Concrete Calculator
How much concrete your slab needs, as volume and as bags of premix.
How the volume is calculated
Volume is simply length × width × thickness, converted to cubic yards (the unit ready-mix suppliers quote) or cubic meters. Bag counts use standard yields: an 80 lb bag makes about 0.60 cubic feet, a 60 lb bag about 0.45, and a 25 kg bag roughly 0.011 m³.
Order more than the math says
Pros order 5 to 10% extra. Subgrades are never perfectly level, forms bow, and spillage happens. Running short mid-pour is expensive (a cold joint weakens the slab), while a little extra is cheap insurance. For anything above about a cubic yard, ready-mix delivery usually beats mixing bags: a 12 × 10 ft slab at 4 inches is 1.5 yards, which is around 68 bags of 80 lb premix.
Typical thicknesses
Patios and walkways: 4 inches (10 cm). Driveways: 5 to 6 inches (12 to 15 cm). Shed bases: 4 inches. Garage floors: 6 inches. Thickness drives volume linearly, so an extra inch adds 25% to a 4-inch estimate.