Concrete calculator
Slabs, footings, post holes and steps, in yards, metres and bags
Pick the shape and type its size. The example is a 10 ft by 10 ft slab, 4 inches thick.
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- Measured volume
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- Same volume, other units
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- Bags of premix
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- To order ready-mix
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An estimate of quantity from the sizes you typed, not an order list or a price. Check structural work, and anything covered by a building code, with your local code or a qualified person.
How the volume is worked out
Concrete is ordered by volume, so every shape is turned into cubic feet or cubic metres first. Inches and millimetres are changed into feet and metres before multiplying: 4 inches is 0.333 ft and 100 mm is 0.1 m.
- Slab or footing: length x width x thickness.
- Round hole or column: 3.1416 x (diameter / 2)² x depth.
- Steps: each step is a solid block from the ground up, so the bottom step is one rise tall and the top step is all of them. Volume = width x tread x rise x (1 + 2 + ... + number of steps).
The total is multiplied by how many you are pouring, then the waste allowance is added. A cubic yard is 27 cubic feet; a cubic metre is about 1.31 cubic yards.
Bags
Bags are counted from the volume each bag makes once mixed (its yield), rounded up to whole bags. Yields differ between products, so check the bag and type its figure if it is different.
Worked example
A 10 ft x 10 ft slab, 4 in thick: 10 x 10 x 0.333 = 33.3 cu ft, or 1.23 cu yd. Add 10%: 36.7 cu ft (1.36 cu yd). At 0.60 cu ft per 80 lb bag that is 61.1, so 62 bags.
What it does not know
It cannot tell how thick a slab must be, what strength of concrete or reinforcement it needs, or how deep footings must go. Those depend on the load, the soil and the local code, so ask a qualified person.
Defaults and where they come from
Every default is only a starting point. Change any box to match what you are buying or building.
| Default | Value | Source |
|---|---|---|
| US bag yields | 80 lb: 0.60 cu ft; 60 lb: 0.45 cu ft; 40 lb: 0.30 cu ft | Yields printed on US concrete mix bags (for example the Quikrete Concrete Mix 1101 data sheet). Products differ: use your bag's figure. |
| Metric bag yields | 20 kg: 0.0093 m³ (about 108 bags per m³); 25 kg: 0.0116 m³ | Australian makers print 100 to 110 bags of 20 kg per cubic metre (Boral, Cement Australia). The 25 kg figure uses the same kilograms per cubic metre. UK 20 and 25 kg bags are similar; check yours. |
| Waste allowance | 10% (usual 5 to 10%) | Common trade practice for spillage, uneven ground and forms that bow. No standard sets a figure. |
| Conversions | 1 cu yd = 27 cu ft = 0.764555 m³ | Exact, from 1 yd = 0.9144 m (international yard, 1959). |