Enter your slab length, width and depth to get the concrete volume in cubic metres - plus how many 20 kg or 25 kg bags to buy. Works in metres and millimetres, or switch to feet and inches.
Net volume
0.60 m³
Concrete is sold by volume, so the calculator multiplies the three dimensions of your pour, then adds a wastage allowance:
volume = length × width × thickness × wastage
It then divides that volume by the yield of one bag (the figure printed on the bag) to give the number of bags - or leaves it as a cubic-yard or cubic-metre figure to order as ready-mix.
This example follows the unit system you pick in the calculator above.
A 3 m × 2 m slab, 100 mm thick, with 5% wastage:
This table follows the unit system you pick in the calculator above.
| Bag | Yield | Bags per cubic metre |
|---|---|---|
| 20 kg | ~0.009 m³ | ~108 |
| 25 kg | ~0.0115 m³ | ~87 |
A 3 m × 3 m slab at 100 mm thick is about 0.9 m³ of concrete before wastage. That is roughly 100 × 20 kg bags, so at that size it is far easier and cheaper to order ready-mix. Enter your exact dimensions and thickness above for the precise figure.
A cubic metre is about 108 × 20 kg bags or 87 × 25 kg bags, because a 20 kg bag yields roughly 0.009 m³. Once you are past about a third of a cubic metre, ready-mix is usually cheaper and faster than bags.
A 100 mm slab is standard for patios, shed bases and footpaths. Use 125–150 mm for driveways and anything carrying vehicles, over a compacted sub-base. Footings are deeper still. Set your thickness in the calculator to match the job.
Add 5% for a clean formed slab and 10% for footings or an uneven sub-base where concrete slumps into dips and over-dig. Running short mid-pour means a cold joint, so it is always better to round up - the calculator does this for you.
Bags suit small jobs up to roughly a third of a cubic metre - posts, small pads, repairs - where mixing by hand or in a small mixer is manageable. For larger slabs and driveways, ordering ready-mix by the cubic metre is cheaper, faster and gives a more consistent mix. Switch the supply option above to compare.
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