Concrete & Mortar Calculator
Add your pours - slabs, post holes, and strip footings - and instantly see how many bags to buy and how much water to mix. Build a project list with multiple pour types. Nothing uploaded.
Concrete volume and mix design
How concrete volume is calculated
For slabs, volume = length × width × thickness in metres, giving cubic metres (m³). For post holes (cylindrical), volume = π × radius² × depth. For strip footings (rectangular trenches), volume = length × width × depth. A cubic metre of concrete weighs about 2.4 tonnes and requires roughly 400-500kg of cement plus sand, gravel, and water. This calculator handles all three pour types and adds them together to give your total volume and number of bags needed based on your chosen bag size.
Concrete strength and cure time
Concrete strength is measured in MPa (megapascals). C20 concrete reaches 20 MPa at 28 days and is suitable for slabs. C25 reaches 25 MPa and is better for driveways and structures under load. Concrete gains strength rapidly - it reaches about 50% strength in 3-7 days, 90% in 14 days. Wait at least 7 days before loading a slab heavily, and 28 days for full strength. Temperature matters - in cold weather, concrete cures much slower and may not reach design strength at all.
FAQ
Can I use a 25kg bag instead of a 20kg bag?
Yes. Both are ready-mix concrete - the difference is only weight. A 25kg bag yields about 0.0137 m³, slightly more than a 20kg bag (0.011 m³). The calculator lets you choose your available bag size, so pick whichever is cheaper or easiest to source locally.
Should I order ready-mix bags or have a truck deliver concrete?
For volumes over 2-3 m³, truck delivery (typically 3m³ minimum) is much cheaper and faster. Mixing 100+ bags by hand is exhausting. For small repairs under 1m³, bags are more practical. The calculator shows the volume, so you can compare cost with local suppliers.
Why do I need a sub-base under a concrete slab?
A compacted sub-base of 100mm hardcore or MOT Type 1 provides drainage and prevents the slab from sinking into soft ground as loads settle. Without it, your slab will crack and shift. Always compact the sub-base with a plate compactor before pouring.