How the Slab Estimate Works
The calculator multiplies length × width × thickness after converting every input to metres. A 3m × 3m slab entered at 100mm thick has a geometric volume of 3 × 3 × 0.1 = 0.90m³. If you select a 10% allowance, the planning quantity becomes 0.99m³.
The bag count multiplies the quantity including the selected allowance by 108 and rounds up to a whole bag. In the example above, 0.99 × 108 = 106.92, so the result is 107 bags.
Measure the actual pour
Measure the inside faces of the formwork, not a nominal shed, patio or garage size. Take several depth measurements if the base is uneven. For stepped or irregular slabs, divide the shape into rectangles, calculate each section, then add the volumes.
Enter a designed thickness
The calculator intentionally does not recommend thickness, reinforcement or concrete strength. Those choices depend on the purpose, loads, ground conditions and approved design. Use dimensions from the relevant plans, manufacturer or qualified professional.
Check the ordering assumptions
The 108-bag conversion follows Boral's 20kg Concrete Mix guide. Other products can yield a different volume. The cost range uses an indicative Australian retail range reviewed on 24 August 2026; confirm the local shelf price and obtain a ready-mix quote before ordering.
See the complete methodology and source list.