How much soil does a raised bed need?
Soil volume for a raised bed, split into the three parts of Mel’s Mix and converted to bags.
Soil to buy
What it weighs
Enter the bed’s interior dimensions and you also get the weight, before you carry it anywhere.
Worked example: a 4 × 8 ft bed, 12 in deep
The most common raised bed there is. With a 2 in drainage layer the soil depth is 10 in, and because soil settles after watering, the amount to buy is larger than the arithmetic volume.
Why the settling allowance matters
Bagged soil is sold by dry volume. Once it is watered in and worked around roots it compresses by roughly 10 to 15%, so a bed filled level with dry soil ends up an inch or two short after the first real watering. This calculator adds 12% for that, which is why the number to buy is larger than the geometry suggests. It is far cheaper than a second trip for two more bags.
The three parts, by volume
Mel’s Mix is equal parts by volume: a bucket of each, not a pound of each. For this bed that works out as:
| Part | Share | Litres | Cubic feet |
|---|---|---|---|
| compost | 33% | 282 | 10.0 |
| peat moss or coir | 33% | 282 | 10.0 |
| vermiculite or perlite | 33% | 282 | 10.0 |
The compost is the part that feeds the plants, so use two or three different kinds if you can: a manure-based bag, a mushroom compost, and your own if you have it. Peat moss holds water and keeps the mix light; coir does the same job without the peat-extraction concerns but needs re-wetting. Vermiculite or perlite is the drainage and aeration part, and it is the one people are tempted to skip. Don’t: a mix of compost and peat alone compacts into something roots struggle in.
Common bed sizes
How much soil each needs at 12 in deep with a 2 in drainage layer. Widths over 4 ft are noted because you cannot reach the middle from one side.
| Bed | Surface area | Soil to buy | Cubic feet | Note |
|---|---|---|---|---|
| 4 × 4 ft | 16 sq ft | 423 L | 14.9 | Reachable from all sides; the classic square-foot bed. |
| 4 × 8 ft | 32 sq ft | 846 L | 29.9 | The standard lumber build: two 8 ft boards per side, no cutting. |
| 4 × 12 ft | 48 sq ft | 1,269 L | 44.8 | Long enough to need a path or a plank to reach the middle. |
| 3 × 6 ft | 18 sq ft | 476 L | 16.8 | Good for narrow spaces and for children. |
| 2 × 8 ft | 16 sq ft | 423 L | 14.9 | Balcony or against-a-wall bed. |
| 5 × 10 ft | 50 sq ft | 1,321 L | 46.7 | Too wide to reach the centre from one side. |