How to Calculate Concrete
Calculating the right amount of concrete for your project prevents costly over-ordering and frustrating mid-pour shortages. Whether you are pouring a patio slab, setting fence post footings, or building a retaining wall, the process follows the same basic principle: calculate the volume of the space you need to fill.
Slab / Rectangular Formula
For rectangular shapes (slabs, walls, footings), concrete volume is calculated by multiplying three dimensions together:
Since concrete is sold in cubic yards in the US, and slab depth is typically measured in inches, you need to convert:
Column / Cylinder Formula
For cylindrical shapes like post holes, sonotubes, piers, and round columns, use the cylinder volume formula:
This calculator handles unit conversions automatically. Enter your measurements in inches (or centimeters for metric) and select the Column shape tab.
Wall and Footing Calculations
Walls and footings use the same rectangular formula but with different typical dimensions:
- Walls- length and height in feet, thickness in inches (commonly 6" or 8")
- Footings- length in feet, width and depth in inches (typically 12"×12" or 16"×8")
Why Add a Waste Factor?
In practice, you always need more concrete than the theoretical volume. Several factors contribute to material loss:
- Uneven subgrade - the ground beneath is never perfectly flat or level
- Form deflection - wooden forms bow slightly outward under the pressure of wet concrete
- Spillage - concrete left in the mixer drum, wheelbarrow, or delivery chute
- Over-excavation - digging slightly deeper or wider than planned is common
Bags vs. Ready-Mix Concrete
Choosing between bagged concrete and ready-mix delivery depends on project size:
| Method | Best For | Cost Range |
|---|---|---|
| Bagged (40-80lb) | Under 1 cubic yard ~45 bags of 80lb | $4-6 per bag |
| Ready-Mix Truck | 1+ cubic yards Min order usually 1 yd | $100-150 per cu yd |
| Short-Load Truck | 1-3 cubic yards Smaller trucks, less waste | $150-200 per cu yd |
Standard Bag Yields
Each bag of pre-mixed concrete yields a specific volume when mixed with water. These are the standard sizes available at most hardware stores:
Curing Time
After pouring, concrete needs time to reach its full strength. The curing process is gradual:
- 24-48 hours - safe for foot traffic
- 7 days - reaches ~70% strength, safe for vehicles
- 28 days - reaches 99% design strength (fully cured)