Concrete Slab Calculator — Australian Metric
Enter slab dimensions
Typical: pathways 75–100 mm · driveways 100 mm · house slabs 100–150 mm
Result
This concrete slab calculator estimates volume in cubic metres (m³) and the number of pre-mix bags for any rectangular slab. Enter length and width in metres and slab thickness in millimetres — the standard way Australian concreters and hardware stores measure. The calculator uses the standard Australian yield: 20 kg bags produce approximately 10 litres (0.010 m³) of finished concrete; 25 kg bags produce approximately 12.5 litres (0.0125 m³). A 10% wastage allowance is recommended for all residential slabs to account for uneven sub-bases, spillage during pour, and minor overfill. Both Boral and Cockburn publish these yield figures on their product data sheets.
Frequently Asked Questions
How many bags of concrete do I need for 1 m³?
You need approximately 100 × 20 kg bags or 80 × 25 kg bags to produce 1 m³ of finished concrete. This is based on the standard Australian pre-mix yield of 10 litres per 20 kg bag and 12.5 litres per 25 kg bag, as published by major Australian manufacturers including Boral and Cockburn Cement.
What thickness should a concrete slab be in Australia?
Under the National Construction Code (NCC), the typical minimum thicknesses for residential construction are: garden paths 75 mm, driveways and shed floors 100 mm, house slabs on ground 100–150 mm depending on soil classification. Always confirm with a structural engineer for any load-bearing application or reactive soil sites.
Should I add extra for wastage?
Yes — a 10% wastage buffer is standard practice on Australian sites. Uneven sub-bases absorb more concrete than a flat calculation assumes, and minor spillage during pour is unavoidable. Running short mid-pour creates cold joints that weaken the slab.
What is the difference between 20 kg and 25 kg bags?
Both are widely available at Bunnings, Beaumont Tiles, and trade suppliers across Australia. The 25 kg bag gives more yield per bag and is generally more economical for larger pours. The 20 kg bag is easier to handle on-site solo. For pours over 0.5 m³, consider ordering ready-mix concrete by the cubic metre from a batch plant instead.
Can I use this calculator for footings or post holes?
This calculator is for rectangular slabs. For round post footings or pad footings, use the dedicated Concrete Footing Calculator, which handles cylindrical volumes.