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.
Interior surface area32 sq ft
Soil depth after drainage10 in
Arithmetic volume755 L (26.7 cu ft)
Buy this much, allowing for settling846 L (29.9 cu ft)
Drainage layer151 L of gravel
Bulk equivalent1.11 cubic yards
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.
Soil volume by bed size| 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. |
Frequently asked questions
How much soil do I need for a 4×8 raised bed?
About 846 litres (29.9 cu ft) at 12 in deep with a 2 in drainage layer, allowing for settling. Use the calculator for other depths.
How deep should raised bed soil be?
Six inches grows lettuce, herbs, onions and most greens. Twelve inches suits tomatoes, peppers, squash and most of a vegetable garden. Carrots, parsnips and potatoes want 12 to 18. Over grass or good ground the roots carry on below the bed, so 8 to 12 in is plenty; on paving or hard clay, go deeper.
Is raised bed soil the same as potting soil?
No. Raised bed soil is heavier: it usually contains topsoil or composted bark so it holds its structure over a large volume and several seasons. Potting mix is lighter and peat or coir based, made to drain freely in a container. Potting mix in a big bed is expensive and dries out fast; raised bed soil in a pot compacts and stays wet.
Can raised bed soil be used in pots?
In large planters, half barrels and grow bags, yes. In smaller pots it holds too much water and compacts, because a pot has no soil below it to drain into. For anything under about 5 gallons, use potting mix, or cut raised bed soil with a third perlite.
How long does raised bed soil last?
It does not go bad, but it does sink. The organic matter breaks down and the level drops an inch or two a year, so top it up with compost each spring. Replace it only if it has become compacted, waterlogged or diseased, which is rare in a bed that is topped up and not walked on.
How much soil is a yard?
A cubic yard is 27 cubic feet, about 765 litres. That fills a 4 × 8 ft bed to roughly 9 in deep once it has settled, which is why bulk soil starts to beat bags at about one full bed.
Should I put gravel in the bottom of a raised bed?
A thin layer helps drainage on a solid base or heavy clay, but a deep gravel layer just steals soil depth. Two inches is plenty; on well-drained ground, skip it and use the full depth for soil.
Is Mel’s Mix measured by weight or volume?
Volume: one part compost, one part peat or coir, one part vermiculite, measured with the same bucket. By weight the proportions would be very different and the mix would be wrong.
Can I fill a raised bed with garden soil?
Topsoil alone compacts and drains poorly in a raised bed. If you use it, blend it half and half with compost and add the aeration part.