Concrete calculator
Concrete volume for a slab, footing or post hole, in cubic metres and bags.
How it works
Calculates the volume of concrete for a rectangular slab, a footing or a cylindrical post hole, then converts it to bags of dry premix.
The bag conversion is the part worth checking. A bag of premix yields far less than its dry weight suggests, because water and compaction reduce the volume \u2014 a 25 kg bag makes roughly 0.011 m³, so a modest 2 m × 1 m × 100 mm slab needs about eighteen bags. This is why mixing by hand stops being sensible somewhere around half a cubic metre; past that, ready-mix delivered is usually both cheaper and better.
Concrete weighs about 2,400 kg per cubic metre, so that same small slab is close to half a tonne. Formwork and any surface it sits on need to be up to it.
Add five to ten per cent for spillage, uneven substrate and over-dig. Running out mid-pour creates a cold joint, which is a permanent weakness rather than a cosmetic one.
This tool runs entirely in your browser. Nothing you enter is sent to a server, logged or stored, and the page keeps working with the network disconnected.
Common questions
- How many bags of concrete do I need per cubic metre?
- Roughly 90 bags of 25 kg premix, since each yields about 0.011 m³. That is the point at which ready-mix becomes the sensible option.
- How much does concrete weigh?
- About 2,400 kg per cubic metre, or 2.4 tonnes. A 100 mm slab weighs roughly 240 kg per square metre.
- Why not run out and buy more mid-pour?
- Because concrete poured against a partly set surface forms a cold joint, which is a permanent structural weakness rather than a visible blemish.
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Sources
- Typical range compiled from manufacturer specifications — At One Place