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.
Volume (Cubic Yards)
2.96 cu yd
Volume (Cubic Yards) vs Slab Length
How to Calculate Concrete for a Slab
Concrete is ordered in cubic yards. To figure out how many you need, calculate the volume of the slab in cubic feet (length x width x thickness), then divide by 27 (since there are 27 cubic feet in a cubic yard). A standard 10x10 slab at 4 inches thick needs about 1.23 cubic yards before waste.
The Formula
Cubic Yards = (Length x Width x Thickness in feet) / 27
If your thickness is in inches, divide by 12 first to convert to feet. A 4-inch slab is 0.333 feet thick.
Always Add a Waste Factor
Order 5-10% extra. Concrete doesn't pour perfectly into forms. Some spills, some sticks to the truck, and ground surfaces aren't perfectly level. For a 3 cubic yard pour, order 3.3 yards. Running short mid-pour is much worse than having a little left over. Most suppliers won't deliver less than 1 cubic yard, and a short load fee applies if you order less than their minimum (usually 3-4 yards).
Typical Slab Thickness
Thicker isn't always better. A well-compacted 4-inch slab on proper base material can handle most residential loads. But if the subgrade is poor (clay, organic soil, or uncompacted fill), going thicker helps.
Common Mistakes
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.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?
- 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?
- 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?
- 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.