Concrete / Cement Calculator
Calculate the volume of concrete needed for slabs, footings, columns, and stairs. Converts to bags of ready-mix cement (40 kg, 25 kg, and 60 lb bags).
What Is the Concrete / Cement Calculator?
This concrete calculator estimates the volume of concrete needed for slabs, footings, columns, and circular pads. It converts the volume to the number of pre-mixed bags (available in 25 kg, 40 kg, and 60 lb sizes) and to cubic metres and cubic yards. Always add a 5–10% waste allowance when ordering.
Formula
How to Use
Select the pour type (slab, footing, column, or circular slab). Enter the dimensions in your preferred unit. The calculator converts to volume in m³ and yd³ and shows the number of bags required at standard mix ratios. Use the waste percentage slider to add an ordering buffer.
Example Calculation
Concrete driveway: 4 m long, 2.5 m wide, 100 mm thick. Volume = 4 × 2.5 × 0.1 = 1 m³. With 10% waste: 1.1 m³. At 0.02 m³ per 40 kg bag: 55 bags.
Understanding Concrete / Cement
Concrete is the most widely used construction material in the world, with roughly 10 billion tonnes produced annually. Its versatility — workable when wet, strong and durable when set — makes it indispensable for foundations, slabs, columns, and infrastructure.
The compressive strength of concrete depends on the water-cement ratio: less water (down to workable limits) produces stronger concrete. Standard residential mixes are typically 25–32 MPa compressive strength (N25–N32 in Australian standards, C25–C30 in European standards, 3500–4500 psi in US).
Concrete shrinks slightly as it cures (~0.04–0.08%). For slabs larger than about 3 m × 3 m, control joints (saw cuts) are needed to prevent random cracking by directing shrinkage cracking to predetermined lines.
Curing — keeping the concrete moist for 7–28 days — is critical for full strength development. Curing at 28 days reaches approximately 99% of the design strength. Rapid drying in hot or windy conditions causes surface cracking and weakens the slab.
Frequently Asked Questions
How many bags of concrete per cubic metre?
A standard 40 kg bag of ready-mix concrete yields approximately 0.020 m³ of set concrete. So 1 m³ requires about 50 bags (40 kg). A 25 kg bag yields about 0.013 m³ (≈77 bags/m³). A 60 lb (27 kg) US bag yields about 0.014 m³.
How thick should a concrete slab be?
For foot traffic (footpaths, patios): 75–100 mm (3–4 in). For driveways with light vehicles: 100–125 mm (4–5 in). For driveways with heavy vehicles: 150 mm (6 in). For structural slabs, consult an engineer.
Should I add a waste allowance?
Always add 5–10% extra for waste, spillage, and slight over-pouring. For irregular shapes or surfaces, 10% is safer. For simple rectangular slabs with good formwork, 5% is usually sufficient.
What is the difference between concrete and cement?
Cement is one ingredient in concrete — the binding agent. Concrete is a mixture of cement, sand (fine aggregate), gravel (coarse aggregate), and water. Ready-mix bags contain a pre-measured blend; you just add water.