Concrete Volume Calculator
Work out the volume of concrete needed for a rectangular pour, with an allowance for wastage.
Formula
Volume = Length × Width × Thickness, then multiplied by (1 + wastage ÷ 100).
- Net volume (m³) = L (m) × W (m) × T (m)
- Total volume (m³) = Net volume × (1 + wastage % ÷ 100)
Worked example
A driveway slab 5 m long, 4 m wide and 150 mm thick, with 5% wastage.
- Convert the thickness: 150 mm = 0.15 m
- Net volume = 5 × 4 × 0.15 = 3.00 m³
- Wastage = 3.00 × 0.05 = 0.15 m³
- Total = 3.00 + 0.15 = 3.15 m³
Order 3.15 m³, which in practice means booking 3.5 m³ from the plant.
Assumptions and limitations
- The pour is a plain rectangular prism of constant thickness. Steps, thickenings, downstands and haunches must be calculated separately and added.
- No deduction is made for reinforcement. Rebar displaces well under 2% of the volume in normal slabs and is conventionally ignored.
- Wastage is a planning allowance, not a guarantee. Actual loss depends on ground conditions, formwork and placing method.
What this calculator does not do
- Does not design the pour. Thickness, grade, reinforcement and joint layout are design decisions taken elsewhere.
- Does not handle irregular shapes, falls or tapers. Break the pour into rectangular parts and add them.
- Does not size a delivery schedule. Truck capacity, pour rate and the number of loads depend on your supplier and your placing method.
Practical tips
- Measure the thickness at several points. A subgrade that dips by 20 mm across a large slab can add more concrete than your whole wastage allowance.
- Ready-mix is normally cheaper to slightly over-order than to call back for a short load. A second part-load usually carries a minimum-charge penalty.
- For an irregular shape, split it into rectangles, run each through the calculator, and add the results.
Frequently asked questions
How many cubic metres of concrete do I need for a slab?
Multiply length by width by thickness in metres. A 5 m × 4 m slab at 150 mm thick needs 3 m³ before wastage. Convert the thickness from millimetres to metres first — this is the most common mistake.
How much wastage should I allow for concrete?
Between 5% and 10% for most slabs on prepared ground. Go higher for footings poured against uneven excavation, where the sides can over-break significantly.
Does this calculator work in cubic yards?
Yes. Switch the result unit to yd³, or set the input units to feet and inches. 1 m³ is about 1.308 yd³.
Should I subtract the volume taken up by rebar?
No. Reinforcement typically occupies 1–2% of a slab volume, which is well inside the wastage allowance, so it is standard practice to ignore it.
Related calculators
- Concrete Slab Calculator Size a floor or ground slab: volume, ready-mix load, weight and area, including an edge thickening if there is one.
- Concrete Footing Calculator Calculate concrete for strip footings or a set of identical pad footings.
- Concrete Bags Calculator Convert a concrete volume into the number of pre-mixed bags needed, for small pours where ready-mix is not practical.
- Excavation Calculator Calculate excavation volume in bank measure and the swelled volume that has to be hauled away.
- Rebar Weight Calculator Calculate the weight of reinforcement bars from diameter, length and quantity.
Estimating tool, not a design. This calculator works out quantities from the dimensions you enter. It does not size any structural member, verify a design, or confirm compliance with any building code. Check every result against your project specification and applicable local standards, and involve a qualified engineer where the work requires one.