Raised Bed Soil: How Much and What Mix
Soil volumes for common raised bed sizes, simple mixes that work, and whether to buy bags or bulk.
Raised beds hold more soil than most people expect. A single 4 × 8 bed takes over a cubic yard once it’s 16” deep. Here’s how much you need and what to fill it with.
Soil for common bed sizes
Heights match beds built from two (11”) or three (16½”) rows of 2×6 boards. Figures include 10% for settling. Bags are 1.5 cu ft.
| Bed (ft) | 11” tall | Bags | 16½” tall | Bags |
|---|---|---|---|---|
| 2 × 8 | 14.7 cu ft (0.54 yd) | 10 | 22.7 cu ft (0.84 yd) | 17 |
| 3 × 6 | 16.5 cu ft (0.61 yd) | 12 | 25.6 cu ft (0.95 yd) | 18 |
| 4 × 4 | 14.7 cu ft (0.54 yd) | 10 | 22.7 cu ft (0.84 yd) | 17 |
| 4 × 8 | 29.3 cu ft (1.09 yd) | 20 | 45.5 cu ft (1.68 yd) | 32 |
| 4 × 12 | 44 cu ft (1.63 yd) | 30 | 68.2 cu ft (2.53 yd) | 47 |
The formula
Soil (cu ft) = length (ft) × width (ft) × fill depth (in) ÷ 12. Then add about 10% because new soil settles. Divide by 27 for cubic yards.
What to fill it with
You want soil that holds moisture, drains well and stays loose. Plain garden soil packs down in a bed; pure compost shrinks a lot and can be too rich. Simple mixes that work:
- 60% screened topsoil, 40% compost. Affordable in bulk and a good all-round choice.
- Equal thirds: topsoil, compost and an aerating material such as perlite, pumice or coarse bark fines. Lighter and better draining.
- A bagged raised-bed mix. The easiest for one or two small beds.
Avoid “fill dirt” (subsoil with little organic matter) and anything that might contain weed seeds or herbicide residue. Ask suppliers about their compost source if you’re growing food.
Filling deep beds more cheaply
For beds deeper than about 18”, many gardeners fill the bottom third with untreated logs, branches and leaves (sometimes called hugelkultur) or with coarse organic material, then add proper soil on top. It saves soil, but it settles a lot over the first year, so expect to top up.
Bags or bulk?
Up to about a cubic yard, bags are convenient. Beyond that, bulk soil from a landscape supplier is usually far cheaper per yard, even after delivery. Order a little extra; topping up later in small bags costs more.
Topping up each year
Beds settle and the organic matter breaks down. Most gardeners add an inch or two of compost each spring rather than refilling.
Work out several beds at once, with the soil split into your mix and the lumber to build them, in the Raised Bed, Soil & Mulch Planner.