home
Paint Calculator
Find out how many tins of paint you need.
Concrete Calculator
Calculate how much concrete you need for slabs, post holes, and footings. Get results in cubic metres, cubic yards, and pre-mixed bag equivalents (25kg and 50kg bags).
Shape
Dimensions (metres)
How to use the Concrete Calculator
Select the shape
Choose 'Slab / footing' for rectangular pours (paths, patios, foundations) or 'Post hole (cylinder)' for fence posts, deck posts, or sign posts.
Enter dimensions in metres
For a slab: enter length, width, and depth (thickness). For a post hole: enter the hole diameter and depth. All dimensions should be in metres.
Set the quantity
Enter how many slabs or post holes you need to fill. The calculator multiplies accordingly.
Read your results
See the total volume in cubic metres and cubic yards, plus the equivalent number of 25kg and 50kg pre-mixed bags. Always add 10% extra for spillage.
About this Concrete Calculator
Concrete is one of the most used materials in construction, from simple fence post footings to full house foundations. Getting the quantity right is important — ordering too little means a second trip to the builder's merchant and a potential cold joint in your pour; ordering too much wastes money and leaves you with a hardened problem to dispose of. The calculation is straightforward once you know the shape: - **Slabs and footings** are rectangular prisms: volume = length × width × depth - **Post holes** are cylinders: volume = π × radius² × depth The volume in cubic metres then converts to bag counts. A standard 25kg bag of ready-mix dry concrete (just-add-water) yields approximately 12.5 litres (0.0125 m³) of mixed concrete. Bags vary by brand — check your bag for the stated yield. For larger pours (over 0.5 m³), it's usually more economical to order ready-mixed concrete from a batching plant. Ready-mix is delivered by the cubic metre and is often stronger and more consistent than bagged mix. For small jobs like individual fence posts, bagged mix is perfectly adequate and much more convenient. Common standard concrete strengths for DIY use: C20/C25 for paths and patios, C30 for driveways and foundations. If you're unsure what mix is required for your project, consult a structural engineer or building inspector.
Frequently Asked Questions
For a slab: multiply length × width × depth. For a cylindrical post hole: π × radius² × depth. Our calculator handles both shapes and converts the volume into cubic metres, cubic yards, and the equivalent number of pre-mixed bags.
A 25kg bag of ready-mix concrete typically yields about 0.012 cubic metres (12 litres) of mixed concrete. So 1 cubic metre requires roughly 84 × 25kg bags. A 50kg bag yields about 0.024 m³, so you'd need about 42 bags per cubic metre.
A standard residential concrete slab for patios and paths is 100mm (4 inches) thick. Driveways for cars typically need 100–150mm. Structural slabs and those bearing heavy loads should be 150–200mm. Always check local building codes for required depths.
A general rule is to bury at least one-third of the post length in concrete — so a 6-foot fence post should have 2 feet in the ground. The concrete collar should extend to ground level and ideally be slightly domed to shed water. For soft soil or heavy-load posts, bury more.