How to calculate concrete volume
For a rectangular slab, volume = length × width × thickness. Make sure all measurements use the same unit before multiplying: for example, convert the thickness from centimetres to metres. A 5 m × 4 m slab that is 10 cm thick needs 5 × 4 × 0.10 = 2 m³.
Add a waste allowance
Always order a little extra. Uneven ground, spillage and formwork that is slightly larger than planned all use more concrete than the theoretical volume. An allowance of 5 to 10% is common, which is why the calculator includes a waste percentage.
Useful conversions
- 1 cubic metre = 1.308 cubic yards.
- 1 cubic yard = 27 cubic feet = 0.7646 cubic metres.
Suppliers sell ready-mix concrete by the cubic metre or cubic yard. For small jobs, bagged mix is common, so check the yield printed on the bag. For structural work, follow local building regulations and ask a professional.
Frequently asked questions
How much extra should I order?
Between 5% and 10% is typical, more for uneven ground.
Does it work for round or irregular shapes?
This tool is for rectangular slabs. For other shapes, split the area into rectangles and add the results.
Are the results exact?
They are estimates. Confirm quantities with your supplier.