Concrete Bags Calculator Australia — Bag Count from Volume
Bags required
This concrete bags calculator converts a known volume in cubic metres (m³) directly into a bag count for 20 kg, 25 kg, or 40 kg pre-mix bags — the standard sizes stocked by Bunnings, Mitre 10, and most Australian trade suppliers. Yields used are: 20 kg = 0.010 m³, 25 kg = 0.0125 m³, 40 kg = 0.020 m³, consistent with Boral and Cockburn Cement product data sheets. A 10% wastage buffer is applied by default to account for uneven sub-bases and pour spillage. If you haven't yet calculated your volume, use the Concrete Slab Calculator first.
Frequently Asked Questions
How many 20 kg bags do I need for 1 m³ of concrete?
You need 100 × 20 kg bags to produce 1 m³ of finished concrete, based on the standard Australian yield of 10 litres (0.010 m³) per bag. With a 10% wastage allowance, order 110 bags per m³.
What is the yield of a 25 kg bag of concrete in Australia?
A standard 25 kg pre-mix concrete bag yields approximately 12.5 litres, or 0.0125 m³, of finished concrete. This figure is consistent across Boral General Purpose Concrete, Cockburn Cement, and comparable products available from major Australian hardware chains.
When should I use pre-mix bags instead of ready-mix concrete?
Pre-mix bags are practical for pours under approximately 0.5–1.0 m³ where site access is awkward or a truck minimum charge isn't worthwhile. For larger pours — house slabs, driveways, footpaths — ordering ready-mix concrete by the cubic metre from a batch plant is significantly more cost-effective and gives a more consistent mix.
What are 40 kg bags of concrete used for?
40 kg bags are a trade-size option that reduces handling trips per m³. They're popular on commercial sites and with concreters who mix in a drum mixer. At 0.020 m³ per bag you need only 50 bags per m³, reducing labour on larger hand-mixed pours.
How accurate are bag yield estimates?
The yield figures used are the manufacturer's nominal values and are accurate for properly mixed concrete. Actual yield can vary slightly depending on the water-to-cement ratio, aggregate variation, and mixing method. Over-watering a mix to make it more workable reduces final yield and weakens the finished concrete.