How much soil does a 4×8 raised bed need?

At 12 inches deep: 32 cubic feet, about 1.19 cubic yards, or 22 bags of 1.5 cu ft. At 6 inches, half that. Use the raised bed calculator for other dimensions.

Equal parts by volume: one-third compost, one-third peat moss or coco coir, one-third coarse vermiculite. For 32 cu ft that is about 10.67 cu ft of each component.

Bags or bulk — which is cheaper?

Bulk usually wins past about one cubic yard. Under a yard, bags can be simpler. Compare price per cubic foot, not per bag, and include delivery.

How deep should a raised bed be?

Six inches can work for shallow crops; 12 inches suits most vegetables; 18 inches helps carrots, parsnips, and beds over hardpan or concrete (crop-depth guidance — our calculator sizes volume from the depth you enter). Roots can also explore native soil if the bed is open-bottomed.

How Much Soil Do You Need for a Raised Bed?

Raised beds fail the budget when you guess the fill. Volume is just length × width × depth: our calculator uses feet for length and width and inches for depth, then converts with 27 cu ft = 1 cu yd. A classic 4 × 8 ft bed at 12 in is 32 cu ft ≈ 1.185 cu yd — the locked worked example — about 22 bags of 1.5 cu ft. Confirm any layout in the raised bed soil calculator.

The volume formulas

Cubic feet = length (ft) × width (ft) × (depth inches ÷ 12)
Cubic yards = cu ft ÷ 27
Bags = ceil(cu ft ÷ bag size); common bags in our data are 1.5 and 2.0 cu ft

Bed (L × W × depth)	Cu ft	Cu yd	1.5-cu-ft bags	2-cu-ft bags
4 × 4 × 12 in	16	0.59	11	8
4 × 8 × 6 in	16	0.59	11	8
4 × 8 × 12 in	32	1.19	22	16
4 × 8 × 18 in	48	1.78	32	24
4 × 12 × 12 in	48	1.78	32	24
Two 4 × 8 × 12 in	64	2.37	43	32

Order a little extra the first year — many gardeners plan about 10% for settling (planning habit noted on our calculator page, not a separate constant in the volume file). Top-dress with compost later rather than overfilling forever.

Mel’s mix in real numbers

Mel’s mix (Square Foot Gardening) is ⅓ compost + ⅓ peat or coco + ⅓ coarse vermiculite by volume.

For the 32 cu ft bed:

Component	Share	Cu ft
Compost	1/3	≈ 10.67
Peat or coco coir	1/3	≈ 10.67
Coarse vermiculite	1/3	≈ 10.67

That blend is light, drains well, and stays workable. It is also spendy at garden-center bag prices. Some growers swap in high-quality compost-heavy blends or screened topsoil mixes for deeper beds while keeping the top tier fluffy for seeders — just avoid filling a frame with raw clay or hot fresh manure.

Bags vs bulk

Price every option per cubic foot. An illustrative $8 bag of 2 cu ft is $4/cu ft; an illustrative $60 bulk yard delivered is $60/27 ≈ $2.22/cu ft before you shovel — use your local quotes. Rule of thumb from our calculator copy (not a separate data constant): under about 1 yd³ bags can win on hassle; over 2 yd³ bulk usually wins on money.

Get bulk delivered as close to the beds as possible. Wheelbarrow math destroys Saturdays.

Filling cheaply without wrecking the bed

Open bottoms over decent native soil let roots dive and reduce how deep you must buy fill — still plan enough depth for the crops you grow.

Hugelkultur-style bases (logs and brush in the bottom of deep beds) displace purchased soil. Our calculator page treats that as a volume you can subtract when you bury wood in the lower third of deep frames — best in beds 18 in+, and avoid allelopathic woods like black walnut. Fresh wood ties up some nitrogen at the interface; compost-rich layers above help. This is a thrift tactic, not a requirement.

Compost you make yourself cuts the Mel’s mix bill. Size piles with the compost calculator — finished volume is about ⅓ of the starting pile in our data — so a big spring fill may need last year’s pile started now.

Mulch on top after planting slows evaporation so the expensive fill lasts. The mulch calculator sizes depth over bed area.

Depth by crop, not by Instagram

Depth is a crop choice, not a volume constant. Practical starting points many gardeners use:

~6 in: lettuce, shallow herbs, some greens — monitor drought.
~12 in: tomatoes, peppers, brassicas, most mixed beds.
~18 in+: long carrots, parsnips, beds on concrete or compacted subsoil.

Deeper is not automatically better if you cannot afford quality fill — a 12-inch bed of decent mix beats 24 inches of anaerobic sludge.

Multiple beds and staging

Two 4 × 8 × 12 beds are 64 cu ft ≈ 2.4 yd³. That is a bulk delivery. Stage fills: build frames, weed the paths, then schedule soil so bags do not bake in the driveway for a month.

If you garden in phases, fill the bed you will plant next month first. Empty frames collect weeds and neighborhood cats.

Paths, edges, and the soil you do not buy

Wide paths mulched with chips keep feet out of the expensive mix. Edging that leans inward sheds path mud into the bed and wastes fill. When you build a U-shaped kitchen-garden layout, total the volumes bed by bed — three “small” frames often equal one bulk delivery.

Hardware cloth under the bed stops voles in some regions; it also stops roots from exploring deep native soil, so you may want the full depth in purchased mix. Decide before the truck arrives.

Reuse last year’s Mel’s mix when you rebuild a frame: screen out roots, amend with finished compost (about ⅓ of a compost pile’s start volume becomes finished material in our compost data), and top up rather than dumping everything. Keep a written note of what you filled each bed with — next spring you will not remember whether Bed B was Mel’s mix or bulk triple-mix.

Bottom line

4 × 8 × 12 in = 32 cu ft ≈ 1.19 cu yd ≈ 22 × 1.5-cu-ft bags. Mel’s mix splits that into three ~10.67 cu ft parts. Use ÷ 27 for yards, ceil for bags, consider ~10% settling on year one, and run every custom size through the raised bed soil calculator before you swipe a card at the nursery.