How Much Soil Do I Need?
Work out the exact volume for any raised bed or planter — plus how many bags to buy, the soil blend to mix, and what it'll cost.
What to Actually Put In the Bed
Volume is only half the answer. Filling a raised bed with pure topsoil compacts badly; filling it with pure potting mix is expensive and dries out fast. The blend most growers settle on is roughly 60/30/10.
- 60% topsoil — the bulk and mineral content, cheapest by volume
- 30% compost — nutrients, structure, and biology
- 10% potting or soilless mix — keeps it light and free-draining
Proportions are approximate — soil volume varies between suppliers, and bagged soil settles once watered. Buying 5–10% more than the calculator says is normal and saves a second trip.
How deep does a raised bed need to be?
| Depth | What it suits |
|---|---|
| 4–6" | Lettuce, spinach, herbs, radishes — shallow-rooted only |
| 8–10" | Most vegetables: beans, peppers, cucumbers, brassicas |
| 12"+ | Carrots, parsnips, potatoes, full-size tomatoes |
| 18"+ | Beds on concrete or hard surfaces with no soil beneath |
If your bed sits on open ground, loosen the soil underneath with a fork before filling. Roots will use that depth too, so a 6" bed over loosened earth behaves more like a 12" one.
Can you reuse last year's soil?
Yes. Top up the level with fresh mix, then add compost at roughly 1–2 inches across the surface and work it into the top few inches. Most beds drop an inch or two a season as organic matter breaks down — that's the soil doing its job, not a fault.