Concrete Slab & Footing Calculator
How to Calculate Concrete for Your Project
Whether you are pouring a patio, a driveway, or a foundation for a shed, accurately calculating the amount of concrete needed is crucial to avoid running out mid-pour or overspending on materials. This calculator helps determine exactly how much premix concrete or ready-mix truck delivery volume you require.
The Formula Used
The standard formula for calculating concrete volume is:
Length (ft) × Width (ft) × Thickness (ft) = Cubic Feet
Since most projects measure thickness in inches, we convert that dimension to feet by dividing by 12. Once we have the total cubic feet, we convert it to Cubic Yards (dividing by 27), which is the standard unit for ordering from a concrete truck.
Understanding Pre-Mix Bags
If you are mixing the concrete yourself using bags from a hardware store, you need to know the yield per bag:
- 80lb Bag: Yields approximately 0.60 cubic feet.
- 60lb Bag: Yields approximately 0.45 cubic feet.
Our calculator adds your specified safety margin (default 10%) to account for spillage, uneven subgrades, and slab variations, ensuring you have enough material to finish the job correctly.
Why Include a Safety Margin?
Professional contractors typically order 5% to 10% more concrete than the exact mathematical calculation. This accounts for spillage, forms that might bow slightly under pressure, or a sub-base that isn't perfectly flat. It is always cheaper to buy a few extra bags upfront than to make a second trip to the store while your wet concrete begins to cure.