Free calculator
Concrete calculator
Enter the size of your slab, footing or post holes and get the volume of concrete with a waste allowance, plus the number of bags for the bag size you are buying.
Result
- Concrete volume
- 1.2 m³ (1,200 L)
- With 10% waste
- 1.32 m³ (1,320 L)
- Bags to buy (25 kg bag (about 12 L))
- 110
Bag yield is the typical figure printed on the bag; check yours. For a ready-mix truck, order the volume with waste.
How it works
A rectangular slab, footing or wall is length × width × thickness. A round post hole or column is π × (diameter ÷ 2)² × height. The result is multiplied by how many you are pouring and then by the waste allowance.
Bags are calculated from the yield printed on the bag: roughly 0.60 cubic feet for an 80 lb bag, 0.45 for 60 lb, 0.30 for 40 lb, and about 12 litres for a 25 kg bag of pre-mixed concrete. Yields differ slightly between brands, so check the bag and round up.
For anything larger than a few cubic metres, a ready-mix truck is usually cheaper and gives a consistent mix. Order the volume including waste; short loads cost extra.
Frequently asked questions
- How much concrete do I need for a 10 x 10 ft slab, 4 inches thick?
- About 33 cubic feet, which is 1.23 cubic yards or 0.94 cubic metres. Without waste that is 56 bags of 80 lb; with 10% waste, 62 bags.
- How thick should a concrete slab be?
- Paths and patios are commonly 100 mm (4 in). Driveways and garage floors are usually thicker, and anything structural should follow your local building code and an engineer's drawing.
- Why add a waste allowance?
- Ground is rarely perfectly level, formwork bows, and some concrete stays in the mixer and on the tools. Ten percent is a common allowance for small jobs.
- Does the calculator include reinforcement or sub-base?
- No. It only gives the concrete volume. Sub-base (compacted hardcore) and reinforcement mesh are separate items.
Guides
- How many bags of concrete for a 10x10 slab?
Bags of 80, 60 and 40 lb concrete for a 10 x 10 ft slab at 4, 5 and 6 inches with 10% waste, plus the metric 3 x 3 m version in 25 kg and 20 kg bags.