Enter your slab length, width and depth to get the concrete volume in cubic yards or cubic metres - plus how many 40/60/80 lb or 20/25 kg bags to buy. Works in feet/inches or metres/millimetres.
Net volume
23.3 cu ft
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 (printed on the bag, e.g. 0.6 cu ft for an 80 lb bag) to give the number of bags - or leaves it as a cubic-yard / cubic-metre figure to order as ready-mix.
This example follows the unit system you pick in the calculator above.
A 10 ft × 7 ft slab, 4 inches thick, with 5% wastage:
| Bag | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | ~90 |
| 60 lb | 0.45 cu ft | ~60 |
| 80 lb | 0.60 cu ft | ~45 |
| 20 kg | ~0.009 m³ | ~108 per m³ |
A 10ft × 10ft slab at 4 inches thick is about 1.23 cubic yards (0.95 m³) of concrete before wastage. That is roughly 56 × 80 lb bags, or far easier to order as ready-mix at that size. Enter your exact dimensions and thickness above for the precise figure.
A cubic yard is about 45 × 80 lb bags, 60 × 60 lb bags, or 90 × 40 lb bags, because an 80 lb bag yields roughly 0.6 cubic feet and there are 27 cubic feet in a yard. In metric, a cubic metre is about 108 × 20 kg bags. Once you are past about half a cubic yard, ready-mix is usually cheaper and faster than bags.
Four inches (100 mm) is standard for patios, shed bases and footpaths. Use 5–6 inches (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 half a cubic yard - 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 yard or metre is cheaper, faster and gives a more consistent mix. Switch the supply option above to compare.
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