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Estimating Concrete for Slabs, Footings & Columns

Concrete is purchased by volume, measured in Cubic Yards in the United States or Cubic Meters (m³) globally. One cubic yard is equal to a 3-foot cube (27 cubic feet), which weighs approximately 4,000 pounds (2 US tons).

Formulas by Project Type

1. Rectangular Slabs, Patios & Footings: Volume (cu yd) = (Length_ft × Width_ft × (Thickness_in / 12)) / 27

2. Round Columns, Pier Holes & Sonotubes: Volume (cu yd) = (π × (Diameter_in / 24)² × Height_ft) / 27

3. Concrete Steps & Stairs: Divide the staircase into individual rectangular step prisms, calculate each step’s volume, and sum them together: Total Volume = Sum(Step Width × Step Run × Cumulative Step Height)

Concrete slab – calculation example

Let’s estimate the concrete needed for a typical 12 ft by 12 ft patio slab that is 4 inches thick:

1. First, enter the slab length: length = 12 ft.
2. Enter the slab width: width = 12 ft.
3. Enter the thickness: thickness = 4 inches.
4. The calculator provides the exact volume: volume = 1.78 cubic yards (48 cu ft).
5. Adding a standard 10% safety margin: recommended order = 1.96 cubic yards.
6. If using bagged premix: you will need 89 bags (60-lb) or 67 bags (80-lb).

💡 Always order at least 10% extra concrete to account for uneven ground excavation, form deflection, and minor spillage.

Pre-Mixed Bag Requirements Breakdown