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.
How to use this calculator
- Enter the concrete volume in m³. If you haven't already worked out the volume, use the Concrete Slab Calculator to get this figure first.
- Select your bag size — 20 kg, 25 kg, or 40 kg. Most Bunnings and Mitre 10 stores stock all three. The 40 kg bag is a trade size not always available at retail.
- Check the wastage box to add 10% to the order quantity. This is recommended for any pour where the sub-base may be uneven, or where spillage is likely.
- Click Calculate Bags to see the bag count and total purchase weight.
Worked example: You need 0.54 m³ of concrete for a garden path. With 10% wastage the order volume is 0.594 m³. Using 20 kg bags (0.010 m³ yield): ceil(0.594 ÷ 0.010) = 60 bags, totalling 1,200 kg. Using 25 kg bags: ceil(0.594 ÷ 0.0125) = 48 bags, totalling 1,200 kg. Both yield the same weight — the 25 kg option saves 12 handling trips.
Understanding your results
The bag count shown is always rounded up to a whole bag — you cannot buy half a bag, and under-ordering mid-pour risks a cold joint. The total weight figure is useful for planning delivery: a standard ute tray carries around 500–700 kg comfortably, and most small trailers are rated to 750 kg. If your total weight exceeds this you will need multiple trips or a larger vehicle.
The "volume to order" row shows the volume after the wastage factor is applied. This is the figure you should quote to a trade supplier if ordering a custom batch, or use as a cross-check when buying bagged product.
Common mistakes: Entering the volume in litres instead of m³ (divide by 1000 to convert); selecting the wrong bag size (a 40 kg rapid-set bag has a higher yield than a standard 40 kg general purpose bag — check the product data sheet); and forgetting that bags must be mixed in a drum mixer for pours above about 20 bags — hand mixing is too slow and risks the first batches setting before the pour is complete.
Choosing and buying bagged concrete in Australia
Australian hardware stores carry several types of bagged concrete, and picking the right product matters as much as getting the bag count right. The main categories are:
General Purpose Concrete (GP): The standard product from Boral, Cockburn, and Blue Circle. Suitable for footings, paths, shed slabs, and post holes. Sets in 24–48 hours depending on temperature. Available in 20 kg, 25 kg, and sometimes 40 kg bags.
Rapid Set Concrete: Reaches usable strength in 15–60 minutes. Ideal for setting fence posts and sign posts where you can't wait overnight. Not suitable for large pours — the fast set time means sections poured minutes apart won't bond properly. Bunnings stocks the 20 kg Boral Rapid Set and the Quikrete equivalent.
High Strength Concrete: Formulated to reach 40–50 MPa at 28 days versus the standard 20–25 MPa of GP mixes. Used where heavy loads or thin sections require more strength. Priced at a premium.
Buying tips for Australian conditions: Check the manufacture date stamped on bags — cement older than 6 months may be partially hydrated from humidity and will produce weaker concrete. In summer, store bags in shade and use cool water when mixing. In winter, don't pour if temperatures are expected to drop below 5°C overnight — early concrete must be protected from frost. Most Bunnings stores will accept returns of unopened bags within 30 days, so it's better to buy slightly more than you need and return the excess than to run short.
For very large projects, get a quote from a local batch plant. In most Australian capital cities, ready-mix concrete starts from around 0.3–0.5 m³ minimum. Regional areas often have higher minimums of 1–2 m³. The cost per m³ from a truck is almost always lower than equivalent bagged product once labour is factored in.
Australian standards and references
- Boral General Purpose Concrete product data sheet — nominal yield: 20 kg = 10 L (0.010 m³), 25 kg = 12.5 L (0.0125 m³).
- Cockburn Cement General Purpose Concrete product data sheet — equivalent yield figures to Boral for pre-mix bags.
- AS 3600:2018 — Concrete Structures: Australian design standard specifying minimum concrete strength grades for structural applications.
- NCC (National Construction Code) Volume 2 — specifies minimum concrete grade (N20) for residential footings and slabs.
- AS 1379:2007 — Specification and Supply of Concrete: covers ready-mix concrete ordering, strength grades, and consistency classes.