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Concrete Calculator

Concrete volume and bags needed for a slab, with waste allowance.

Enter values

m
m
cm
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The formula

volume = length × width × thickness, plus 10% waste

Concrete volume is simply length × width × thickness, with thickness converted to the same unit. The waste allowance is not optional in practice: uneven sub-base, spillage and over-dig routinely absorb extra, and running short mid-pour creates a cold joint you cannot undo.

All calculations run locally in your browser and update instantly as you type.

Example calculations

Common questions this calculator answers — select one to load its values.

How much concrete for a 4 m by 3 m slab, 10 cm thick? Concrete needed (with 10% waste) 1.32 m³ How much concrete for a 2 m by 2 m slab, 15 cm thick? Concrete needed (with 10% waste) 0.66 m³

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Frequently asked questions

How many bags of premix per cubic metre?+
This tool assumes a 40 kg premix bag yields about 0.0185 m³ (18.5 litres) of mixed concrete — roughly 54 bags per m³. Check your bag’s label, as yields vary slightly by brand.
Why add 10% waste?+
Uneven ground, spillage and over-excavation almost always consume extra concrete. Ordering 10% more avoids running short mid-pour.
All calculations run locally in your browser. Nothing you type is sent to a server.