The four-step answer
- Measure the inside length and width of the bed in feet.
- Decide the soil depth in inches. Most people fill to an inch or two below the top edge.
- Multiply: length × width × depth ÷ 12 = cubic feet.
- Convert: cubic feet ÷ 27 = cubic yards, or cubic feet ÷ bag size = bags.
Example: a 4 × 8 ft bed filled 10 inches deep is 4 × 8 × 10 ÷ 12 = 26.7 cubic feet. That is 0.99 cubic yards, or 18 bags of 1.5 cubic feet. With 10% extra for settling it becomes 29.3 cubic feet, or 20 bags.
Prefer to skip the arithmetic? The raised bed soil calculator does it for any number of beds, including round ones.
Raised bed soil chart in cubic feet
No extra included. Read across for your depth.
| Bed size | 6 in | 8 in | 11 in | 12 in | 18 in | 24 in |
|---|---|---|---|---|---|---|
| 2 × 4 ft | 4 | 5.3 | 7.3 | 8 | 12 | 16 |
| 2 × 8 ft | 8 | 10.7 | 14.7 | 16 | 24 | 32 |
| 3 × 6 ft | 9 | 12 | 16.5 | 18 | 27 | 36 |
| 4 × 4 ft | 8 | 10.7 | 14.7 | 16 | 24 | 32 |
| 3 × 8 ft | 12 | 16 | 22 | 24 | 36 | 48 |
| 4 × 6 ft | 12 | 16 | 22 | 24 | 36 | 48 |
| 4 × 8 ft | 16 | 21.3 | 29.3 | 32 | 48 | 64 |
| 4 × 10 ft | 20 | 26.7 | 36.7 | 40 | 60 | 80 |
| 4 × 12 ft | 24 | 32 | 44 | 48 | 72 | 96 |
The 11-inch column is there because two stacked 2×6 boards make a bed 11 inches tall, a very common build.
Bags needed at 12 inches deep
Bag counts round up. No extra included.
| Bed size | Cubic yards | 0.75 cu ft bags | 1 cu ft bags | 1.5 cu ft bags | 2 cu ft bags |
|---|---|---|---|---|---|
| 2 × 4 ft | 0.30 | 11 | 8 | 6 | 4 |
| 4 × 4 ft | 0.59 | 22 | 16 | 11 | 8 |
| 3 × 6 ft | 0.67 | 24 | 18 | 12 | 9 |
| 4 × 6 ft | 0.89 | 32 | 24 | 16 | 12 |
| 4 × 8 ft | 1.19 | 43 | 32 | 22 | 16 |
| 4 × 10 ft | 1.48 | 54 | 40 | 27 | 20 |
| 4 × 12 ft | 1.78 | 64 | 48 | 32 | 24 |
Three 4 × 8 beds at 12 inches need 96 cubic feet, about 3.6 cubic yards. At that size a bulk delivery is usually worth pricing.
How much extra to allow
New raised bed soil is fluffy. Watering, rain and the slow breakdown of compost all make it sink, often by an inch or more in the first season. Allow:
- About 10% for a bed mostly of topsoil, or for shallow beds.
- 15–20% for deep beds filled with a compost-heavy or very light mix.
- A yearly top-up of 1 to 2 inches of compost. For a 4 × 8 bed, 1 inch is 2.7 cubic feet and 2 inches is 5.3 cubic feet. The compost calculator works this out for any bed.
What to fill a raised bed with
Most gardeners fill raised beds with a blend rather than a single material. Topsoil on its own can pack down and drain slowly in a box, and pure compost shrinks a lot as it breaks down. A common starting point is roughly 60% topsoil, 30% compost and 10% something coarse such as bark fines. The raised bed soil mix calculator splits your volume into whatever recipe you choose.
Filling a very deep bed
Beds 24 inches or taller take a lot of soil: a 4 × 8 bed that deep needs 64 cubic feet, more than two cubic yards. A common approach is to put coarser, cheaper material in the bottom third, such as rough unscreened soil, leaves or small branches. Keep these points in mind:
- Woody material and leaves rot and sink, so the bed will need topping up more often.
- Keep at least 12 inches of good soil on top, where most vegetable roots grow.
- If the bed sits on soil rather than paving, leave the bottom open so roots and worms can move down.
To decide how deep your bed needs to be in the first place, see how deep should a raised bed be.
These charts are for the inside size of the bed. If you measured the outside of a wooden bed, take 3 inches off each dimension for 1.5-inch boards before you use the chart.
Questions gardeners ask
- How much soil do I need for a 4x8 raised bed?
- 32 cubic feet at 12 inches deep (1.19 cubic yards), 16 cubic feet at 6 inches, 48 at 18 inches and 64 at 24 inches. Add about 10 percent for settling.
- How much soil for a 4x4 raised bed?
- 16 cubic feet at 12 inches deep, which is 16 bags of 1 cubic foot, 11 bags of 1.5 cubic feet or 8 bags of 2 cubic feet.
- Is it cheaper to buy bags or bulk soil for raised beds?
- It depends on local prices and delivery fees. Work out the price per cubic foot both ways: bag price divided by the bag size, and the bulk price per yard plus delivery divided by 27 times the number of yards.
- Can I fill the bottom of a tall raised bed with something else?
- Many gardeners put a coarser, cheaper layer in the bottom of very deep beds. Woody material settles as it rots, so keep at least 12 inches of good soil on top and expect to top up.