Concrete Calculator
Estimate the concrete for a slab and the number of bags to buy.
Sizing a concrete slab
A slab is a flat box, so the volume is length × width × thickness. Because concrete is sold by volume, it is easy to underorder, so allow extra for uneven ground, spillage and a slightly thicker pour.
Example
A 10 ft by 10 ft slab that is 4 inches thick needs 33.3 cubic feet. With 10% extra that is about 36.7 cubic feet, or 1.36 cubic yards. In bagged mix that is about 62 bags of 80 lb, or 82 bags of 60 lb, using typical yields of 0.60 and 0.45 cubic feet.
Before you buy
- Bag yields differ between brands, so check the label.
- For large jobs, ready-mix delivered by truck is usually easier than bags.
- This estimates volume only. Depth, reinforcement and base preparation depend on what the slab is for, so follow local guidance.
Frequently asked questions
How much extra should I allow?
Ten per cent is a common allowance. Use more on rough or uneven ground.
How do I convert cubic yards to cubic metres?
Multiply cubic yards by 0.7646.
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