Concrete Calculator
Estimate precisely how much material your project requires to avoid waste or shortages.
Concrete Calculator
Ordering too little concrete stops your project cold. Ordering too much wastes money. This concrete calculator figures out exactly how many cubic yards — or bags — you need before you place that call to the ready-mix company.
What Does This Calculator Do?
Enter the dimensions of your slab, footing, column, or wall and the calculator converts them to cubic yards of concrete. It also tells you how many 40 lb, 60 lb, or 80 lb bags of pre-mix you'd need for smaller jobs.
How to Use This Calculator
- Select your shape — slab, footing, column, or tube.
- Enter the length, width, and depth (or diameter and height for columns).
- Click Calculate to see cubic yards and equivalent bag counts.
- Add 5–10% extra for waste and spillage.
Concrete Volume Formula
For a rectangular slab:
Volume (ft³) = Length × Width × Depth
Cubic Yards = Volume (ft³) ÷ 27
One cubic yard = 27 cubic feet. Standard ready-mix is ordered by the cubic yard.
Example Calculation
You want to pour a driveway slab that is 20 ft long × 10 ft wide × 4 inches thick.
- Convert depth: 4 inches = 0.333 ft
- Volume = 20 × 10 × 0.333 = 66.67 ft³
- Cubic yards = 66.67 ÷ 27 = 2.47 cubic yards
- Add 10% buffer → order ~2.75 cubic yards
- Equivalent: about 55 bags of 80 lb pre-mix
Why Use This Calculator?
Concrete is priced by the cubic yard for ready-mix and by the bag for smaller DIY projects. Knowing the exact volume prevents under-ordering (which causes cold joints) or over-ordering (which wastes money). This tool handles the math so you can focus on the pour.
Common Mistakes to Avoid
- Not converting inches to feet — depth is usually given in inches but the formula needs feet. Divide inches by 12.
- Forgetting the 10% buffer — always order slightly more than calculated to account for spillage and uneven subgrade.
- Confusing cubic yards and cubic feet — ready-mix trucks measure in cubic yards; bags are measured in cubic feet. The calculator handles both.
- Ignoring rebar displacement — for heavily reinforced slabs, the actual concrete volume is slightly less, but the difference is small enough to ignore for most residential work.
Frequently Asked Questions
How many bags of concrete are in a cubic yard?
Approximately 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb pre-mix concrete equal one cubic yard.
How thick should a concrete slab be?
Walkways: 4 inches. Driveways: 4–6 inches. Patios: 4 inches. Garage slabs: 4–6 inches. Check local building codes for specific requirements.
When should I use ready-mix vs. bagged concrete?
For jobs under 1 cubic yard, bagged concrete is more practical. For larger pours, ready-mix from a truck is faster and more cost-effective.
Can I use this for footings?
Yes. For a rectangular footing, enter the length, width (usually 8–16 inches), and depth. The calculator works the same way.
What is the PSI strength I should use?
3,000 PSI is standard for most residential slabs. 4,000 PSI is used for driveways and floors with heavier loads.
Conclusion
Getting your concrete order right the first time saves time, money, and a whole lot of frustration. Use this calculator before every pour — whether it's a small stepping stone or a full driveway slab.
Related: Asphalt Calculator | Gravel Calculator | Square Footage Calculator
Pro Tip
For large slabs, it's often cheaper to order a ready-mix truck if you need more than 1 cubic yard. Most trucks carry up to 10 yards.
