How this calculator works
Bed volume is length × width × depth. That volume is then split by the percentages you set: compost, an optional amendment, and topsoil for the remainder. Each part is converted to bags separately because you buy them separately.
Volume = Length × Width × Depth(in)/12 × Beds
Part = Volume × Share% ÷ 100
Bags = round up( Part ÷ Bag size ) (for each part)
Assumptions and tips
- A mix of roughly 60% topsoil and 40% compost is a common starting point; adjust for your plants and local soil.
- Bag sizes vary by brand — check the label.
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Bed length | 8 ft |
| Bed width | 4 ft |
| Soil depth | 12 in |
| Number of beds | 2 |
| Compost share | 40 % |
| Other amendment share | 0 % |
| Bag size | 1.5 cu ft |
| Result | Value |
|---|---|
| Total soil | 64 cubic feet |
| Total soil | 2.37 cubic yards |
| Topsoil (60%) | 38.4 cu ft · 26 bags |
| Compost (40%) | 25.6 cu ft · 18 bags |
| Total 1.5 cu ft bags | 44 bags |
Step by step:
- Volume = 8 × 4 × 12/12 × 2 = 64 cu ft
- Topsoil 60% = 38.4 cu ft; compost 40% = 25.6 cu ft
- Each part ÷ 1.5 cu ft bags, rounded up
Soil for common raised bed sizes
One bed, 12 in deep.
| Bed | Cubic feet |
|---|---|
| 2 × 4 ft | 8 cubic feet |
| 3 × 6 ft | 18 cubic feet |
| 4 × 4 ft | 16 cubic feet |
| 4 × 8 ft | 32 cubic feet |
| 4 × 12 ft | 48 cubic feet |
Common mistakes to avoid
- Filling beds with bagged potting mix only — it is costly and shrinks.
- Forgetting that beds settle 10–20% in the first season.
Frequently asked questions
How much soil for a 4×8 raised bed?
At 12 in deep, one 4×8 bed holds 32 cubic feet — about 1.2 cubic yards.
Can I fill the bottom with something cheaper?
Deep beds are sometimes partly filled with logs and branches (hugelkultur) or extra topsoil. Plants mostly root in the top 8–12 in.