Worked examples: three slabs at 100 mm
All three are 100 mm deep in a 1:2:4 mix. The ready-mix column already includes the calculator's 5% over-order, because ready-mix is batched by volume and cannot be topped up on site.
| Slab | Wet volume | Dry material | Ready-mix | Cement | Sand | Ballast |
|---|---|---|---|---|---|---|
| 3 m × 2 m | 0.600 m³ | 0.924 m³ | 0.63 m³ | 8 bags | 17 bags | 32 bags |
| 4 m × 3 m | 1.200 m³ | 1.848 m³ | 1.26 m³ | 16 bags | 34 bags | 64 bags |
| 5 m × 4 m | 2.000 m³ | 3.08 m³ | 2.10 m³ | 26 bags | 57 bags | 106 bags |
Bags are 25 kg. The dry material is larger than the wet volume because the sand and aggregate bulk when they are loose.
Depth is the whole calculation
Volume is the area multiplied by the depth, so the depth is worth checking twice. The 4 m × 3 m slab at 100 mm is 1.200 m³; the 5 m × 4 m slab is 2.000 m³. Get the depth wrong by half and the order is wrong by half — and a slab that runs out mid-pour is a cold joint.
Ready-mix or bags?
The ready-mix volume is always a little above the wet volume: 0.63 m³ for a 0.600 m³ slab, 1.26 m³ for 1.200 m³ and 2.10 m³ for 2.000 m³. Against that, mixing on site for the largest slab means 26 bags of cement, 57 of sand and 106 of ballast. Above a couple of cubic metres the wagon is usually the cheaper and far faster route; for a small slab like 3 m × 2 m, bags are easier to handle.
Reinforcement and joints
A ground slab wants A142 mesh, held on spacers so it sits in the middle of the concrete rather than on the sub-base. The 3 m × 2 m slab takes 1 sheet and the larger slabs take 2, with 24 spacers and 14 m of perimeter joint filler on the 4 m × 3 m.
How the calculator works
- Enter the slab length, width and depth; the depth stays in millimetres.
- The wet volume is the area multiplied by the depth.
- Choose the mix and the dry material follows, then the whole 25 kg bags.
- The ready-mix volume adds a small over-order for the wagon.
- Mesh, spacers and joint filler come from the slab area and perimeter.
Where to go next
Concrete FAQs
How much concrete do I need for a 4 m × 3 m slab?
1.200 m³ at 100 mm deep. Ordered ready-mixed, allow 1.26 m³; mixed on site in a 1:2:4 mix it is 16 bags of cement, 34 of sharp sand and 64 of ballast.
How do I work out concrete volume?
Length × width × depth gives the wet volume. A 4 m × 3 m slab at 100 mm deep is 1.200 m³ — the figure the concrete calculator returns for that slab.
Is ready-mix cheaper than mixing my own?
Above a couple of cubic metres it usually is, and it is far faster: the 5 m × 4 m slab would otherwise be 106 bags of ballast to mix. For a small slab like 3 m × 2 m, bagged material is easier to handle and waste less.
How much extra concrete should I order?
The ready-mix figure already carries a 5% over-order — 0.63 m³ for a 0.600 m³ slab. Ordering is by volume off a wagon, so a little spare is far cheaper than a second load.
Do I need steel mesh in a concrete slab?
A ground slab wants A142 mesh on spacers — 1 sheet for a 3 m × 2 m slab and 2 for larger ones. A post hole or a narrow strip footing does not need mesh.
Guidance only. The quantities above are planning estimates from the ProjectCalc calculators, not a quote and not a specification. Check your own measurements and your supplier's current product data before ordering.