How we work it out
Length × width × depth gives cubic metres, the same sum as mulch or decking, just with a much heavier material. The judgment call is depth. 75 mm suits a path or garden edge, 100 mm is standard for a shed slab or patio, and driveways want 125–150 mm depending on whether it's carrying a car or something heavier. Skimp on depth to save a few bags and you're setting yourself up for cracking.
Worked example: a 4 m × 3 m shed slab at 100 mm needs 1.2 m³. That's 130 bags of 20 kg premix, roughly $1,170 to $1,560 at the hardware shop, or about $380 to $480 delivered by a ready-mix truck. Past around half a cube, ready-mix nearly always wins.
Bagged or ready-mix?
Bagged premix suits small jobs: footings, post holes, a garden edge, anything under about half a cubic metre. Beyond that, mixing by hand or barrow becomes a full day's labour for a result a truck delivers already blended in one pour. Ready-mix suppliers apply a short-load surcharge under roughly a cubic metre, so a driveway or shed slab is usually the point where it's worth calling one.
Don't forget the mixer
The bagged total above is materials only. Past a handful of bags, mixing by hand in a wheelbarrow is slow going, and most people hire a mixer instead. A small electric cement mixer runs about $45 to $65 for the first day at an independent hire yard: Bendigo Smart Hire lists its electric mixer at $60 for a full day, and Geoffs Hire prices its small and large electric mixers at $45 and $55. A bigger pour, past about half a cubic metre in one go, realistically needs the mixer for more than a single day, and extra days cost less than the first: Central Hire Echuca's own rate card runs $45 for day one, $80 for two, and about $35 more for every day after that. Geoffs Hire's weekly rate ($220 for its small electric mixer, against $45 a day) works out cheaper again once a job runs past about five days. The "Bagged + mixer hire" line above scales with the job's own size using this same day-then-week structure, instead of adding one flat day's rate no matter how much there is to mix.
Common questions
How many 20kg bags of concrete make a cubic metre?
108, per Boral's own product data sheet for General Purpose Concrete Mix. A calculator that rounds this to 100 is using a convenient number, not the manufacturer's actual yield figure.
How deep should a concrete slab be?
75 mm for a path or edging, 100 mm for a shed floor or patio, and 125–150 mm for a driveway, more at the top end if trucks or heavy vehicles will cross it.
Is ready-mix concrete cheaper than bags?
Per cubic metre, yes, substantially: bagged premix works out at roughly $850 to $1,300 a cube once you're buying 100-plus bags, against about $320 to $400 delivered by truck. The catch is most suppliers charge a minimum-order or short-load fee under about a cubic metre, which is why bags still make sense for small jobs.
Do I need to hire a mixer for a DIY concrete job?
Past a handful of bags, yes, mixing by hand in a wheelbarrow is too slow to keep pace with the concrete setting. A small electric cement mixer runs about $45 to $65 for the first day at an independent hire yard, worth adding to the bagged total above rather than treating the bags as the whole cost. A pour past about half a cubic metre often needs it for longer than one day; extra days cost roughly $30 to $40 more each, cheaper again by the week once a job runs past about five days.
How long before you can drive on new concrete?
About 7 days for a light vehicle, driven and parked gently, per CCAA's own published curing data, which is only around two-thirds of the concrete's 28-day design strength. Save anything heavier, a loaded trailer, caravan or truck, for the full 28 days. Walking on it only needs 24 to 48 hours. See our full curing timeline guide for the day-by-day breakdown.
Sources
Electricity rates: AER Default Market Offer (NSW, SA, south-east QLD), Victorian Default Offer, and each remaining state's regulated tariff, current for 2026–27. Appliance figures: the energyrating.gov.au registration database (July 2026 snapshot). Methods and assumptions are described on each page.