Concrete Slab Calculator
Calculate exactly how much concrete you need for any slab — floors, driveways, pathways or footings. Get volume in cubic metres or cubic yards, premix bag count and cost estimate instantly. Works in metric and imperial.
🔨 Concrete Slab Calculator
📐 How It Works
Concrete volume is calculated by multiplying the length, width and thickness of the slab. Thickness is converted from millimetres to metres before calculation. A waste factor is added to account for spillage, ground irregularities and over-pour — 10% is standard for a flat residential slab, 15% for irregular shapes or sloped ground.
📋 Worked Example
A 6m × 4m driveway slab at 125mm thickness: Volume = 6 × 4 × 0.125 = 3.0 m³. With 10% waste: 3.0 × 1.10 = 3.3 m³. At $220/m³ ready-mix: 3.3 × $220 = $726. In 20kg premix bags: 3.3 ÷ 0.0095 = 347 bags — at this volume, ready-mix concrete is far more economical than bagged mix.
❓ Frequently Asked Questions
Residential foot traffic slabs: 85–100mm. Driveways for cars: 100–125mm. Driveways for heavy vehicles or trucks: 150–200mm. Commercial floors: 150mm minimum. Pathways: 75–100mm. Always check local building codes and soil conditions with your engineer.
At 100mm thick: 1 m³ covers 10 m². At 125mm: 1 m³ covers 8 m². At 150mm: 1 m³ covers 6.67 m². Use the formula: Coverage (m²) = Volume (m³) ÷ Thickness (m).
Ready-mix concrete is more economical for pours over 0.5 m³ and gives better quality control. Bagged premix suits small jobs under 0.3 m³ where a truck delivery is impractical. For medium pours (0.3–1.0 m³), get quotes for both.
Ground is never perfectly level, forms are never perfectly rigid, and some concrete is always lost to spillage. A 10% waste factor is standard for flat slabs. Add 15% for sloped ground, irregular shapes or if the sub-base is rough or uneven.