Concrete Calculator
Work out how much concrete you need for a slab — volume in cubic metres, 40 kg bags, and the delivered cost — from the length, width, and thickness. Free, instant, no sign-up.
Slab dimensions
Waste & price
Your concrete estimate
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How the concrete slab calculation works
The volume of a slab is simply its area times its thickness. Enter dimensions in metres and millimetres and the calculator converts thickness to metres for you:
area (m²) = length × width raw volume (m³) = area × (thickness_mm ÷ 1000) order volume = raw volume × (1 + waste%) → rounded up to 0.5 m³ concrete cost = order volume × price per m³
Ready-mix is ordered by the cubic metre and typically supplied in half-cubic-metre steps, so the order quantity is rounded up. For small jobs you can hand-mix from 40 kg bags — about 55 bags per cubic metre.
Concrete calculator FAQ
- How do I calculate how much concrete I need for a slab?
- Multiply the slab length by its width to get the area, then multiply by the thickness in metres to get the raw volume in cubic metres (m³). Add a waste allowance of 5–10% and round up to the nearest 0.5 m³, which is how ready-mix is ordered. This calculator does all of that for you.
- How much concrete do I need for a 10m² slab at 100mm thick?
- A 10 m² slab at 100 mm (0.1 m) thick is 1.0 m³ of concrete before waste. With a 7% waste allowance you would order about 1.5 m³ once rounded up to the nearest half cubic metre.
- How many 40 kg bags of concrete are in a cubic metre?
- A 40 kg bag of pre-mixed concrete yields roughly 0.018 m³, so about 55 bags make up one cubic metre. Bags suit small hand-mixed pours; for anything larger, ready-mix delivered by the cubic metre is cheaper and faster.
- How much waste should I allow for concrete?
- Allow 5–10% for spillage, uneven sub-base, and over-excavation. Thin slabs on rough ground need more; a well-prepared, screeded base needs less. The default here is 7%.
- Does this include rebar, labour, and finishing?
- This tool focuses on concrete volume and cost. To add reinforcing steel, labour hours, and a finish allowance — then turn it into a professional quote — create a free BuilderBuddi account and use the full slab estimator.
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