Soil is usually the most expensive part of a new raised bed. Buy for filled soil depth, not for the outside of the lumber.
The formula
Measure the inside planting area. Frame thickness does not hold soil. Convert every measurement to feet (12 inches = 1 foot; 6 inches = 0.5 feet). Then:
Cubic feet
cubic feet = inside length (ft) × inside width (ft) × fill depth (ft) × number of beds
Cubic yards = cubic feet ÷ 27. Bag counts divide cubic feet by the bag’s cubic-foot size and round up — stores do not sell a fraction of a bag.
Worked example: 4 × 8 ft, 12 inches of fill
4 × 8 × 1 = 32 cubic feet. 32 ÷ 27 ≈ 1.19 cubic yards before you round any purchase quantity. At 1.5 cubic feet per bag that is 22 bags. Two identical beds are 64 cubic feet.
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 |
Frame height is not fill depth
A tall accessibility frame does not automatically need that much mix. Open-bottom beds can use loosened native soil as part of the rooting depth. Closed-bottom and elevated planters depend entirely on the growing medium you put in the box.
Bags vs bulk
For larger volumes, bulk topsoil is generally more economical than the same amount in individual bags. Bags still win for a single small bed, a balcony, or topping off after settling. Compare ingredients, not bag names. This page does not list prices.
Settling and headspace
Fresh mix drops after the first deep watering, and bags are not always filled to the printed volume. Compost-heavy fills often settle more than mineral-heavy ones. There is no single overage percentage that fits every mix. Water the fill in, then top off. Do not compact soil by stomping it.
What to fill with
Do not assume the whole box should be pure compost. A practical raised-bed mixture can be about one-half to two-thirds topsoil and one-third to one-half plant-based compost. Volume is still 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 the Balanced and More-topsoil calculator presets, 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.
Sources
- Raised bed gardens — University of Minnesota Extension
- How to use compost in gardens and landscapes — Oregon State University Extension
- Building a Raised Bed on a Budget — University of Georgia Extension