How this soil calculator works
The maths is straightforward: length × width × depth = volume in cubic metres. A 3 m by 1.2 m garden bed filled to 200 mm deep needs 3 × 1.2 × 0.2 = 0.72 cubic metres of soil.
For circular beds or round planters, the formula is π × radius² × depth. The calculator handles that automatically — just enter the diameter.
We add a 5% compaction margin to the recommended order quantity. Garden soil settles about 5-10% after watering and a few weeks of use, so ordering slightly more means you won't come up short when the bed settles. It's a smaller margin than mulch because soil doesn't blow away or disappear the same way loose mulch does.
How deep should garden soil be?
Depth depends on what you're growing:
- Turf underlay / lawn prep: 100–150 mm. Deep enough for grass roots to establish.
- Ornamental garden beds: 150–200 mm. Most shrubs and flowers thrive here.
- Vegetable gardens: 200–300 mm. Deep-rooted veg (tomatoes, carrots, potatoes) need the extra depth.
- Raised garden beds: 300–500 mm. Fill the whole bed, not just the top.
- Tree planting: 500+ mm, or dig a hole twice the root ball width and depth.
For raised beds, don't skimp on depth. Shallow soil limits root development and makes plants more prone to drying out in hot Australian summers.
Choosing a soil type
- Topsoil — screened natural soil. The all-purpose workhorse for filling, levelling and general garden beds. Good for most plants, economical for large areas.
- Garden mix — blend of topsoil, compost and organic matter. Ready for planting straight away, ideal for ornamental beds and new gardens.
- Veggie mix — specifically blended for vegetables with extra nutrients and good drainage. Best for dedicated veggie patches and raised beds.
- Premium compost — high in organic matter, lightweight. Use as a top-dress or to enrich existing soil, not as the sole growing medium.
- Turf underlay — engineered for lawns. Right balance of drainage and water retention for establishing grass.
- Sandy loam — drains exceptionally well. Best for natives, Mediterranean plants, and anything that hates wet feet.
- Fill soil — unscreened, heavier. For structural fill where you're not planting directly (under paths, levelling land). Cheapest option.
Bulk vs bags: which is cheaper?
Soil is heavier than mulch, which changes the bulk-vs-bags calculation. For anything over about 0.3 m³, bulk delivery is usually the cheaper option, often by 40-50% per cubic metre. Bags from Bunnings or Mitre 10 are convenient for small top-ups (a single planter, a bit of levelling) but get expensive fast.
A 6×4 trailer safely carries about 0.5 m³ of soil when filled level — noticeably less than mulch because soil is about 3× heavier per cubic metre. Overloading a trailer with wet soil is both dangerous and illegal. If you need more than half a cube, get it delivered.
Bulka bags (1 m³ in a large woven sack) are a good middle-ground option — delivered to your driveway, you shovel at your own pace, no rush to unload before the truck leaves.