Concrete Slab Calculator
Calculate how much concrete you need for any slab. Choose between rectangular and circular shapes, enter your dimensions and thickness, and get results in cubic yards, bags, and estimated weight.
How to Calculate Concrete for a Slab
The formula depends on your slab shape:
- Rectangular slab: Length (ft) × Width (ft) × Thickness (in) ÷ 12 ÷ 27 = Cubic Yards
- Circular slab: π × (Diameter ÷ 2)² × Thickness (in) ÷ 12 ÷ 27 = Cubic Yards
- Add 10% waste factor to account for spillage, uneven ground, and form irregularities.
- Convert to bags: One 80lb bag yields about 0.022 cubic yards (0.6 cu ft).
Slab Thickness Recommendations
- 4 inches: Patios, sidewalks, shed floors, and light-use areas
- 5 inches: Residential driveways for standard vehicles
- 6 inches: Garage floors, heavy-use driveways, and RV pads
- 8 inches: Commercial slabs and areas with heavy equipment
Rebar Spacing Guide
- 12-inch grid: Heavy-duty driveways and commercial slabs
- 18-inch grid: Standard driveways and garage floors
- 24-inch grid: Patios and light-use areas
- Wire mesh (6×6): Walkways and patio slabs under 4 inches thick
Concrete Curing Tips
- Keep the surface moist for at least 7 days after pouring
- Use curing compound or plastic sheeting to retain moisture
- Avoid pouring in extreme heat (above 90°F) or freezing temperatures
- Do not walk on fresh concrete for at least 24 hours
- Wait 28 days before placing heavy loads on the slab
Frequently Asked Questions
How thick should a concrete slab be?
Standard concrete slab thickness depends on use: 4 inches for patios and sidewalks, 5-6 inches for driveways and garage floors, and 6-8 inches for heavy equipment or commercial applications. Always check local building codes for minimum requirements.
Do I need rebar in a concrete slab?
Rebar is recommended for driveways, garage floors, and any slab over 5 inches thick. For standard 4-inch patios and walkways, wire mesh or fiber reinforcement is usually sufficient. Rebar is spaced in a grid pattern, typically 12-18 inches apart, placed in the lower third of the slab.
How long does it take for a concrete slab to cure?
Concrete reaches about 70% of its full strength in 7 days and 100% in 28 days. You can walk on a slab after 24-48 hours, drive on it after 7 days, and place heavy loads after 28 days. Keep the slab moist during the first 7 days for proper curing.