Concrete calculator
Concrete for one slab, path or shed base of even thickness: whole bags from the yield on the bag, or cubic metres to order.
How the number is worked out, a worked example and questions
How the number is worked out
- Measure the slab. Its length and width, or draw its shape if it is not a rectangle. The area is what is left after the formwork is in.
- Enter the thickness you have decided on. BuildBatch suggests none: the thickness depends on what the slab is for and what it sits on.
- Add the allowance. The area times the thickness is the volume; a visible percentage is added once for an uneven base and spillage.
- Count bags, or order by volume. For bags, the volume is divided by what one bag makes, as printed on the bag, and rounded up once to whole bags. For ready-mixed concrete it is the cubic metres to order, rounded up to the supplier's step if you know it.
This is a material quantity, not a design. It does not decide the thickness, what goes under the slab, whether a membrane or reinforcement is needed, or whether building control must be told. Mixing from cement and ballast is not covered.
A worked example
A shed base 2.4 m × 1.8 m is 4.32 m². At 100 mm thick, with 10% extra, and bags that each make 0.01 m³ and weigh 20 kg. These are generic example figures, not a product or a design.
| Working | Figure | Result |
|---|---|---|
| Slab area | 2.4 m × 1.8 m | 4.32 m² |
| Volume at the thickness | 100 mm | 0.432 m³ |
| With 10% extra | 10% | 0.4752 m³ |
| Divide by 0.01 m³ a bag and round up | 47.52 | 48 bags |
| To carry and mix | 20 kg a bag | 960 kg |
| Ready-mixed instead, in 0.25 m³ steps | 0.25 m³ | 0.5 m³ |
The yield and the water differ between products: read them off the bag you buy, and buy all the bags from one batch.
Stop and check first
- The thickness and the ground. A base for a shed and a floor that takes a load are different slabs. The product's instructions, a designer or building control decide the thickness, the sub-base and any reinforcement.
- What is under it. Concrete goes on a prepared, compacted base, never on soil or turf. Sub-base is sold by weight, and the tonnes depend on the material's density, which this calculator does not assume.
- A membrane. A damp-proof membrane under the slab depends on what the slab is for; allow for laps and a turn-up at the edges if one is used.
- The weather and the working time. The bag's instructions give temperature limits, how long the mix can be worked, and how long to protect the new concrete. Have everything ready before the first mix.
- Carrying it. Forty-eight bags is nearly a tonne to move and mix by hand. A mixer or a ready-mixed delivery is the usual answer for anything but the smallest base.
Questions
How thick should the slab be?
BuildBatch does not say. The example uses 100 mm because it is a round figure for the sum, not a recommendation. The right thickness depends on the load, the ground and what the slab is for; the product's instructions, a designer or building control decide it.
How much does one bag make?
It is printed on the bag, and it differs between products and bag sizes. The generic example uses 0.01 m³ for a 20 kg bag. Enter the figure from the bag you are buying, and the calculator counts whole bags from it.
Bags or ready-mixed?
Choose in the calculator. Bags are counted and weighed; ready-mixed concrete is a volume to order, rounded up to the step your supplier sells in if you enter it. Suppliers also have a minimum load: ask.
Can I work out cement and ballast instead?
Not yet. A mix from cement and ballast needs a maker's mix table, which has not been read, so the calculator leaves it out rather than guess.
The base is not a rectangle. What do I do?
Draw its shape: the calculator takes a drawn slab and uses its area. Or split it into rectangles and plan each on its own.