Concrete Slab Calculator
Calculate the exact volume and number of premix bags needed for your concrete slab project.
Project Totals
Total Volume (Cubic Yards):
–
Total Volume (Cubic Feet):
–
Pre-Mix Bags Required
80lb Bags (Yield ~0.6 ft³):
–
60lb Bags (Yield ~0.45 ft³):
–
How to Calculate Concrete for a Slab
Planning a patio, driveway, or shed foundation requires precise calculations to ensure you order enough material without excessive waste. Concrete is typically measured in cubic yards when ordering from a ready-mix truck, or by the number of bags when mixing it yourself.
The Concrete Formula
To find the volume of your slab, use the following formula:
Volume = Length × Width × Thickness
Note: Since length and width are usually in feet and thickness is in inches, you must convert the thickness to feet (divide by 12) before multiplying.
Why Include a Safety Margin?
Our calculator defaults to a 5% safety margin (waste factor). This is critical for several reasons:
- Uneven Subgrade: The ground is rarely perfectly flat; dips require more concrete.
- Spillage: Some material is always lost during mixing and pouring.
- Form Deflection: Wooden forms may bow slightly outward under the weight of wet concrete.
Pro Tip: For complex shapes or deeper excavations, increase your safety margin to 10%. It is much more expensive to order a "short load" truck for a tiny remaining amount than to order slightly too much initially.
Bag vs. Truck Delivery
When to use bags: If your project requires less than 1 cubic yard (approx. 45-60 bags), mixing by hand or with a small rental mixer is often cost-effective.
When to order a truck: If you need more than 1 cubic yard, ordering ready-mix concrete is usually cheaper and saves significant labor. A standard concrete truck holds about 9 to 10 cubic yards.
Standard Thickness Guidelines
- 4 Inches: Standard for sidewalks, patios, and shed bases.
- 5-6 Inches: Recommended for driveways accommodating passenger vehicles.
- 6+ Inches: Heavy equipment pads or RV parking.