Concrete Slab Calculator
Calculate concrete for any slab or pad. Enter length, width and thickness to get cubic yards, bag count and cost estimate. Free slab calculator — accurate to contractor standards.
Dimensions
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Estimation Results
| Cubic yards = Cubic feet ÷ 27
| With waste = Cubic yards × (1 + Waste% ÷ 100)
How to Calculate Concrete for a Slab
Figuring out the exact amount of concrete needed for your pad is a straightforward process once you know the dimensions. The first step is to ensure all your measurements are in the same unit. Since concrete is sold by the cubic yard, you will want to measure the length and width in feet, and convert the slab thickness from inches to feet by dividing by 12.
Next, multiply the length, width, and depth together. This will give you the total volume of your pour in cubic feet. Because concrete is ordered in yards, you then divide that cubic feet number by 27. Finally, it's highly recommended to add a 10% waste margin to account for spillage, an uneven base, or minor measuring errors. Our concrete slab calculator handles all of these conversions automatically so you don't have to worry about the math.
Recommended Slab Thickness Guide
| Application | Recommended Thickness |
|---|---|
| Walkway / Sidewalk | 4 inches |
| Residential Patio | 4 inches |
| Garage Floor | 4–6 inches |
| Residential Driveway | 4–6 inches |
| Heavy Equipment Areas | 6+ inches |
Concrete Slab Formula
Cubic yards = (Length ft × Width ft × Thickness in ÷ 12) ÷ 27
With waste: Final yd³ = Cubic yards × 1.10 (for 10% waste)
Real-World Example
A 12 × 14 ft garage floor at 4 inches thick: 12 × 14 × (4÷12) ÷ 27 = 2.07 yd³. With 10% waste you'd order 2.28 yd³. At $150 per yard that's approximately $342 in concrete alone.
Frequently Asked Questions
How many cubic yards of concrete do I need for a slab?
Multiply length × width (in feet) by thickness in feet (inches ÷ 12), then divide by 27. Add 10% for waste. For a 10×10 ft slab at 4 inches, that's 1.23 yd³ before waste, or 1.36 yd³ with 10% waste added.
How many 80lb bags for a concrete slab?
One 80lb bag covers 0.60 cubic feet. A 10×10 ft slab at 4 inches (33.3 ft³) requires about 56 bags before waste, or 62 bags with 10% waste added.
What is the minimum concrete slab thickness?
For foot traffic and light residential use, 4 inches is the standard minimum. Driveways should be 4 to 6 inches. Areas subject to heavy vehicle loads require 6 inches or more.
How much does a concrete slab cost per square foot?
In materials only, a standard 4-inch concrete slab costs roughly $1.50 to $2.50 per square foot at current ready-mix prices. A 10×10 ft slab runs approximately $150 to $250 in concrete materials.