Concrete Slab Calculator — How Much Concrete Do I Need?
Calculate the exact amount of concrete needed for your slab, patio, or driveway. Includes waste factor.
How to Use This Calculator
- Enter the length and width of your slab in feet
- Enter the desired thickness in inches (4" is standard for patios)
- Adjust the waste factor slider (10% is recommended)
- Enter the number of slabs if you're pouring multiple
- Click "Calculate" to see your results
How the Calculation Works
volume_cubic_feet = length_ft × width_ft × (thickness_in / 12)
volume_cubic_yards = volume_cubic_feet / 27
volume_with_waste = volume_cubic_yards × (1 + waste_factor)
bags_80lb = ceil(volume_cubic_feet / 0.6)
bags_60lb = ceil(volume_cubic_feet / 0.45) The calculation converts all dimensions to feet, calculates the volume in cubic feet, then converts to cubic yards. A waste factor is added to account for spillage and uneven surfaces. For small projects, bag quantities are calculated based on standard bag sizes.
Variables:
- length_ft: Length of the slab in feet
- width_ft: Width of the slab in feet
- thickness_in: Thickness of the slab in inches
- waste_factor: Percentage of extra material needed (typically 10%)
Pro Tips
- Order 10% extra for waste and spillage
- For driveways, use minimum 4" thickness
- For garages or heavy loads, use 5–6" thickness
- Temperature affects curing — below 50°F, use fast-setting mix
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Frequently Asked Questions
How much concrete do I need for a 10x10 slab?
For a 10×10 foot slab at 4 inches thick, you need approximately 1.23 cubic yards of concrete.
How many bags of concrete for a small slab?
A 4x4 slab at 4" thick needs about 9 bags (80lb) of concrete mix.
Should I add extra concrete for waste?
Yes, always add 10% for waste and spillage to ensure you have enough material.
What thickness should a concrete slab be?
4" for patios and walkways, 5–6" for driveways and garages.
How much does a cubic yard of concrete cost?
Ready-mix: $100–$150/yard. Bags: more expensive per yard but better for small jobs.