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.
Cement, sand and stone per cubic metre come from the Concrete Society of Southern Africa’s mix leaflet: 290 kg of cement with 0.65 m³ each of sand and stone for low-strength concrete (house foundations), 385 kg with 0.62 m³ each for medium strength (house floors, footpaths and driveways), and 460 kg with 0.60 m³ each for high strength (heavy-duty floors). The leaflet notes that these batches don’t guarantee a particular strength; where a strength is specified, have cubes tested.
Worked example
With these inputs:
- Slab shape
- Rectangle / square
- Length
- 6 m
- Width
- 4 m
- Thickness
- 100 mm
- Number of slabs
- 1
- Supply as
- Mix on site (cement, sand and stone)
- Concrete mix
- Medium strength (house floors, footpaths, driveways)
- Waste allowance
- 10%
the result is:
- Concrete needed
- 2.64 m³
- Slab area
- 24 m²
- Net volume (no waste)
- 2.4 m³
- Cement (50 kg bags)
- 21 bags
- Concrete sand
- 1.64 m³
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.
- Ask your ready-mix supplier for the smallest load they deliver and any charge for a small load.
- Batch by the whole bag of cement. It is more accurate and wastes less than measuring cement in tins or drums.
Common questions
How thick should a concrete slab be?
It depends on the load and the ground. Follow the slab thickness on your plans or from your engineer; house floors must meet SANS 10400.
How many bags of cement make a cubic metre of concrete?
About 5.8 bags of 50 kg for low-strength concrete (house foundations), 7.7 for house floors, footpaths and driveways, and 9.2 for heavy-duty floors, each with 0.6 to 0.65 m³ of sand and the same of stone (Concrete Society of Southern Africa).
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and SANS 10400 (the National Building Regulations) and your local municipality before ordering.