How much concrete do I need?
The free Australian concrete calculator — volume, bags, cost and materials, with local pricing and wastage built in.
How it's worked out
Concrete is sold by volume — cubic metres (m³). You measure the space you're filling, work out its volume, add a bit for wastage, then convert that to bags or a premix order.
Volume by shape
Every dimension is entered in millimetres (how building plans are dimensioned) and converted to metres before the maths — so a 4 m slab edge is 4000 mm. Pouring more than one identical thing? Multiply by the quantity.
Wastage
Real sites lose concrete to spillage, an uneven sub-base and over-dug holes. A 10% allowance is standard — bump it up for rough ground, drop it for tidy formwork. Your total to order is volume × (1 + wastage%).
Bags or premix
A 20 kg bag of pre-mixed concrete yields about 9.8 litres (0.0098 m³), so it takes roughly 102 bags to fill a cubic metre. We round bags up — running short mid-pour means a cold joint. Premix from a truck is sold by the cubic metre in 0.1 m³ steps, so the order is rounded up to the next 0.1 m³. The Best buy line compares the cost both ways and flags the cheaper option; under about 0.5 m³ most plants charge a short-load fee, so bags often win.
Steel reinforcement
Turn on reinforcement to size the steel. For slabs we use standard 6.0 × 2.4 m mesh sheets (SL72 for paths and shed floors, SL82 for driveways, SL92 for heavy use) and add 10% for laps, plus bar chairs at roughly 1.5 per m² to hold the mesh at the right height. Footings use trench mesh in 6 m lengths with a 15% lap allowance. Always defer to your engineer's drawings for anything structural.
Local prices
Pick your state to load an indicative metro price for premix and bags — then edit it to match your own quote. These are starting points, not a guarantee.
Weight assumes 2400 kg/m³. Costs use the prices you enter and are estimates only — confirm with your supplier.
Concrete FAQs
How many 20 kg bags of concrete are in a cubic metre?
About 102 bags. A 20 kg bag yields roughly 0.0098 m³ (9.8 litres), so 1 ÷ 0.0098 ≈ 102. Always round up and keep a spare bag or two on hand — it's cheaper than a failed pour.
How thick should a concrete slab be?
A typical domestic path or shed slab is 100 mm. Driveways that take cars are usually 100–125 mm; heavier loads (trucks, large sheds) go to 150 mm or more. Check your council and engineering requirements for anything structural.
Should I use bags or order premix?
As a rough guide, under about 0.5 m³ (a few post holes or a small pad) bags are easiest. Above that, mixing by hand becomes a slog and premix from a truck is cheaper per cubic metre and far less work. This calculator costs both so you can compare.
How much extra concrete should I allow for wastage?
10% is the standard allowance for spillage, an uneven sub-base and over-dug footings. On rough ground or hand-dug holes, 15% is safer. Tidy formwork on a level base can drop to 5%.
How much reo mesh do I need for a slab?
Reo mesh comes in 6.0 × 2.4 m sheets (14.4 m²). Divide your slab area by 14.4 and add ~10% for overlaps — so a 15 m² slab needs 2 sheets. Turn on reinforcement above to get sheets, bar chairs and an indicative cost. Use SL72 for paths and shed floors, SL82 for driveways.
How much does a cubic metre of concrete weigh?
About 2.4 tonnes (2400 kg). That's why even a modest slab is a serious amount of material — and why getting the order right matters.