How to Calculate Concrete for a Slab in Australia
Calculating the right amount of concrete for your slab ensures you have enough material to complete the job without excessive waste or costly shortfalls. Whether you're pouring a shed base, driveway, patio, or garage floor, understanding the calculation process helps you plan and budget your project accurately.
The Basic Formula
The volume of a rectangular slab is calculated using a simple formula: Length × Width × Thickness, with all measurements in metres. For example, a 4m × 3m slab at 100mm (0.1m) thickness equals 4 × 3 × 0.1 = 1.2 cubic metres. This gives you the base volume before accounting for wastage.
Standard Slab Thickness in Australia
The most common residential slab thickness is 100mm, which suits most applications including shed bases, patios, and light-duty areas. Driveways typically require 100 to 150mm thickness depending on expected vehicle loads, with 125mm being a popular choice for standard residential driveways. Garage floors follow similar guidelines, usually 100 to 125mm with appropriate reinforcement mesh.
Thickness Guide by Application
Garden paths and stepping stones can be as thin as 75mm since they only support foot traffic. Patios and entertainment areas should be 100mm minimum with SL72 reinforcement mesh. Shed bases at 100mm thickness provide adequate support for most garden sheds and workshops. Driveways need 100mm for light vehicles, increasing to 150mm if you're parking caravans, boats, or heavy vehicles regularly.
Why Add 10% Wastage
Adding 10% wastage to your calculations accounts for several unavoidable factors. Uneven ground often requires extra concrete in low spots. Spillage occurs during mixing, transporting, and pouring. Some concrete inevitably sticks to wheelbarrows, mixers, and tools. Minor variations in formwork and measurements also consume concrete. For complex shapes or sloping sites, consider increasing wastage to 15%.
Converting to Bags
Boral's guideline of approximately 108 bags per cubic metre provides a reliable estimate for 20kg pre-mixed concrete bags. This accounts for typical mixing conditions and the yield you can expect from bagged concrete. For a 1m³ slab, you'd need roughly 108 bags, which translates to about 2.16 tonnes of material to handle and mix.
Bags vs Ready-Mix Decision
For small slabs under 0.5m³, bagged concrete is often practical and cost-effective. Between 0.5m³ and 1m³, the decision depends on your physical capability and timeline, as mixing over 50 bags is significant labour. Above 1m³, ready-mix delivery usually makes more sense, offering consistent quality, faster placement, and often competitive pricing when you factor in the labour saved.
Common Slab Projects and Volumes
A standard 3m × 3m shed slab at 100mm needs approximately 0.9m³ or 97 bags. A single car driveway of 5m × 3m at 125mm requires around 1.9m³. A 4m × 4m patio at 100mm comes to 1.6m³. A double garage measuring 6m × 6m at 100mm needs 3.6m³, a volume that strongly favours ready-mix delivery over bagged concrete.
Popular Concrete Calculations
Use our pre-calculated guides for common Australian projects: