Concrete Calculator
Calculate the volume of concrete needed for slabs, footings, columns, or steps. Get results in cubic metres, cubic feet, and bags of ready-mix.
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How to use this calculator
Volume in cubic metres (m³). A standard 25 kg bag of ready-mix concrete covers approximately 0.012 m³; a 40 kg bag covers 0.019 m³.
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Enter the length, width, and depth of your concrete slab or footing in metres.
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A 10% waste allowance is standard — increase for complex shapes.
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Bag counts assume standard pre-mix concrete bags; adjust for your supplier's bag size.
Frequently asked questions
How thick should a concrete slab be?
Residential driveways: 100–150 mm. Garage floors: 100 mm. Footpaths: 75–100 mm. Structural slabs and commercial floors: 150–200 mm. Always check local building codes and soil conditions.
How many bags of concrete do I need?
A 25 kg bag of standard pre-mix concrete yields about 0.012 m³ when mixed. For 1 m³ you need approximately 85 bags. Ordering ready-mix truck concrete is more economical for volumes above 0.5 m³.
What is a good concrete mix ratio?
A general-purpose mix is 1 part cement : 2 parts sand : 3 parts aggregate (1:2:3 by volume). For stronger concrete (driveways, structural): 1:1.5:3. Always add the minimum water needed — excess water weakens concrete.
Concrete Calculator — Slabs, Footings & Bag Counts
When to order ready-mix vs bags
For small pours under 0.5 m³ (e.g., fence posts, small pads), bagged pre-mix is more practical. For volumes above 0.5–1 m³, ordering ready-mix concrete by truck is faster, stronger (quality-controlled mix), and often cheaper per m³ despite the minimum delivery charge.
Curing concrete properly
Concrete gains most of its strength in the first 28 days. Keep it moist for at least 7 days (cover with wet hessian or plastic sheeting) and avoid foot traffic for 24 hours, vehicle traffic for 7 days. Temperature affects curing: avoid pouring below 5°C or above 35°C without special precautions.
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Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →