Concrete Calculator
Calculate the volume of concrete needed for any slab, footing, or pour. Get cubic metres, cubic yards, and number of bags.
Concrete Calculator
Typical slab: 10 cm / 4 in | Driveway: 15 cm / 6 in
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The concrete calculator tells you how much concrete you need for any rectangular slab, footing, path, or pad. Enter the length, width, and thickness — the calculator returns the volume in cubic metres and cubic yards, plus the number of 20 kg and 40 kg premix bags to buy. Getting the quantity right matters on both sides. Running short mid-pour means stopping work while fresh concrete continues to cure — a problem that can leave visible cold joints and weak spots in the finished slab. Over-ordering is wasteful and costly. Premixed bags can cost $8–15 each; over-estimating by 20% on a large pour wastes real money. This calculator covers rectangular slabs, footings, paths, and any other rectangular concrete pour. For circular piers or columns, the volume is π × radius² × depth — a different formula. For L-shaped areas, split into two rectangles and add the bag counts together.
How to Use the Concrete Calculator
The Concrete Calculator is designed to give you an accurate answer in seconds. Follow these steps:
- Step 1: Enter your details into the input fields above.
- Step 2: Adjust any settings, toggles, or unit selections to match your situation.
- Step 3: Your results appear automatically — review the output and adjust any inputs to explore different scenarios.
No account or sign-up required. All calculations run locally in your browser — nothing is stored or transmitted to any server.
How It Works
Volume (m³) = Length × Width × Thickness (cm ÷ 100) | Bags (20kg) = Volume × 110
Formula: Volume (m³) = Length (m) × Width (m) × Thickness (m) Thickness is entered in centimetres and divided by 100 to convert to metres. The waste buffer adds a percentage to account for spillage, over-excavation, and uneven sub-base. Premix bag estimates: - 20 kg bag yields approximately 0.009 m³ of finished concrete → 1 m³ ≈ 111 bags → use 110 as a round figure - 40 kg bag yields approximately 0.018 m³ → 1 m³ ≈ 55 bags Example: 3 m × 3 m slab, 10 cm thick, 10% waste buffer. Volume = 3 × 3 × 0.10 = 0.9 m³ With 10% waste = 0.99 m³ 20 kg bags = 0.99 × 110 = 109 → round up to 109 bags 40 kg bags = 0.99 × 55 = 55 bags For large pours (more than 1–2 m³), ordering ready-mix concrete delivered by truck is almost always cheaper and more practical than bags. At roughly $150–200 per cubic metre for delivered ready-mix vs $8–15 per 20 kg bag (equivalently around $880–1650 per m³ in bags), the crossover point is around 0.5–1 m³. Use bags for small jobs and patches; use a ready-mix truck for anything larger.
Frequently Asked Questions
How many bags of concrete do I need for a 3m × 3m slab?
For a 3 m × 3 m slab at 10 cm depth, you need 0.9 m³ of concrete. That is approximately 99 × 20 kg bags or 50 × 40 kg bags before adding a waste buffer. With 10% waste: 109 × 20 kg bags or 55 × 40 kg bags. Enter your exact dimensions above for a precise figure with your chosen waste allowance.
How thick should a concrete slab be?
Standard residential slabs are 10 cm (4 inches) thick for paths, patios, and shed bases. Garage floors and driveways typically need 10–15 cm due to vehicle loads. Structural slabs (house foundations, load-bearing floors) are usually 15–20 cm and should be designed by a structural engineer. Decorative garden paths can be as thin as 7–8 cm if the ground is stable and well-compacted.
How many cubic metres of concrete do I need for a driveway?
A standard single-car driveway is approximately 3 m × 6 m = 18 m². At 10 cm depth, that is 1.8 m³ of concrete — better ordered as ready-mix than bags at this volume. A double driveway (5 m × 6 m = 30 m²) at 10–15 cm requires 3–4.5 m³. Get at least two ready-mix quotes for pours over 1 m³.
When should I use bags vs ready-mix concrete?
Use premix bags for small jobs under 0.5 m³ — garden steps, fence post footings, small patches, and pads for sheds or clotheslines. Use ready-mix truck delivery for anything larger. Bags cost roughly $880–1,650 per m³ in materials alone, not counting labour for mixing, versus $150–250 per m³ for delivered ready-mix. The convenience of bags suits small jobs; the economy of scale favours ready-mix from about half a cubic metre upward.
Is the concrete calculator free?
Yes — free with no sign-up needed. Enter your slab dimensions and the calculator instantly returns cubic metres, cubic yards, and bag counts for both 20 kg and 40 kg bags. Adjust the waste buffer slider to match your project complexity. All calculations run in your browser and no data is stored.