How Much Soil Does a Raised Garden Bed Need?
A raised bed is a simple box, so the soil math is straightforward: length times width times depth. The trick is using the interior dimensions, choosing a sensible depth, and planning a mix that suits what you grow.
Use the interior dimensions
Measure the inside of the bed — the soil fills the space between the walls, not the outside footprint. A bed built from 2-by-6 lumber has walls about 1.5 inches thick, so a nominally 4 by 8 foot bed has an interior closer to 45 by 93 inches. For a typical 4 by 8 bed filled 12 inches deep, the interior volume is about 4 × 8 × 1 = 32 cubic feet, or 1.19 cubic yards.
Choose a depth
Deeper beds hold more soil and suit deeper-rooted crops, but they cost more to fill. A 6-inch bed works for shallow-rooted greens and herbs; 10 to 12 inches suits most vegetables; 18 inches or more is used for root crops and raised planters on hard ground. Soil settles after the first waterings, so fill slightly above the rim and leave a little headroom for mulch.
| Depth | Volume (4×8 bed) |
|---|---|
| 6 in | 16 cu ft (0.59 cu yd) |
| 10 in | 26.7 cu ft (0.99 cu yd) |
| 12 in | 32 cu ft (1.19 cu yd) |
| 18 in | 48 cu ft (1.78 cu yd) |
Multiple beds
For several identical beds, work out the volume of one and multiply by the count. Three 4 by 8 beds at 12 inches each need 3 × 32 = 96 cubic feet, or 3.56 cubic yards. The Raised Bed Soil Calculator does this and converts to bag counts for the bag size you choose.
Planning a soil mix
There is no single correct raised bed mix. A common starting point is roughly equal parts quality topsoil, compost, and an aeration amendment such as perlite, vermiculite, or coarse sand — then adjusted for what you grow. Leafy greens and root crops prefer different drainage; heavy native clay changes how much aeration you want; wet climates benefit from more drainage, dry ones from more water-holding organic matter. The calculator's optional mix planner breaks a total into percentages you set, but treat it as a convenience, not a recipe.
Worked example
Two 4 by 8 foot beds filled 12 inches deep: each holds 4 × 8 × (12 ÷ 12) = 32 cu ft; total 64 cu ft = 2.37 cu yd. In 2 cu ft bags: 64 ÷ 2 = 32 bags. With a 40/40/20 mix: 25.6 cu ft topsoil, 25.6 cu ft compost, and 12.8 cu ft amendment.
Practical cautions
- Use interior dimensions — wall thickness quietly shrinks the fill volume.
- Soil settles 10 to 20% after the first waterings; fill slightly high and top up later.
- Line the bottom with cardboard or weed barrier over grass to smother weeds before filling.
- For buying decisions, see Bulk Topsoil vs. Bagged Soil and How Much Topsoil Do I Need?
Once beds are filled, a mulch layer conserves moisture — the Mulch Calculator sizes that for you.