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.
Garden soil calculator — frequently asked questions
How much soil do I need for a raised garden bed?
Multiply the length by the width by the depth of your bed in metres. A standard 2.4 m × 1.2 m raised bed filled to 300 mm deep needs about 0.86 cubic metres of soil. The calculator handles this automatically — just enter your dimensions.
What is the best soil mix for Australian garden beds?
A good general mix is 60% quality loam, 30% compost and 10% coarse sand. For veggie beds, increase the compost to 40%. Avoid using straight topsoil — it compacts badly in raised beds and drains poorly in the Queensland climate.
How many bags of garden soil make a cubic metre?
Most bagged garden soil or potting mix in Australia comes in 25 or 30 litre bags. You need about 40 of the 25 L bags or 33 of the 30 L bags to make one cubic metre. For anything over 0.5 m³, bulk delivery from a landscape yard is significantly cheaper.
How deep should a raised garden bed be?
300 mm is the minimum for most vegetables — enough for root development without hitting the base. 400–450 mm is better for root vegetables like carrots and parsnips. Herb beds and flower beds can get away with 200 mm if the drainage underneath is good.
Will a cubic metre of garden soil fit in a 6×4 trailer?
Just about — a 6×4 trailer holds roughly 0.7 to 1 cubic metre loaded level. Garden soil is heavier than mulch, typically 900 kg to 1.2 tonnes per cubic metre depending on moisture content, so check your trailer's ATM rating before loading.
How much does a cubic metre of garden soil cost in Australia?
Bulk garden soil from a landscape yard typically costs $60–$120 per cubic metre depending on the mix and your location. Premium veggie mix or compost-rich blends sit at the higher end. Delivery adds $50–$100 depending on distance. Bagged soil from Bunnings or hardware stores works out two to three times more expensive per cubic metre.