Concrete Volume Calculator
Concrete for a slab is straightforward volume — length times width times thickness — but the real-world number always needs a waste allowance on top, since spillage, formwork imperfections and uneven ground all eat into a razor-thin order.
Inputs
Result
3.888 m³
≈ 9.33 tonnes of concrete
How the concrete volume calculator works
Base volume = length × width × thickness.
Final volume = base volume × (1 + waste allowance ÷ 100).
Approximate weight = volume × 2.4 tonnes/m³, a typical density for standard concrete.
Worked example: 6 m × 4 m slab, 0.15 m thick, 8% waste
- Base volume = 6 × 4 × 0.15 = 3.6 m³.
- With 8% waste: 3.6 × 1.08 = 3.888 m³.
- Approximate weight: 3.888 × 2.4 ≈ 9.33 tonnes.
Common mistakes to avoid
Ordering exactly the calculated volume with no waste allowance
Real pours almost always use somewhat more than the theoretical volume due to ground irregularities, formwork seepage and minor spillage — a 5-10% allowance is standard practice, not padding.
Mixing up thickness units (cm vs m)
This formula expects thickness in metres. A 150 mm slab is 0.15 m, not 150 — entering 150 directly would produce a wildly overstated volume.
Frequently asked questions
How much waste allowance should I actually use?
5-10% is common for straightforward, well-prepared slabs; rougher or uneven ground, or complex formwork, often justifies allowances toward the higher end of that range.
Is 2.4 tonnes per cubic metre accurate for all concrete mixes?
It's a reasonable general-purpose figure for standard concrete, but density varies somewhat with mix design and aggregate type — check your supplier's specific mix density for critical projects.
How is concrete typically ordered from a supplier?
Most ready-mix suppliers sell by the cubic metre, often with a minimum order quantity — this calculator's final volume (including waste allowance) is the number to quote them.
Does this work for non-rectangular slabs?
Not directly — for irregular shapes, break the area into rectangular sections, calculate each separately, and sum the volumes before applying the waste allowance.
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