About this calculator
Work out how much concrete a slab, patio, shed base or driveway needs, then see it as a ready-mix order in cubic metres, a bag count, or a site-mix recipe. Add the result to a job and it carries straight into your quote.
How it’s calculated
Volume = length × width × thickness. A circular slab uses π × radius² × thickness. The waste allowance is then added on top.
Bag yield used: a 20 kg bag of concrete mix makes roughly 0.0095 m³ (about 9.5 litres), so a cubic metre takes about 105 bags. Check the yield on your brand’s bag.
Worked example
With these inputs:
- Slab shape
- Rectangle / square
- Length
- 6 m
- Width
- 4 m
- Thickness
- 100 mm
- Number of slabs
- 1
- Supply as
- Pre-mixed bags
- Bag size
- 20 kg
- Waste allowance
- 10%
the result is:
- Concrete needed
- 2.64 m³
- Slab area
- 24 m²
- Net volume (no waste)
- 2.4 m³
- Premix order (to 0.2 m³)
- 2.8 m³
- 20 kg bag
- 278 bags
Tips from the trade
- Measure thickness at several points on the sub-base. A slab that is 10 mm thicker than planned across the whole area adds up fast.
- 5–10% waste is normal for a formed slab on a level base. Use 10–15% if the ground is uneven or the edges are hand-dug.
- Premix suppliers usually take orders in 0.2 m³ steps (some go to 0.1 m³). Expect a short-load fee under about 3–4 m³.
- Past about 1 m³ (around 105 bags of 20 kg), ready-mix or a mini-mix or mix-on-site truck is usually cheaper and much faster than bags.
Common questions
How thick should a concrete slab be?
Paths, patios and car driveways are typically 100 mm thick on a compacted base, reinforced with SL72 or SL82 mesh. Driveways for heavier vehicles go to 125–150 mm. House slabs are designed to AS 2870 for your site classification, and the engineer’s design takes priority.
How many bags of concrete do I need for a slab?
Divide the volume by your bag yield. The calculator shows every common bag size at once so you can compare prices.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and the NCC and Australian Standards before ordering.