Free tool
Concrete Calculator
Work out how much concrete you need for a slab, base or footing. Enter the length, width and depth and get the volume in cubic metres, plus a sensible waste allowance and an estimate of how many 20kg bags of ready-mix that is.
How the concrete volume is calculated
Volume is simply length × width × depth, with the depth converted from millimetres to metres. We then add a 10% allowance for waste, spillage and uneven ground, and estimate bags using a yield of about 0.0095 m³ per 20kg bag of ready-mix concrete. For larger pours, ready-mixed delivery is usually cheaper than bags.
- Volume (m³) = length × width × (depth ÷ 1000)
- +10% added for waste and uneven sub-base
- Bags estimated at ~0.0095 m³ per 20kg bag
Estimates are guidance only — always confirm critical figures against your own measurements, supplier quotes and, for tax, your accountant or HMRC.
Common questions
How much concrete do I need for a slab?
Multiply the slab's length × width × thickness (in metres) to get cubic metres. For a 5m × 3m slab at 100mm thick that's 1.5 m³. Always add ~10% for waste — this calculator does that for you.
How many 20kg bags of concrete in a cubic metre?
Roughly 105–110 bags of 20kg ready-mix per cubic metre, depending on the mix. This tool uses about 0.0095 m³ per bag. For anything more than around half a cubic metre, ready-mixed delivery is usually more economical.
Should I add a waste allowance?
Yes — always. Ground is rarely perfectly level and some concrete is lost to spillage and over-dig. A 10% allowance (included here) is a sensible default.
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