Concrete Slab Calculator
Calculate how much concrete you need for a slab, patio, or driveway. Get results in cubic yards, bags, and estimated cost.
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How to Calculate Concrete for a Slab
Calculating the right amount of concrete for a slab is straightforward once you understand the formula. Measure your slab's length and width in feet, then the depth in inches. The calculator converts depth to feet (dividing by 12), multiplies all three dimensions to get cubic feet, then divides by 27 to convert to cubic yards — the standard unit for ordering concrete.
The formula is: Volume (ft³) = Length × Width × (Depth ÷ 12). Then divide by 27 to get cubic yards. Always add a waste factor (typically 10%) to account for spillage, over-excavation, and form inconsistencies.
Tips for a Successful Concrete Slab Pour
- Prepare the subgrade properly. Compact the soil and add a 4-inch gravel base for drainage. Poor subgrade is the #1 cause of cracking and settling.
- Use fiber mesh or wire mesh for reinforcement. For slabs over 4 inches thick or any slab that will bear vehicle weight, reinforcement prevents cracking.
- Plan for control joints. Score or cut joints every 8–12 feet in both directions to control where cracks form. The rule of thumb: joint spacing in feet should not exceed 2–3 times the slab thickness in inches.
- Order 5–10% more than calculated.Running short mid-pour creates cold joints, which are both unsightly and structurally weak. It's always better to have a little extra.
- Cure the concrete properly. Keep the slab moist for at least 7 days after pouring. Use curing compound or cover with plastic sheeting. Proper curing increases strength by up to 50%.
Common Mistakes When Ordering Concrete
- Not accounting for uneven ground.If your excavation isn't perfectly level, you'll need more concrete to fill low spots. Measure depth at multiple points and use the deepest measurement.
- Forgetting form thickness.If you're using 2×4 or 2×6 lumber for forms, the interior dimensions will be slightly smaller than the exterior. Measure inside the forms.
- Ordering exact quantities with no buffer. Always round up. A shortfall mid-pour is far worse than having a small amount left over.
Frequently Asked Questions
- How thick should a concrete slab be?
- For a standard patio or walkway, 4 inches is sufficient. Driveways should be at least 5–6 inches thick to support vehicle weight. Garage floors are typically 4–6 inches. If heavy trucks or equipment will use the surface, go with 6–8 inches.
- How many bags of concrete do I need for a 10×10 slab?
- A 10×10 slab at 4 inches thick requires approximately 1.24 cubic yards of concrete, or about 56 80-lb bags of premixed concrete. With a 10% waste factor, plan for around 62 bags. For larger projects, ordering ready-mix by the yard is more practical and cost-effective.
- Should I use bags or ready-mix concrete?
- For small projects under 1 cubic yard (about 45 80-lb bags), bags are practical. For anything larger, ready-mix delivery is faster, more consistent, and usually cheaper per yard. The break-even point is typically around 1–1.5 cubic yards.
- What waste factor should I use?
- A 10% waste factor is standard for most slab projects. Increase to 15% if the site has irregular edges, poor subgrade preparation, or if you're a first-time pourer. Experienced contractors on clean sites may use 5–7%.
- How much does a concrete slab cost?
- Ready-mix concrete typically costs $125–$175 per cubic yard delivered. Total project cost including labor, forming, and finishing runs $6–$12 per square foot for a basic slab. Decorative or stamped concrete can double or triple that price.