Free Concrete Slab Calculator
Calculate exactly how much concrete you need for a rectangular slab. Enter dimensions including thickness in inches to get cubic yards.
feet
feet
inches
Volume (Cubic Yards)
2.96 cu yd
Slab Area240.0 sq ft
Volume (Cubic Feet)80.00 cu ft
With 10% Waste3.26 cu yd
Volume (Cubic Yards) vs Slab Length
How to Calculate Concrete for a Slab
Formula
Volume (cu ft) = Length (ft) x Width (ft) x Thickness (ft)
Volume (cu yd) = Volume (cu ft) / 27
First convert thickness from inches to feet by dividing by 12. Standard slab thicknesses are 4 inches for patios and walkways, 4-6 inches for driveways, and 6-8 inches for heavy-duty applications.
Example Calculation
A patio slab measuring 20 ft by 12 ft with a 4-inch thickness.
- 01Convert thickness: 4 inches / 12 = 0.333 ft
- 02Volume = 20 × 12 × 0.333 = 80 cu ft
- 03Cubic yards = 80 / 27 = 2.96 cu yd
- 04With 10% waste = 2.96 × 1.10 = 3.26 cu yd
- 05Order 3.5 cubic yards from the concrete plant.
Example Calculations
How much concrete for a 10×10 slab?
1.23 cubic yards
- 1.Dimensions: 10 ft × 10 ft × 4 inches
- 2.Convert thickness: 4 inches = 0.333 feet
- 3.Volume in cubic feet: 10 × 10 × 0.333 = 33.3 cu ft
- 4.Convert to cubic yards: 33.3 ÷ 27 = 1.23 cu yd
- 5.With 10% waste factor: 1.36 cu yd
How much concrete for a 20×20 slab?
4.94 cubic yards
- 1.Dimensions: 20 ft × 20 ft × 4 inches
- 2.Convert thickness: 4 inches = 0.333 feet
- 3.Volume: 20 × 20 × 0.333 = 133.3 cu ft
- 4.Convert: 133.3 ÷ 27 = 4.94 cu yd
- 5.With 10% waste factor: 5.43 cu yd
How much concrete for a 12×24 driveway?
3.56 cubic yards
- 1.Dimensions: 12 ft × 24 ft × 4 inches
- 2.Volume: 12 × 24 × 0.333 = 96 cu ft
- 3.Convert: 96 ÷ 27 = 3.56 cu yd
- 4.With 10% waste factor: 3.91 cu yd
- 5.Order 4 yards to be safe
How much concrete for a 4×4 post footing?
0.58 cubic feet per footing
- 1.Typical post footing: 18 inches diameter × 36 inches deep
- 2.Radius: 9 inches = 0.75 feet
- 3.Depth: 36 inches = 3 feet
- 4.Volume = π × r² × h = 3.14159 × 0.75² × 3 = 5.3 cu ft
- 5.Per footing: about 0.2 cubic yards
Frequently Asked Questions
Learn More
How to Calculate Concrete Needed for Any Project
Learn how to calculate the exact amount of concrete needed for slabs, footings, columns, and other projects. Covers volume formulas, waste factors, and ordering tips.