Concrete Slab Calculator
Material Estimation
Total Volume (Cubic Yards):
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Total Volume (Cubic Feet):
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Pre-Mix Bags (80 lb):
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Pre-Mix Bags (60 lb):
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How to Calculate Concrete for Slabs
Calculating the correct amount of concrete for your project is crucial to avoid running short during a pour or overpaying for wasted material. Whether you are pouring a patio, a driveway, or a shed foundation, the math essentially boils down to determining the volume of the slab.
The Concrete Formula
The standard formula for calculating concrete volume is:
Length (ft) × Width (ft) × Thickness (ft) = Volume (Cubic Feet)
Since concrete is typically sold by the Cubic Yard, you must divide the total cubic feet by 27 (since there are 27 cubic feet in one cubic yard).
Why Include a Waste Margin?
Professional contractors always order more concrete than the exact mathematical volume implies. This is because:
- Uneven Subgrade: The ground is rarely perfectly flat; dips increase the volume needed.
- Spillage: Some concrete is lost during transport and screeding.
- Form Spread: Wooden forms may bow slightly under the weight of wet concrete.
We recommend adding a 5% to 10% safety margin to your order. Our calculator includes an optional 10% buffer to ensure you have enough material to finish the job.
Thickness Guidelines
Choosing the right thickness determines the longevity of your slab:
- 4 Inches: Standard for residential sidewalks, patios, and garage floors for passenger cars.
- 5-6 Inches: Recommended for driveways holding heavier vehicles (trucks, RVs).
- 6+ Inches: Heavy-duty industrial floors or commercial applications.
Bags vs. Ready-Mix Truck
If your project requires less than 1 cubic yard of concrete, buying pre-mix bags (60lb or 80lb) from a hardware store is usually more cost-effective. For projects requiring more than 2 cubic yards, ordering a Ready-Mix truck is often cheaper and saves significant labor.
Yield Reference:
- One 80lb bag yields approximately 0.60 cubic feet.
- One 60lb bag yields approximately 0.45 cubic feet.
- You need roughly 45 bags (80lb) to equal one cubic yard of concrete.