How many bags or cubic yards of concrete do you need? Covers slabs, footings, and columns.
Concrete is ordered and mixed by volume, typically measured in cubic yards in the United States or cubic meters internationally. To calculate the volume of a rectangular slab, multiply length by width by depth (all in the same unit), then convert to cubic yards by dividing cubic feet by 27 (since 1 cubic yard = 27 cubic feet). This calculator handles slabs, footings, columns, and custom shapes.
The required concrete thickness depends on the application. Residential driveways are typically 4 inches thick. Garage floors are 4 to 6 inches. Sidewalks and patios are 4 inches. Footings for load-bearing walls are typically 8 to 12 inches deep and 12 to 24 inches wide, depending on soil conditions and local building codes. Foundation walls are 8 to 10 inches thick. Commercial and industrial slabs may be 6 to 12 inches or more, often with reinforcing steel or fiber mesh.
Always check local building codes for minimum thickness requirements. In areas with frost, footings must extend below the frost line to prevent heaving. Frost depth ranges from 12 inches in the southern U.S. to 48 inches or more in northern states. Your local building department can specify the required frost depth for your area.
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Ready-mix concrete is sold by the cubic yard and delivered by truck. Most suppliers have a minimum order of 1 cubic yard, with a short-load fee for orders under 3 to 5 yards. A standard truck holds 8 to 10 cubic yards. Add 5 to 10% to your calculated volume to account for waste, spillage, uneven subgrade, and form irregularities. Ordering slightly more is far better than running short mid-pour, which creates a cold joint that weakens the slab.
For small projects (under 1 cubic yard), pre-mixed bags from a hardware store may be more practical. An 80-pound bag of concrete mix yields approximately 0.6 cubic feet when mixed. You would need about 45 bags to make 1 cubic yard. At $5 to $7 per bag, the bagged approach costs $225 to $315 per cubic yard compared to $125 to $175 for ready-mix delivery, so the truck is more economical for anything beyond a few bags.
Concrete strength is rated in PSI (pounds per square inch). Standard residential concrete is 3,000 to 4,000 PSI. Driveways that support heavy vehicles may use 4,000 to 5,000 PSI. Structural applications like bridges and high-rise foundations use 5,000 PSI or higher. Higher PSI mixes use more cement per cubic yard, which increases cost. When ordering ready-mix, specify the PSI requirement and the supplier will adjust the mix design accordingly. The concrete reaches its rated strength after curing for 28 days under normal conditions.