Calculating soil for a raised bed seems trivial — length × width × depth, done — but the practical version involves settling allowances, mix ratios, and the bagged-vs-bulk math that determines whether you save money or waste money.
The basic math
Soil volume is inside dimensions multiplied:
Volume = Length × Width × Depth
The trick is unit conversion. Most raised bed dimensions are mixed: length in feet, depth in inches.
Cubic feet = (Length ft) × (Width ft) × (Depth in ÷ 12)
For a 4 × 8 foot bed at 10 inches deep: 4 × 8 × (10/12) = 26.7 cubic feet.
Convert to cubic yards by dividing by 27 (since 1 yd³ = 27 ft³). So 26.7 ÷ 27 = 0.99 yd³ — call it a full cubic yard.
How much soil do common beds need?
| Bed dimensions | Depth | Cubic feet | Cubic yards |
|---|---|---|---|
| 4 × 4 ft | 8 in | 10.7 | 0.40 |
| 4 × 4 ft | 12 in | 16 | 0.59 |
| 4 × 8 ft | 10 in | 26.7 | 0.99 |
| 4 × 8 ft | 12 in | 32 | 1.19 |
| 3 × 6 ft | 10 in | 15 | 0.56 |
| 2 × 8 ft | 12 in | 16 | 0.59 |
The settling allowance
Fresh soil compacts. After the first heavy watering, the surface drops 1–2 inches. After the first season, it can drop 3–4 inches. Two ways to handle this:
- Order 10–15% extra at fill time. Mound the soil slightly above the bed edges, knowing it will settle to level.
- Top up annually. Order enough to fill, then add a fresh 1–2 inch layer each spring.
Bagged vs bulk: where to switch
- Less than 0.5 cubic yards (14 ft³): Bagged is fine. About 9 standard 1.5-ft³ bags or 7 of the larger 2-ft³ bags.
- 0.5 to 1 cubic yard: Bulk starts to win, but bagged is still convenient.
- Over 1 cubic yard (27 ft³): Bulk delivery almost always cheaper, often by 30–50%.
- Over 2 cubic yards: Get bulk. Bagged becomes absurd at this volume.
Bulk delivery typically has a 1-yard or 2-yard minimum, plus a delivery fee ($75–$150).
Bags per cubic yard:
- 1.5 ft³ bags: 18 bags per yard
- 2 ft³ bags: 14 bags per yard
- 40 L bags: 19 bags per yard
- 50 L bags: 15 bags per yard
What soil to put in
Raised beds need a soil mix engineered for raised beds — not "topsoil," which is unscreened native soil and will compact into a brick within a season.
The classic recipe is "Mel's Mix" from Square Foot Gardening:
- 1/3 compost (ideally from multiple sources)
- 1/3 peat moss or coconut coir
- 1/3 coarse vermiculite
Roughly $200–300 per cubic yard if you assemble it, or $100–150 per yard pre-mixed.
Cheaper alternatives:
- 50% topsoil + 50% compost: $30–60 per yard.
- 40% topsoil + 40% compost + 20% sand: $40–70 per yard.
- Lasagna fill: Cheapest. Cardboard, leaves, food scraps at bottom, soil on top.
Hugelkultur and partial fill
If your bed is deeper than 12 inches, you don't need expensive soil mix all the way down. Most vegetable roots stop at 8–12 inches.
Hugelkultur: Fill the bottom 30–50% with rotting wood, branches, and leaves. As they break down, they release nutrients and improve soil over years. Top with soil mix.
Inverted sod: If placing the bed on grass, flip the sod upside-down at the bottom.
For a 4 × 8 × 18" bed, partial fill cuts soil needs from 48 ft³ to about 24 ft³ — half the cost.
Estimating without a calculator
- A 4×4 bed at 10" deep is just over a third of a yard.
- A 4×8 bed at 10" deep is almost exactly one cubic yard.
- A 4×8 bed at 12" deep is just over one yard (1.2).
- Two 4×8 beds at 10" deep is two yards — minimum for bulk delivery.
The takeaway
Soil is where most beginners over- or under-spend on a raised bed. Underbuy and your bed sits empty for two weeks. Overbuy and you've got an unsightly pile in your driveway for a month. Measure twice, calculate once, and order with a 10% settling allowance.