Concrete Calculator
Work out how much concrete you need for a slab, path, footing or fence posts, and what to buy: ready-to-use bags, cement and ballast to mix yourself, or a ready-mix delivery.
About this tool
The volume is length × width × depth for a slab, π × radius² × depth for a round hole or pad, and side² × depth for a square hole, less the space taken up by the post. The extra percentage covers spills, the ground being lower than you thought and the concrete that sticks to the mixer; 10% is usual, more if the base is rough.
Bag counts assume a 20 kg bag of ready-to-use concrete makes about 9.4 litres once mixed (about 107 bags per cubic metre) and a 25 kg bag about 11.75 litres. Mixing your own, a cubic metre of finished concrete takes roughly 1.85 tonnes of all-in ballast, plus about 12 bags of cement at 1:5 (15–16 at 1:4, 10–11 at 1:6), because the cement and water fill the gaps between the stones rather than adding to the volume. Water is a starting guide; add it gradually until the mix holds its shape. Product yields vary, so check the bag.
For anything over about half a cubic metre, a ready-mix or mix-on-site delivery is usually cheaper and much less work, and the supplier can advise on the right strength (for example C20 for paths and bases, C25 to C30 for driveways). Your figures are remembered on this device.