Soil
Raised Bed Soil Calculator
Enter your bed size and depth to get soil volume in cubic feet and yards, bag counts, and a cost estimate.
Your answer
Suggested blend
Above one cubic yard, bulk delivery is usually far cheaper than bags.
How much soil a raised bed really takes
Raised beds swallow more soil than almost anyone expects, and the reason is that volume scales with three dimensions at once. A 4 ft by 8 ft bed at 6 inches deep needs 16 cubic feet. Double the depth to 12 inches and it needs 32. Add a second bed and you are at 64 cubic feet — nearly two and a half cubic yards, which is a delivery, not a car boot full of bags.
The calculation itself is length x width x depth, with every dimension in the same unit. This tool does the unit conversion, splits the result into a workable blend, and tells you the point at which bulk delivery becomes cheaper than bags.
Quick reference: common bed sizes
| Bed size | 6 in deep | 12 in deep | 18 in deep |
|---|---|---|---|
| 2 x 4 ft | 4 cu ft | 8 cu ft | 12 cu ft |
| 3 x 6 ft | 9 cu ft | 18 cu ft | 27 cu ft (1 yd) |
| 4 x 4 ft | 8 cu ft | 16 cu ft | 24 cu ft |
| 4 x 8 ft | 16 cu ft | 32 cu ft | 48 cu ft |
| 4 x 12 ft | 24 cu ft | 48 cu ft | 72 cu ft |
What to fill it with
The single most common raised bed failure is filling it with bagged topsoil alone. Topsoil is screened mineral soil with little organic matter; in a confined bed with no earthworm traffic from below, it compacts into a dense block within two seasons. The opposite mistake — filling with pure compost — is just as costly: compost is too rich for many crops, holds too much water, and loses a third of its volume as it finishes decomposing.
- 60% screened topsoil or loam — the mineral body that holds nutrients and moisture.
- 30% finished compost — nutrition, biology, and structure.
- 10% coarse material such as bark fines, perlite, or coarse grit — keeps air space open long-term.
- Optional: a handful of balanced organic fertiliser per square foot at fill time, worked into the top 6 inches.
How deep does the bed need to be?
| Crop | Minimum fill depth | Comfortable depth |
|---|---|---|
| Lettuce, spinach, radish, herbs | 6 in / 15 cm | 8 in / 20 cm |
| Bush beans, peppers, chard, onions | 8 in / 20 cm | 12 in / 30 cm |
| Tomatoes, cucumbers, squash, brassicas | 12 in / 30 cm | 18 in / 45 cm |
| Carrots, parsnips, potatoes | 12 in / 30 cm | 18-24 in / 45-60 cm |
| Perennials, small fruit | 18 in / 45 cm | 24 in / 60 cm |
If the bed sits on open soil rather than a hard surface, roots will travel below the frame and the practical depth is greater than the frame height. On a patio, deck, or membrane, the frame depth is all you have — size it for the deepest-rooting crop you intend to grow.
Bags versus bulk
Bagged soil is convenient, consistent, and expensive. Bulk soil from a landscape supplier is typically a third to a half of the price per unit volume, but comes with a delivery charge and a minimum order, and lands as a pile on your driveway that must be moved by wheelbarrow before it rains.
- Under 0.5 cubic yards: bags almost always win once delivery minimums are counted.
- 0.5 to 1 cubic yard: roughly break-even. Consider handling and access.
- Over 1 cubic yard: bulk, clearly. At 2 yards the saving typically covers the delivery several times over.
- Ask the supplier what the mix actually contains. 'Garden soil' and 'vegetable blend' are not regulated terms.
Topping up in later years
Expect the level to fall by one to three inches each year as organic matter mineralises and the mix settles. Top up in early spring with an inch or two of finished compost spread across the surface rather than digging in a fresh load of topsoil — the compost feeds the soil biology that keeps the structure open, and it does not need mixing in.
How much soil do I need for a 4x8 raised bed?
A 4 ft x 8 ft bed 12 inches deep holds 32 cubic feet, or about 1.19 cubic yards — roughly 40 bags of 0.8 cubic feet. At 6 inches deep it halves to 16 cubic feet.
Is it cheaper to buy soil in bags or in bulk?
Bulk is dramatically cheaper above about one cubic yard — often a third of the bagged price per unit volume. Below half a yard, delivery minimums usually make bags the practical choice.
What is the best soil mix for a raised bed?
A workable general mix is roughly 60% screened topsoil or loam, 30% finished compost, and 10% coarse material such as bark fines or perlite for structure. Pure compost is too rich and slumps; pure topsoil compacts.
Why does my raised bed soil level drop every year?
Organic matter decomposes and the mix settles, typically losing 1-3 inches a year. Top up each spring with compost rather than refilling with fresh topsoil.
Can I put anything in the bottom of the bed to use less soil?
Yes, on deep beds. Coarse woody material in the lower third (a hugelkultur layer) reduces the soil volume needed and holds moisture, but it settles significantly in the first two seasons.
Last reviewed February 2026