Multiply the bed's length by width by depth in feet to get cubic feet of soil. A 4 by 8 foot bed that is 1 foot deep needs 32 cubic feet, about 1.2 cubic yards (there are 27 cubic feet in a cubic yard). Fill it with a mix of quality topsoil and compost, and plan for the soil to settle, so buy about 10 to 20% extra.
Filling a raised bed takes more soil than most people expect. Here is how to calculate it and what to use.
The calculation
Length (ft) × Width (ft) × Depth (ft) = cubic feet
Convert inches to feet by dividing by 12 (for example, 10 inches = 0.83 ft).
Cubic feet ÷ 27 = cubic yards
Common bed sizes
| Bed size | Depth | Cubic feet | Cubic yards |
|---|---|---|---|
| 4 × 4 ft | 12 in | 16 | 0.6 |
| 4 × 8 ft | 6 in | 16 | 0.6 |
| 4 × 8 ft | 12 in | 32 | 1.2 |
| 4 × 8 ft | 18 in | 48 | 1.8 |
| 3 × 6 ft | 12 in | 18 | 0.7 |
Add 10 to 20% for settling.
Bags or bulk?
- Bags: convenient for one or two small beds. Divide cubic feet by the bag size (often 1 to 2 cubic feet).
- Bulk: usually cheaper for several beds, delivered by the cubic yard.
What soil mix to use
A common mix is roughly equal parts quality topsoil and compost, sometimes with an aerating material for drainage. Many garden centers sell ready-made raised bed blends.
Saving soil in deep beds
For very deep beds, some gardeners fill the bottom with logs, branches or leaves (a technique related to hugelkultur) to reduce soil needs. Expect extra settling as it breaks down.
Each season
Top up beds with an inch or two of compost each year to replace nutrients and settled soil.
Common questions
How much soil do I need for a 4x8 raised bed?
For a bed 12 inches deep, 4 x 8 x 1 = 32 cubic feet, or about 1.2 cubic yards. For a 6-inch-deep bed, half that: 16 cubic feet.
How many bags of soil fill a raised bed?
Divide the cubic feet needed by the bag size. A 32 cubic foot bed needs about 22 bags of 1.5 cubic feet, before allowing for settling.
What soil should I put in a raised bed?
A blend of good topsoil and compost is a common, reliable choice. Avoid filling beds with heavy clay or plain garden soil, which compacts.