Concrete Calculator
Enter the length, width and depth of your slab, path, shed base or footing to get the concrete volume in cubic metres. Choose metres or centimetres for each dimension, add a waste allowance, and see both ready-mix volume and an approximate bag count.
Enter your measurements
Your m³ of concrete result will appear here instantly.
How to use this calculator
- 1
Measure the length and width of the area to be concreted.
- 2
Enter the slab depth in millimetres — Australian slabs are commonly 100 mm, and driveways 125–150 mm.
- 3
Keep the 10% waste allowance for most jobs; increase it for rough ground or irregular shapes.
- 4
Order ready-mix in 0.1 m³ increments and round up to the nearest half metre for trucks.
- 5
For small jobs under about 0.5 m³, compare the bag estimate against the cost of a mini-mix delivery.
How the calculation works
Volume
Length × width × depth gives the volume in cubic metres. All measurements are converted to metres first, so a 100 mm depth is entered as 0.1 m.
Waste allowance
Real-world pours always use a little more than the theoretical volume — the subgrade absorbs or displaces concrete, and edges are rarely perfectly square. The waste percentage tops up the theoretical volume.
Bag estimate
A 20 kg bag of premixed concrete yields roughly 0.009 m³. The bag count is your volume divided by that yield, rounded up.
Worked example
A garden shed slab 3 m × 2.5 m at 100 mm thick with 10% waste.
Volume = 3 × 2.5 × 0.1 = 0.75 m³.
With 10% waste: 0.75 × 1.1 = 0.825 m³.
Order approximately 0.85–0.9 m³ of ready-mix concrete (trucks usually supply in 0.1 m³ increments), or about 92 × 20 kg bags of premix if mixing by hand.
Frequently asked questions
How thick should my slab be?
Common Australian guidance: 100 mm for garden sheds and pedestrian paths, 125–150 mm for driveways and car ports. Reinforcement (slab mesh) and the prepared base matter as much as thickness. For anything structural or vehicle-bearing, confirm the specification with an engineer or your local council requirements.
Ready-mix or bags of premix?
As a rule of thumb, jobs over roughly 0.5–1 m³ are far easier with a ready-mix truck — one wheelbarrow holds under 0.05 m³, so a 1 m³ pour means 20+ barrow loads if mixed by hand. Bags suit small footings, post holes and repairs.
How much extra should I order?
10% covers the usual losses from an uneven excavation and spillage. Order more for very rough ground, steep sites, or shapes with many edges. Concrete suppliers will ask you to sign for the amount poured, so decide before the truck arrives.
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Results are estimates only. Always confirm material requirements with your supplier or a qualified professional before purchasing.