Concrete Slab Volume Calculator
Before ordering ready-mix concrete for a slab, you need the volume in cubic yards. This calculator takes the length and width of your slab in feet and the thickness in inches, then converts to cubic yards using the standard formula: Volume = L x W x T / 324. The 324 factor combines the 12-inch-per-foot and 27-cubic-feet-per-cubic-yard conversions. Add a 5 to 10 percent waste allowance for spillage, uneven subgrade, and over-pour. The American Concrete Institute (ACI) recommends a minimum 4-inch slab thickness for residential driveways and 3.5 inches for patios.
Concrete slab volume formula
Volume (yd3) = Length (ft) x Width (ft) x Thickness (in) / 324
With waste: Volume x (1 + waste% / 100)
The number 324 equals 27 cubic feet per cubic yard multiplied by 12 inches per foot. Dividing the product of length, width, and thickness (in inches) by 324 converts directly to cubic yards, which is the standard ordering unit for ready-mix concrete.
Concrete ordering tips
- Ready-mix concrete is sold in cubic yards. Most trucks carry 8 to 10 yards.
- Always add at least 5 percent waste for small pours; 10 percent for larger pours with uneven subgrade.
- A standard 4-inch residential driveway uses approximately 1.23 cubic yards per 10 square feet of slab.
- Concrete strength for driveways should be at least 4,000 psi per ACI 332.
- Cure concrete for a minimum of 7 days by keeping it moist, per ACI 308.
Concrete slab calculator: frequently asked questions
How do I calculate cubic yards of concrete for a slab?
Multiply the length (ft) by the width (ft) by the thickness (in), then divide by 324 to get cubic yards. The formula is: Volume (yd3) = L * W * T / 324, where T is in inches.
Why divide by 324?
One cubic yard equals 27 cubic feet, and one cubic foot equals 12 inches of depth. So 27 * 12 = 324. Dividing length (ft) * width (ft) * thickness (in) by 324 gives cubic yards directly.
How much waste factor should I add?
Most concrete contractors recommend adding 5 to 10 percent for waste, spillage, and uneven subgrade. For a 4-inch slab, 10 percent extra is a common practice.
What is the standard thickness for a residential concrete slab?
Per ACI 332 and common residential practice, driveways are typically 4 inches thick, patios 3.5 to 4 inches, and structural slabs 4 to 6 inches. Always consult local building codes.
Can I use this calculator for a concrete driveway?
Yes. Enter the driveway length and width in feet and the planned slab thickness in inches. Add 10 percent waste. The calculator outputs the volume in cubic yards, which is how ready-mix concrete is ordered.
Official sources
- American Concrete Institute: ACI 332 Residential Code Requirements for Structural Concrete.
- USDA Forest Products Laboratory: FPL Homepage.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.