Concrete Slab & Bag Calculator
Calculate cubic yards and premix bags needed for your project
Project Requirements
*Includes the selected safety margin.
Complete Guide to Calculating Concrete Slabs
Whether you are pouring a new patio, a driveway, or a shed foundation, calculating the correct amount of concrete is the most critical step in the planning process. Ordering too little results in a "cold joint" and structural weakness, while ordering too much wastes money. This Concrete Slab Calculator helps you determine exactly how many cubic yards or premix bags you need, including a safety margin for spillage and uneven subgrades.
How to Calculate Concrete Volume
Concrete is measured by volume, specifically in cubic yards (for truck deliveries) or cubic feet (for bagged concrete). The formula to calculate the concrete volume for a rectangular slab is straightforward:
Since most people measure thickness in inches, you must first convert the thickness to feet by dividing by 12. For example, a 4-inch slab is 0.33 feet thick.
Converting to Cubic Yards
If you are ordering from a ready-mix truck, you will need to know the yardage. There are 27 cubic feet in 1 cubic yard. Simply divide your total cubic footage by 27 to get the required cubic yards.
Bagged Concrete: 80lb vs 60lb Bags
For smaller projects (typically under 1 cubic yard), it is often more economical to mix bagged concrete yourself. The yield of these bags depends on the mix density, but standard pre-mix concrete (like Quikrete or Sakrete) generally follows these yields:
- 80lb Bag: Yields approximately 0.60 cubic feet.
- 60lb Bag: Yields approximately 0.45 cubic feet.
To calculate the number of bags needed, divide your total project volume (in cubic feet) by the yield per bag. Always round up to the nearest whole bag.
Why You Need a Safety Margin
Professional contractors never order the exact mathematical amount of concrete. Real-world variables always require extra material. We recommend adding a 5% to 10% safety margin to account for:
- Uneven Subgrade: If your dirt base is slightly lower in some spots, you will need more concrete to fill it.
- Spillage: Concrete is heavy and messy; some will inevitably be spilled during mixing or pouring.
- Form Bowing: Wooden forms often bow slightly outward under the pressure of wet concrete, increasing the volume.
Standard Slab Thicknesses
Choosing the right thickness is vital for the longevity of your slab:
- 4 Inches: Standard for walkways, patios, and residential shed floors.
- 5-6 Inches: Recommended for driveways and garages hosting passenger vehicles.
- 6+ Inches: Required for heavy equipment or RV parking pads.
Frequently Asked Questions
How many 80lb bags of concrete make a yard?
It takes approximately 45 bags of 80lb concrete to equal one cubic yard (27 cubic feet / 0.60 yield = 45). If you are mixing more than a yard, it is usually cheaper and less labor-intensive to order a truck.
Does this calculator include rebar volume?
No. While rebar takes up some space, it is negligible for general volume calculations and serves as an additional safety buffer.