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.
How to use this calculator
- Enter the length and width of your slab in metres. If your slab is irregular, break it into rectangles and add the results together.
- Enter the thickness in millimetres. The default is 100 mm — change it to match your project (e.g. 75 mm for a path, 150 mm for a house slab).
- Check the wastage box to add a 10% buffer. This accounts for uneven sub-bases, spillage, and overfill. Leave it unchecked only if you are ordering ready-mix and the batch plant adds their own margin.
- Click Calculate. The result shows volume in cubic metres plus bag counts for both 20 kg and 25 kg pre-mix bags.
Worked example: A garden shed slab measuring 4.5 m × 3.0 m at 100 mm thick has a net volume of 4.5 × 3.0 × 0.1 = 1.35 m³. With 10% wastage that becomes 1.485 m³. You would need 149 × 20 kg bags (ceil(1.485 ÷ 0.010)) or 119 × 25 kg bags (ceil(1.485 ÷ 0.0125)). At typical Bunnings retail prices, the 25 kg bags cost less per m³, so larger jobs benefit from buying the bigger bags or switching to a ready-mix truck for pours above 1 m³.
Understanding your results
The calculator returns two volume figures: net volume (pure geometry) and volume to order (net plus wastage). Always order to the second figure. When buying bagged concrete, round bag counts up to the nearest full bag — you can always return unopened bags, but running short mid-pour creates a cold joint that permanently weakens the slab.
For pours above 1 m³, compare the bag count against a ready-mix quote. In most Australian metro areas, a 1 m³ ready-mix truck load is competitively priced once you factor in the labour cost of mixing individual bags. Regional areas may have higher truck minimums (typically 2–3 m³).
Common mistakes: Measuring in centimetres instead of metres (will produce a result 100× too large or small); forgetting to convert thickness from mm to m when double-checking manually; using 40 kg rapid-set bags which have a different yield (approximately 0.019 m³ per bag) not covered by this calculator.
Concrete slab construction in Australia
A well-built concrete slab starts long before the concrete is ordered. In Australia, residential slab design is governed by AS 2870 (Residential Slabs and Footings), which classifies sites from Class A (stable, non-reactive soil) through to Class P (problem sites requiring engineering). Your site's soil class determines the required slab thickness and reinforcement — confirm this with a geotechnical report if you are unsure.
Sub-base preparation is the most critical and most commonly skipped step. The ground must be compacted to at least 95% standard proctor density. Soft or loose fill must be removed and replaced with clean compacted material. In termite-prone zones (most of Australia), a termite management system must be installed before the slab is poured in accordance with AS 3660.1.
Formwork should be set to the correct level with a spirit level or laser. Timber formwork must be oiled or dampened before the pour to prevent it absorbing water from the concrete. Check for squareness by measuring diagonals — they must be equal.
Reinforcement mesh (typically SL72 or SL82 for residential slabs) must be placed at the correct depth within the slab — usually in the bottom third for ground-supported slabs, held up by bar chairs or concrete blocks. Mesh resting on the ground provides zero structural benefit.
Pouring and finishing: Pour concrete in rows and compact with a vibrator to eliminate air pockets. Screed to the formwork top, then float and finish to the required surface. In hot or windy weather, place concrete early in the morning to slow the set.
Curing is essential and often neglected. Concrete gains strength through a chemical hydration reaction that requires moisture. In Australia's climate, slabs should be cured for a minimum of 7 days using wet hessian, curing compounds, or plastic sheeting. Curing increases 28-day strength by up to 50% compared to uncured concrete.
When to call a professional: Any house slab or structural slab should be designed by a structural engineer and inspected by a building certifier. DIY concrete work is generally limited to paths, garden slabs, and shed floors.
Australian standards and references
- AS 2870:2011 — Residential Slabs and Footings: sets minimum slab thickness and reinforcement requirements by soil classification.
- NCC (National Construction Code) Volume 1 & 2 — Building Code of Australia requirements for concrete construction including minimum strength grades (N20, N25, N32).
- AS 3600:2018 — Concrete Structures: the primary design standard for reinforced concrete in Australia.
- Boral Concrete Pre-Mix product data sheet — confirms 20 kg bag yield of 10 L (0.010 m³) and 25 kg yield of 12.5 L (0.0125 m³).
- Cockburn Cement product data sheet — confirms equivalent bag yields for their pre-mix range.
- AS 3660.1:2014 — Termite Management: New Building Work — relevant for slab construction in termite-prone areas.