Concrete Slab & Calculator
Pre-Mixed Bag Equivalents
How to Calculate Concrete for Your Slab
Planning a patio, driveway, or shed foundation requires precise calculations to ensure you order enough concrete without overspending. Concrete is typically measured in cubic yards (for truck delivery) or cubic feet (for pre-mixed bags). This calculator helps you determine exactly how much material you need based on standard dimensions.
The Concrete Formula
To determine the volume of concrete required for a rectangular slab, use the following formula:
Length (ft) × Width (ft) × Thickness (ft) = Cubic Feet
Note: Since thickness is usually measured in inches, you must divide the inches by 12 to get the foot equivalent before multiplying.
Converting to Cubic Yards
Ready-mix concrete trucks deliver by the cubic yard. To convert cubic feet to cubic yards, divide your total cubic feet by 27.
- 1 Cubic Yard = 27 Cubic Feet
- Standard Truck Capacity = 8 to 10 Cubic Yards
Pre-Mixed Bags vs. Ready-Mix Truck
If your project is small (less than 1 cubic yard), buying pre-mixed bags (like Quikrete or Sakrete) from a home improvement store is usually more economical. For larger projects, ordering a ready-mix truck saves time and labor.
Bag Yields:
- 80lb Bag: Yields approximately 0.60 cubic feet.
- 60lb Bag: Yields approximately 0.45 cubic feet.
Why Include a Waste Factor?
It is industry standard to add a "safety margin" or waste factor of 5% to 10% to your calculation. This accounts for:
- Spillage during pouring.
- Uneven subgrade depth.
- Settlement of the formwork.
Running out of concrete mid-pour creates a "cold joint," which weakens the slab structurally. It is always cheaper to buy slightly extra concrete than to pay for a second delivery (short load) to finish a small section.
Common Slab Thicknesses
- 4 Inches: Standard for sidewalks, patios, and residential driveways (passenger cars).
- 5-6 Inches: Reinforced heavy-duty driveways or garage floors.
- 6+ Inches: Heavy equipment pads or commercial foundations.