of grit.
Pure compost is the commonest mistake: it holds too much water, shrinks by a third as it decomposes, and is too rich for root crops. A 50/50 topsoil-compost blend is acceptable for hungry crops like courgettes but still benefits from grit in a wet climate.
Depth matters more than area for some crops
- 6 inches — salads, herbs, radishes, spring onions.
- 8 to 10 inches — beans, peas, onions, most annual flowers.
- 12 inches — tomatoes, peppers, carrots, beetroot.
- 18 inches — potatoes, parsnips, dwarf fruit trees.
Deeper is not automatically better: below about 12 inches the soil is often anaerobic and roots do not use it. If the bed sits on compacted clay or concrete, add drainage holes or a gravel layer, and expect the fill to settle more in the first year.