How much concrete do you need?
Pick a shape, enter your measurements, get cubic yards and bag count instantly.
How is concrete volume calculated?
Concrete is ordered by volume, not by area. Multiply length × width × thickness, convert to cubic yards (or cubic metres), then add a waste allowance. A 10 ft × 10 ft slab at 4 inches thick works out to about 1.23 cubic yards before waste — roughly 1.35 after a 10% allowance.
How many bags of concrete in a cubic yard?
It depends on bag size, since each bag lists a yield rather than a volume. A standard 80 lb bag yields about 0.60 cubic feet, a 60 lb bag about 0.45, and a 50 lb bag about 0.375. That works out to roughly 45 × 80 lb bags, 60 × 60 lb bags, or 72 × 50 lb bags per cubic yard. In metric, a 25 kg bag yields about 0.0125 m³, so a cubic metre takes around 80 bags.
Bagged concrete or ready-mix?
Bagged mix wins on small pours — fence posts, small pads, repairs — because there is no delivery fee, no truck minimum and you can work at your own pace. The break-even sits around one to one and a half cubic yards; above that ready-mix pulls clearly ahead. The catch is that the quoted price per yard is rarely the final invoice: short-load fees, delivery, fuel surcharges and waiting time all get added, and some suppliers bill their minimum load whether you use it or not. Ask for a written delivered total before comparing.
How much waste should I allow?
Between 5% and 15%, depending on the pour. A flat slab on a well-compacted, level base wastes least. Uneven ground eats concrete quickly — half an inch of dip across a four-inch slab is over 10% extra on its own. Columns, steps and anything with complex formwork waste more, and small pours are less forgiving of small errors. When in doubt, round up: running short mid-pour is far worse than having a little left over.
What thickness should a slab be?
Four inches (about 100 mm) is typical for a residential patio, shed base or walkway. Driveways and anything carrying vehicle loads usually go to five or six inches, often with reinforcement. Check local building requirements for anything structural — this calculator handles the arithmetic, not the engineering.