Trade & construction Australia
Soil Calculator Australia
Topsoil and garden mix are sold by the cubic metre or tonne, and the volume you need depends on the depth of cover. This calculator turns the area and depth into cubic metres, then tonnes using a typical density, with an allowance for settling.
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Compacted or finished depth in millimetres.
Typical for loose material; ask your supplier for their figure.
Tonnes to order
—Cubic metres
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Cubic metres with allowance
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20 kg bags equivalent
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Small trailer loads (about 1 m³)
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Estimated cost
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The working
How much soil do I need?
Multiply length by width by depth in metres to get cubic metres, add an allowance, and multiply by the density to get tonnes. A 6 × 4 m area at 150 mm deep is 3.60 m³, which is about 5.4 tonnes of soil at 1.3 t/m³ with a 15% allowance.
Working out the depth
For a new lawn, 100 to 150 mm of topsoil is typical. Garden beds often take 200 to 300 mm, and topping up an existing bed may need only 50 mm.
To find the concrete you need for a slab or footing, use the concrete calculator.
Buying in bulk or bags
Bags suit small jobs, but bulk soil by the tonne or cubic metre is far cheaper per tonne once you need more than a few bags. Many suppliers deliver a minimum quantity, and a delivery fee applies, so check whether a trailer collection suits a small job.
Check the delivery access: a tipper truck needs a clear space to unload, and the load goes where it is dropped.
How this is calculated
The rule, step by step, so you can check any figure by hand.
volume = length × width × depth ÷ 1,000 (depth in mm)volume with allowance = volume × (1 + allowance)tonnes = volume with allowance × density (typically about 1.3 t/m³ for loose soil)bags = tonnes × 1,000 ÷ 20
Density is the number that changes the order. Loose soil weighs about 1.3 tonnes per cubic metre, but moisture, grading and compaction all move it. Wet material can weigh 10% to 20% more, which matters when you are buying by weight and the truck has a limit.
The allowance exists because loose material settles once it is spread and compacted, and some is lost at the edges. Ten to fifteen percent is common for a simple area; use more for uneven ground.
A worked example
A 6 × 4 m area spread 150 mm deep.
- Volume6 × 4 × 0.150 = 3.60 m³
- With 15% allowance4.14 m³
- Tonnes at 1.3 t/m³5.38 t
- 20 kg bags269
About 5.4 tonnes, or 4.1 cubic metres.
Bulk delivery is usually cheaper than bags above about half a cubic metre.
Depth and use
The right depth depends on the use. Garden soil is bought loose and settles after watering, so a 15% allowance is common, and a little more for clay sites. Always check the supplier's recommendation for the product you buy, since grades and blends differ.
What it leaves out
Edging, geotextile, base preparation and compaction equipment are not included. If you are building under a driveway or load, the base needs a proper design.
What this calculator does not do
- Flat areas of constant depth
- Density is typical, not a product specification
- Compaction reduces volume by an amount that depends on the material
- Price is your quote
Common questions
How much soil do I need for a garden bed?
Multiply length, width and depth in metres. A 6 × 4 m bed at 150 mm is 3.6 m³ before allowance, about 5.4 tonnes with 15% at 1.3 t/m³.
How deep should topsoil be for a lawn?
Around 100 to 150 mm over a prepared base is typical.
How many bags in a tonne?
50 bags of 20 kg weigh one tonne.
Where these numbers come from
- Your Home — Australian Government guide to sustainable homes Independent guidance on insulation, materials and design
- National Construction Code (ABCB) The code that sets minimum building requirements in Australia
Method last reviewed 2026-10-28 This calculator uses fixed rules rather than rates that change, and its method is re-checked against the sources above at least once a year. Out of date? Tell us and we will fix it.
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