A small slab for a shed base, a bin store or a step is one of the most common reasons to buy bagged concrete. The count comes from three numbers: the size of the slab, how thick it is, and how much concrete one bag makes once it is mixed.
The last number is the one people skip. A bag is sold by weight, but the job needs volume, and the volume one bag makes is printed on the bag as its yield.
Start with the volume
Volume is length times width times thickness, with the thickness turned from millimetres into metres. A 4 m by 2.5 m slab at 100 mm thick is 4 × 2.5 × 0.1, which is 1 cubic metre.
Add a margin for an uneven base, spillage and the bit left in the barrow. Ten per cent is a common starting point, which makes that slab 1.1 m³ to buy.
Divide by the bag yield
Wickes lists Blue Circle 20 kg ready-to-use concrete at about 0.01 m³ a bag. The Concrete Society uses 0.010 m³ for a 25 kg bag in its own example and says the printing on the bag wins. In Australia, the Cemix datasheet gives 0.01 m³ for 20 kg, 0.012 m³ for 25 kg and 0.019 m³ for 40 kg.
At 0.01 m³ a bag, one cubic metre is 100 bags. The 1.1 m³ slab above is 110 bags. Round up once, for the whole job, rather than per barrow.
| Thickness | Volume per m² | Bags per m² |
|---|---|---|
| 75 mm | 0.075 m³ | 7.5 |
| 100 mm | 0.100 m³ | 10 |
| 150 mm | 0.150 m³ | 15 |
A few worked sizes
A 3 m by 3 m base at 100 mm is 0.9 m³, or 0.99 m³ with 10% waste: 99 bags. A 2 m by 2 m pad at 75 mm is 0.3 m³, or 0.33 m³ with waste: 33 bags.
With Australian 40 kg bags at 0.019 m³, the same 1.1 m³ is 57.9 bags, so 58. Bigger bags mean fewer trips to the mixer, not less concrete.
When bags stop making sense
Over about one cubic metre, bags become hard work. 110 bags of 25 kg is 2.75 tonnes to lift, carry and mix by hand. The calculator flags this and suggests ready-mix delivery for larger slabs.
Depth matters more than people expect. Going from 100 mm to 150 mm adds half as much again, and an uneven dig can add more. Measure the depth in a few places and use the average.
What the count does not include
The count is concrete only. It does not include the sub-base under the slab, the formwork, reinforcement mesh or a membrane. The Gravel and Tonnes calculators cover the layer underneath.
None of this is structural advice. A slab that carries a vehicle, a wall or a building has its own specification; follow that, and use these numbers only to plan the shopping.
Figures are estimates for a shopping list. Check the pack or datasheet before you buy.