Concrete Slab Calculator
Calculate concrete volume, wastage-adjusted volume, bags or premix required, estimated cost and approximate weight for any rectangular slab shape.
Your results
Assumptions
- Concrete density is assumed at 2400 kg per cubic metre for the weight estimate.
- Wastage allowance covers spillage, uneven excavation and over-ordering.
- Bag yield and pricing vary by brand and supplier — check the bag label for the actual yield.
How this calculator works
Choose a rectangle, circle or custom area, enter thickness and a wastage allowance, then compare pre-mixed bags against delivered premix pricing. The calculator shows volume, wastage-adjusted volume, bags or cost, and approximate weight based on standard concrete density.
Frequently asked questions
How much wastage should I allow?
A common allowance is 5–10% to cover spillage, uneven excavation and minor over-ordering, but larger or more irregular pours may need more.
What density is used for the weight estimate?
2400 kg per cubic metre, a standard approximation for normal-weight concrete.
Should I choose bags or premix?
Bags suit small jobs like footings or post holes, while delivered premix is usually more cost-effective for slabs larger than about one cubic metre.