From dimensions to bags
The calculator turns your measurements into a volume, adds your waste allowance, and divides by what one bag yields to give a whole number of bags. It also shows the same volume rounded up to the nearest amount a supplier would deliver.
Choose a slab or footing for rectangular shapes, or a round column for posts and piers, then set the units and bag size to match what you will buy.
Getting an accurate measurement
Measure the finished depth, not the depth of the hole, and remember that the ground below a slab is usually covered with a layer of compacted hardcore or gravel that is not concrete. For footings, multiply out each trench section and add them together, then enter the total length as one run.
Round column calculations use the volume of a cylinder: π × radius² × height, multiplied by the number of columns.
Mixing and safety
Follow the instructions on the bag for the amount of water and the working time. Wear gloves and eye protection, because wet cement is caustic. For structural work such as foundations and load-bearing slabs, check the specification with an engineer or your local building authority.
Questions people ask
How do I calculate how much concrete I need?
Multiply length × width × thickness, with all three in the same unit. A 4 m × 3 m slab that is 100 mm (0.1 m) thick needs 4 × 3 × 0.1 = 1.2 m³. In feet and inches, divide the volume in cubic feet by 27 to get cubic yards.
How many bags of concrete are in a cubic metre?
About 80 bags of 25 kg general-purpose mix. In the US, a cubic yard needs roughly 45 bags of 80 lb mix, since each bag yields about 0.6 cubic feet and a yard is 27 cubic feet.
How much extra should I order?
Allow 5–10% for spillage, uneven ground and over-dug trenches, and up to 15% for rough or uneven excavations. The default is 10%. Running short mid-pour is worse than having a little left over.
When is ready-mix cheaper than bags?
Roughly above 0.5 m³ (two thirds of a cubic yard). Mixing dozens of bags by hand is hard work and can make a pour inconsistent, whereas a delivery arrives ready to place. Suppliers often have a minimum order, so check before you decide.
What thickness should a concrete slab be?
It depends on the load. Garden paths and patios are often 75–100 mm (3–4 in). Garage floors and driveways are typically 100–150 mm (4–6 in). Check local building rules or ask a builder for structural work.