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.
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Labour, extra items, margin and tax on top of the material list. Printed with your logo, saved as PDF. Works from the 3D room planner.
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.
- How many bags of concrete per fence post? (40, 50, 60 and 80 lb, 20 and 25 kg)
Bags of 40, 50, 60 and 80 lb concrete and pounds of mix per 4x4 or 6x6 fence post, by hole width and depth, plus 20 and 25 kg bags for 100 mm posts.
- How many bags of concrete for a cubic yard? 45 of 80 lb, 60 of 60 lb, 90 of 40 lb
45 bags of 80 lb make a cubic yard of concrete (0.60 cu ft a bag), 60 of 60 lb, 90 of 40 lb; 50 with 10% waste. An 80 lb bag is 0.022 cubic yards.
- How much concrete for a 12x12 slab? (plus 16x16, 20x20, 24x24)
Cubic yards and bags of concrete for 12x12, 16x16, 20x20 and 24x24 ft slabs at 4 and 6 inches with 10% waste, the 4 x 4 m metric slab, and the point where ready-mix wins.
- How many bags of concrete for a sonotube? 8, 10, 12 and 16 inch
Bags of 80, 60 and 50 lb concrete for 8, 10, 12 and 16 inch sonotubes at 2, 3 and 4 ft, concrete per foot of tube, and 200 to 300 mm forms in 25 kg bags.
- How many bags of concrete for a 4x4 slab? (5x5, 6x6, 8x8 too)
Bags of 80, 60 and 50 lb concrete for 4x4, 5x5, 6x6 and 8x8 ft pads at 4 and 6 inches with 10% waste, for AC units, hot tubs, steps and small sheds, plus metric pads.
- How much concrete for a 4x4 m slab? (3x3 to 6x8 m)
Cubic metres of concrete for 3x3, 4x4, 5x5, 6x6, 4x6 and 6x8 m slabs at 100, 125 and 150 mm with 10% waste, in 25 kg and 20 kg bags, and where ready-mix takes over.
- How much concrete do I need for a driveway?
Cubic yards and truck loads of concrete for 10x20 to 24x40 ft driveways at 4, 5 and 6 inches with 10% waste, the same in 80 lb bags, and metric 3x6 to 6x12 m drives.
- How much concrete for a garage floor? (1, 2 and 3 car)
Cubic yards of concrete for 1, 2 and 3 car garage floors (12x20 to 24x36 ft) at 4, 5 and 6 inches with 10% waste, the 80 lb bag count for comparison, and metric garages.