Concrete Slab Calculator
Calculate the exact amount of concrete needed for your slab project — patios, driveways, foundations, and more. Get volume in cubic yards, cubic meters, bag count, and cost estimates. Supports rectangular, square, and circular slabs.
What Is a Concrete Slab?
A concrete slab is a flat, horizontal surface made of poured concrete. It is one of the most common construction elements, used for foundations, floors, patios, driveways, sidewalks, and more. Slabs can be rectangular, square, or circular, and their thickness varies depending on the intended use.
This calculator estimates the volume of concrete needed for a slab project, helping you order the right amount of material — whether you're pouring a small patio or a large foundation. It also calculates the number of concrete bags required and provides a cost estimate.
How Does the Concrete Slab Calculator Work?
The calculator uses standard geometric formulas for each shape:
Rectangle: Volume = Length × Width × Depth
Square: Volume = Side² × Depth
Circle: Volume = π × (Diameter/2)² × Depth
The base volume is then adjusted for a waste factor (default 10%) to account for spillage, uneven ground, and over-excavation. The calculator also converts between measurement units and estimates the number of pre-mixed concrete bags required based on your selected bag size.
Why Use This Concrete Slab Calculator?
- Three Shapes: Supports rectangular, square, and circular slabs.
- Unit Flexibility: Switch between imperial (feet, inches, yards) and metric (meters, centimeters).
- Bag Estimation: Calculates the number of concrete bags needed for your project.
- Cost Estimation: Optional cost calculation based on price per cubic yard or cubic meter.
- Free & Private: No registration, no data storage.
Common Concrete Slab Applications
- Patios: Outdoor living spaces, typically 4 inches (10 cm) thick.
- Driveways: Vehicle access, typically 4-6 inches (10-15 cm) thick.
- Garage Floors: 6-8 inches (15-20 cm) thick for heavy vehicles.
- House Foundations: 8-12 inches (20-30 cm) thick for structural support.
- Sidewalks: 4 inches (10 cm) thick for pedestrian traffic.
❓ Concrete Slab Calculator FAQ
How do I calculate how much concrete I need for a slab?
Measure the length, width, and depth of the slab. Multiply these dimensions to get the volume. This calculator does the math for you — just select the shape and enter your dimensions.
What is the formula for calculating concrete slab volume?
For rectangular slabs: Volume = Length × Width × Depth. For square slabs: Volume = Side² × Depth. For circular slabs: Volume = π × (Diameter/2)² × Depth.
How many bags of concrete do I need for a slab?
This depends on the bag size and the slab volume. A 60 lb bag of concrete yields about 0.45 cubic feet (0.017 cubic yards). The calculator estimates the number of bags based on your selected bag size and total volume with waste factor.
What is a waste factor and why is it important?
The waste factor (typically 5-10%) accounts for concrete lost to spillage, uneven ground, over-excavation, and minor miscalculations. It ensures you order enough concrete to complete your project without running short.
How thick should a concrete slab be?
Typical slab thicknesses: 4 inches (10 cm) for patios and sidewalks, 4-6 inches (10-15 cm) for driveways, 6-8 inches (15-20 cm) for garage floors, and 8-12 inches (20-30 cm) for house foundations.
What is the difference between cubic yards and cubic meters?
A cubic yard is a volume unit used primarily in the US, equal to 27 cubic feet (0.765 cubic meters). A cubic meter is the metric equivalent, equal to 1.308 cubic yards. This calculator shows both for your convenience.
How much concrete do I need for a 10×10 slab?
For a 10 ft × 10 ft slab at 4 inches thick: Volume = 10 × 10 × 0.333 = 33.3 cubic feet = 1.23 cubic yards. With a 10% waste factor, you'd need about 1.35 cubic yards.
How much concrete do I need for a 20×20 slab?
For a 20 ft × 20 ft slab at 4 inches thick: Volume = 20 × 20 × 0.333 = 133.3 cubic feet = 4.94 cubic yards. With a 10% waste factor, you'd need about 5.4 cubic yards.
What is the yield of a 60 lb bag of concrete?
A 60 lb bag of standard concrete mix yields approximately 0.45 cubic feet (0.017 cubic yards or 0.013 cubic meters) of finished concrete.
What is the yield of an 80 lb bag of concrete?
An 80 lb bag of standard concrete mix yields approximately 0.6 cubic feet (0.022 cubic yards or 0.017 cubic meters) of finished concrete.
What is the difference between concrete and cement?
Cement is a binding agent used in concrete. Concrete is a mixture of cement, aggregates (sand and gravel), and water. Cement is an ingredient; concrete is the finished product.
How do I calculate concrete for a circular slab?
Use the circle shape: Volume = π × (Diameter/2)² × Depth. Enter the diameter and depth in the Circle mode of this calculator.
What is the best concrete mix for a driveway?
A standard driveway mix is typically 4,000 psi concrete with a 3/4-inch aggregate. For cold climates, consider adding air entrainment to resist freeze-thaw damage. Always check local building codes for specific requirements.
How long does concrete take to cure?
Concrete reaches about 70% of its final strength in 7 days and 90% in 28 days. It can be walked on after 24-48 hours, but full curing takes about 28 days for maximum strength.
What is the difference between ready-mix and bagged concrete?
Ready-mix concrete is delivered by truck and is suitable for large projects. Bagged concrete (pre-mixed) is sold in bags and is ideal for small projects, repairs, and DIY work. This calculator estimates both.
What is the cost of concrete per cubic yard?
Concrete prices vary by region, but generally range from $120-$180 per cubic yard for standard mix. This calculator includes an optional cost estimation feature — just enter your local price per cubic yard or cubic meter.
How do I estimate concrete for a slab with multiple sections?
Calculate the volume for each section separately and add them together. This calculator supports one slab at a time — for complex projects, use the rectangular shape for each section and sum the results.