Most concrete mistakes on small South African jobs start with one of three things: a slab that is thicker than anyone measured, a waste allowance that was too tight, or a mix that was never meant for the job. This guide walks through the whole estimate the way a careful builder does it, from the tape measure to the order, and shows where the numbers in BouWise come from.
1. Measure the real shape
Measure inside the shuttering, not off the drawing. Shutters are seldom set exactly to plan, and a few millimetres on each side of a large slab adds up. Split awkward shapes into rectangles, triangles and circles, work out each area and add them together.
Thickness is where estimates go wrong most often. Run a string line or a level across the shutters and measure down to the compacted base at the corners, along the edges and through the middle, then take the average. Look out for thickenings under walls, steps cast with the slab, and falls to a drain, which make one side thicker than the other.
2. Work in metres
Convert every dimension to metres before you multiply. A slab 6 m long, 4 m wide and 100 mm thick is 6 × 4 × 0.1 = 2.4 m³. Writing 100 instead of 0.1 is the classic slip, and it gives a thousand times too much.
Formula: volume (m³) = length × width × thickness, all in metres. For a round slab, volume = π × radius² × thickness.
A useful check: a 100 mm slab takes 1 m³ for every 10 m², and a 75 mm floor slab takes 1 m³ for every 13.3 m².
3. Add waste
Concrete is lost to uneven bases, bulging shutters, spillage and what sticks to the mixer and the barrow. As an estimating allowance we use about 5% on a well-shuttered slab on a level, compacted base, 10% on a typical house slab or path, and 10 to 15% for hand-dug trenches on uneven ground. On very small pours, a spare bag of cement is simpler than a percentage. Running short halfway through a pour costs far more than a little extra.
4. Turn the volume into materials
If you mix on site, the Concrete Society of Southern Africa’s 2024 mixes leaflet gives the materials for one cubic metre of concrete:
| Mix | Typical use | Cement | Concrete sand | Stone |
|---|---|---|---|---|
| Low strength | House foundations | 290 kg | 0.65 m³ | 0.65 m³ |
| Medium strength | House floors, footpaths, driveways | 385 kg | 0.62 m³ | 0.62 m³ |
| High strength | Precast items, heavy-duty floors | 460 kg | 0.60 m³ | 0.60 m³ |
Divide the cement by 50 to get 50 kg bags, and round up. Sand and stone are sold by the cube (one cubic metre) or the half cube, so round those up to the next half cube when you order.
Worked example. The 6 × 4 m slab at 100 mm is 2.4 m³; with 10% waste that is 2.64 m³. At the medium mix: 2.64 × 385 = 1,016 kg of cement, which is 21 bags. Sand and stone are each 2.64 × 0.62 = 1.64 m³, so order 2 cubes of concrete sand and 2 cubes of 19 mm stone. The concrete slab calculator does exactly this sum, and the cement, sand and stone calculator does it for any volume you type in.
5. Where MPa comes in
Concrete strength is specified in megapascals (MPa) at 28 days. The Concrete Society is clear that its site batches “do not guarantee any particular strengths”: they are good, practical mixes for ordinary house work, not a tested strength. When your plans or an engineer call for a strength, such as 25 MPa for a suspended slab, order ready-mix in that strength or use a tested mix.
For a house built under the deemed-to-satisfy rules, two numbers matter most:
- Strip foundations: SANS 10400-H asks for concrete of at least 10 MPa, or a mix of 1 cement : 4 sand : 5 stone by volume, at least 200 mm thick. See strip foundations under SANS 10400-H.
- Floor slabs on the ground: for a slab that doesn’t rest on or pass over the foundation walls, SANS 10400-J allows a plain grade 10 concrete slab, or grade 15 where the slab is the finished floor, at least 75 mm thick on a 250 µm damp-proof membrane. A surface bed resting on the foundation walls follows your plans or an engineer’s design instead. See floor slabs, DPM and mesh.
6. Order it the right way
For ready-mix, give the supplier the volume including waste and the strength from your plans; BouWise rounds ready-mix up to the next 0.1 m³. For a site mix, check that the delivery covers the whole pour: cement, concrete sand (not plaster or building sand) and 19 mm or 26 mm stone, plus water on site. Have enough hands and barrows to place the concrete before it stiffens, and keep it damp while it cures.
Quick checklist
- Measured thickness at several points, not just the edge.
- All dimensions in metres before multiplying.
- Waste added: 5% for a neat slab, 10% for most jobs, more for trenches.
- Mix chosen for the job, and the strength on the plans respected.
- Sand and stone rounded up to half cubes.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check SANS 10400 (the National Building Regulations) and your local municipality’s building control officer for your project.