How Much Soil Does a Raised Bed Need?
The formula is straightforward: Length (ft) × Width (ft) × Depth (ft) = Cubic Feet. Convert depth from inches by dividing by 12. Multiple beds? Multiply by the count at the end.
A single 4×8 bed at 12 inches deep: 4 × 8 × 1 = 32 cubic feet. At 6 inches deep, that same bed only needs 16 cubic feet — half the cost, but only appropriate for shallow-rooted crops.
The Formula, Written Out
cubic_feet = length × width × (depth_inches ÷ 12) × number_of_beds
cubic_yards = cubic_feet ÷ 27
bags_needed = CEILING(cubic_feet ÷ bag_size_cubic_feet)
Always round bags up — you can't buy half a bag, and running short mid-fill is frustrating.
Bags vs. Bulk Delivery: The Break-Even Point
Bagged soil from a garden center runs $5–12 per cubic foot (a 1.5 cu ft bag at $8 = ~$5.33/cu ft). Bulk delivery from a landscape supplier typically costs $35–60 per cubic yard plus a flat delivery fee of $50–100.
The break-even math: if bulk costs $50/yard delivered plus a $75 delivery fee, you're at $50 + ($75 ÷ yards ordered). At 5 cubic yards that's $65/yard = $2.41/cu ft — still cheaper than bags. At 1 cubic yard it's $125/yard = $4.63/cu ft — close to or above bag prices.
Rule of thumb: if you need 3 cubic yards (81 cu ft) or more, call a bulk supplier. Less than that, bags are usually more convenient than cost-prohibitive.
Soil Mix Recipes
Standard 60/30/10
60% topsoil, 30% compost, 10% aeration (perlite or horticultural grit). This is the workhorse mix — affordable, drains well, holds nutrients. Most garden centers sell pre-blended versions. It's the right choice if you're filling more than two beds and budget matters.
Mel's Mix — Equal Thirds
⅓ compost, ⅓ peat moss or coir, ⅓ vermiculite. Popularized by Mel Bartholomew's Square Foot Gardening. The mix is exceptionally light, self-draining, and weed-free. The catch: vermiculite costs $30–60 per cubic foot in bulk, making this mix 3–5× more expensive than the standard blend. Worth it for intensive small beds where yield per square foot matters.
Budget Lasagna / Hügelkultur Fill
33% wood fill or cardboard (the "brown" layer), 40% topsoil, 27% compost. You're filling the bottom third with carbon-rich material that breaks down slowly — logs, wood chips, straw, or layered cardboard work. This dramatically reduces the soil you need to buy. The trade-off: the bed will settle significantly in year one as the wood decomposes, and nitrogen can be temporarily tied up by the decomposition process. Add extra compost in spring.
How Much Will Soil Settle?
Plan on 10–20% settling in the first season. A bed filled to the rim with fresh, fluffy mix will drop 1–3 inches as organic matter compresses under its own weight and begins to decompose.
Fill to the very top of the frame on day one. After the first growing season, top off with 1–2 inches of fresh compost each spring. Over 3–5 years the soil biology will stabilize and settling slows.
Common Bed Size Quick-Reference
Cubic feet of soil needed at three depths:
| Bed Size | 6" Depth | 12" Depth | 18" Depth |
|---|---|---|---|
| 4×4 ft | 8 cu ft | 16 cu ft | 24 cu ft |
| 4×8 ft | 16 cu ft | 32 cu ft | 48 cu ft |
| 3×6 ft | 9 cu ft | 18 cu ft | 27 cu ft |
| 2×8 ft | 8 cu ft | 16 cu ft | 24 cu ft |
At 1.5 cu ft bags: a 4×8 bed at 12 inches needs 22 bags. At 6 inches deep it needs only 11. Depth is the biggest lever on cost — don't go deeper than your crops require.