How much soil does a raised bed need?
Length × width × depth, in feet, is cubic feet; divide by 27 for cubic yards. A 4 × 8 ft bed filled 12 in deep is 32 cubic feet — about 1.2 yards, or a pallet's worth of bags. Two things the box-store bag math skips: fresh mix settles about 10% after the first few waterings, so order that much extra, and several of the ingredients are sold compressed — a 3 cu ft bale of peat makes about 6 loose — which the calculator accounts for so you don't buy double.
4 × 8 × (12 ÷ 12) = 32 cu ft; × 1.1 = 35.2 cu ft to order (1.30 cu yd). As a 60/30/10 that's 21.1 cu ft of topsoil, 10.6 of compost and 3.5 of aeration — 42 bags if you buy it all bagged, or most of a yard of bulk topsoil-and-compost with a couple of bags of perlite stirred in.
Which mix?
| Mix | What it is |
|---|---|
| 60 / 30 / 10 — topsoil, compost, aeration | The extension / Gardener’s Supply default. Cheapest real soil; heavy; drains fine in a bed. |
| Mel’s mix — ⅓ compost, ⅓ peat/coir, ⅓ vermiculite | Square Foot Gardening. Light, weed-free, pricey; needs feeding after year one. |
| 50 / 50 — topsoil and compost | Simple and cheap by the yard; add aeration if your topsoil is clay-heavy. |
| Bagged raised-bed mix | No mixing; pay for convenience. Fine for one or two small beds. |
For most vegetable beds the 60/30/10 (or a plain 50/50 topsoil-compost with a few bags of perlite) is the right answer: real soil holds water and nutrients, drains fine once it's above grade, and costs a third of the alternatives by the yard. Mel's mix is wonderful to work and weed-free, and it's what the Square Foot Gardening book sells — but vermiculite is expensive, peat is a finite resource, and a soilless mix needs feeding every season. Bagged raised-bed mix is the convenient one for a single small bed; past two or three beds the bags cost more than the lumber did.
With example prices: 60/30/10 from bags ≈ $189 (42 bags); Mel's mix from bags ≈ $207 (the vermiculite); bagged raised-bed mix ≈ $240 (24 bags). The same bed filled 50/50 from a bulk yard of topsoil and compost is about $59 of material before delivery — which is why the calculator shows bulk yards next to the bags once you're past the first bed. Prices illustrative; enter yours above.
Bags per bed, common sizes
| Bed | Volume | Order (+10%) | 60/30/10, bags | Bagged mix, 1.5 cu ft bags |
|---|---|---|---|---|
| 4 × 4 ft, 6 in | 8.0 cu ft | 8.8 cu ft · 0.33 yd | 12 | 6 |
| 3 × 6 ft, 10 in | 15.0 cu ft | 16.5 cu ft · 0.61 yd | 20 | 11 |
| 4 × 8 ft, 12 in | 32.0 cu ft | 35.2 cu ft · 1.30 yd | 42 | 24 |
| 4 × 8 ft, 18 in | 48.0 cu ft | 52.8 cu ft · 1.96 yd | 62 | 36 |
| 4 × 8 ft, 24 in | 64.0 cu ft | 70.4 cu ft · 2.61 yd | 83 | 47 |
60/30/10 bags: topsoil in 0.75 cu ft (40 lb) bags, compost in 1 cu ft, perlite/bark in 2 cu ft. Engine figures; the calculator does your beds and your recipe.
How deep should a raised bed be?
Six inches grows lettuce and herbs; 10–12 inches grows almost everything if the ground underneath is open soil the roots can continue into; 18–24 inches is for beds on concrete, over hard clay or tree roots, for carrots and parsnips, or for backs that don't bend. Deeper isn't better for the plants past about 18 in — it's just more soil to buy, which is what the log-fill option is for.
A 4 × 8 × 24 in bed wants 70 cu ft of mix (2.6 yards). Fill the bottom half with logs, branches, leaves, straw and cardboard and it needs 35. The woody layer holds water, breaks down over three to five years into something very like compost, and drops a few inches as it does — top up with compost each spring. Don't do it in a shallow bed (the roots need the depth) and keep the wood untreated.
What not to fill a bed with
- Straight topsoil or "fill dirt." It compacts into a brick above grade and drains badly. Cut it with at least 30% compost and some coarse material.
- Straight compost. Too rich, slumps to half its volume in a season, and many plants sulk in it. Part of the mix, not the mix.
- Potting mix for a whole bed. Formulated for pots — too light and too dear. Bagged "raised-bed mix" is the heavier version made for this.
- Anything from a site with unknown history — free fill off a construction job can carry anything. For food beds, know the source.
Whatever you fill with, the bed eats an inch or two a year as the organic matter breaks down. Budget a few bags of compost each spring to top up — the topsoil calculator in top-dress mode sizes it.
Sources: Mel Bartholomew, All New Square Foot Gardening (Mel's mix); Gardener's Supply and University of Minnesota Extension, Raised bed gardens (60/30/10 and 50/50 guidance); Oregon State Extension, Raised Bed Gardening (depths). Bag sizes and peat expansion from product specifications.