Soil is usually the most expensive part of a new raised bed. Getting the volume right matters more than picking a fashionable mix name on a bag.
The formula
Convert every measurement to feet. Twelve inches is 1 foot. Six inches is 0.5 feet. Then:
Cubic feet
cubic feet = length (ft) × width (ft) × height (ft) × number of beds
Cubic yards = cubic feet ÷ 27. Use the raised bed soil calculator if you do not want to convert units by hand.
Common US bed sizes
| Inside size | Fill height | Cubic feet | Cubic yards | 1.5 cu ft bags |
|---|---|---|---|---|
| 4 × 4 ft | 10 in | 13.3 | 0.49 | 9 |
| 4 × 8 ft | 10 in | 26.7 | 0.99 | 18 |
| 4 × 8 ft | 12 in | 32 | 1.19 | 22 |
| 4 × 8 ft | 16 in | 42.7 | 1.58 | 29 |
| 3 × 6 ft | 12 in | 18 | 0.67 | 12 |
| 2 × 8 ft (narrow) | 12 in | 16 | 0.59 | 11 |
Settling
Fresh mix drops after the first deep watering. Compost-heavy fills can settle 10% or more. Either order a little extra or plan to top off two weeks after filling. Do not compact soil by stomping it; water it in, then add more.
What to fill with
Volume is only half the decision. A 4×8×12-inch bed filled with cheap screened fill dirt will hold plants but drain and feed poorly. See best soil mix for raised beds for mixing ratios, and the buying guide for raised bed soil for what to look for on a bag label.
Frequently asked questions
How many bags of soil for a 4x8 raised bed?
At 12 inches deep you need 32 cubic feet. With 1.5 cu ft bags that is 22 bags; with 2.0 cu ft bags that is 16 bags. Confirm the bag’s actual cubic feet — some “40 lb” bags are not 1 cubic foot.
Can I fill the bottom with logs (hügelkultur)?
Yes. In a 12-inch bed use 2–3 inches of thin branches packed with leaves, then about 9 inches of mix. Logs belong in a taller frame. A 4×8×24-inch bed can skip about half the bags. See the hugelkultur guide and turn it on in the soil calculator.