Concrete Slab & Footing Calculator
Calculate cubic yards and premix bags required
*Includes selected waste margin.
How to Calculate Concrete for Slabs and Footings
Planning a driveway, patio, or shed foundation requires precise measurements to ensure you order enough concrete without overspending. This calculator helps determine the total volume in cubic yards and the number of premix bags needed for smaller DIY projects.
The Concrete Calculation Formula
To find the amount of concrete needed for a rectangular slab, you must first calculate the volume in cubic feet and then convert it to cubic yards, which is the standard unit for ordering ready-mix concrete.
The basic formula is:
- Volume (cu ft) = Length (ft) × Width (ft) × Thickness (ft)
- Note: Since thickness is usually measured in inches, divide it by 12 to convert to feet.
Once you have cubic feet, divide by 27 to get cubic yards (since there are 27 cubic feet in one cubic yard).
How Many Bags of Concrete Do I Need?
If you are mixing the concrete yourself using premixed bags (like Quikrete or Sakrete), the calculation depends on the weight of the bag. Standard concrete yields approximately:
- 80lb bag: ~0.60 cubic feet
- 60lb bag: ~0.45 cubic feet
- 40lb bag: ~0.30 cubic feet
Our calculator determines the total weight required based on a standard density of about 133-145 lbs per cubic foot and divides by your selected bag size.
Tips for a Successful Pour
1. Prepare the Subgrade: Ensure the ground is compacted and level. Loose soil can cause the slab to crack later.
2. Add a Margin for Error: We recommend adding 5-10% to your calculation (the "Waste Margin" in our tool). Spillage happens, and uneven ground often requires more material than the theoretical math suggests.
3. Use Rebar or Mesh: For slabs that will hold weight (like a driveway), reinforcement helps prevent separation if cracks occur.
4. Standard Thicknesses:
– Walkways/Patios: 4 inches
– Driveways: 5-6 inches
– Heavy Equipment Pads: 6+ inches