Concrete Calculator
Estimate how much concrete you need for a rectangular slab, foundation, footing, pad, or other project.
Enter the length, width, thickness, and waste allowance to calculate concrete volume in cubic feet and cubic yards.
Calculate Concrete Needed
Enter the project dimensions in feet and inches.
Project Dimensions
Waste Allowance
Concrete Estimate
How Does the Concrete Calculator Work?
The calculator determines the volume of a rectangular project by multiplying its length, width, and thickness. Because thickness is entered in inches, it is converted to feet before the volume is calculated.
The resulting volume is shown in cubic feet and converted to cubic yards. A waste allowance can then be added to provide a practical ordering estimate.
Volume = Length × Width × Thickness
Cubic Yards = Cubic Feet ÷ 27
Required Concrete = Volume × (1 + Waste %)
Converting Concrete Thickness to Feet
Concrete slabs and similar projects are often measured using thickness in inches, while volume calculations use feet.
To convert inches to feet, divide the thickness by 12.
Thickness in Feet = Thickness in Inches ÷ 12
For example, a 4-inch slab has a thickness of:
4 ÷ 12 = 0.3333 feet
What Is a Cubic Yard of Concrete?
Ready-mix concrete is commonly ordered in cubic yards in the United States. One cubic yard contains 27 cubic feet.
1 Cubic Yard = 27 Cubic Feet
To convert a volume measured in cubic feet to cubic yards, divide the cubic-foot value by 27.
Concrete Calculator Example
Suppose you are pouring a rectangular slab that is 20 feet long, 15 feet wide, and 4 inches thick.
First convert the thickness to feet:
4 ÷ 12 = 0.3333 ft
The concrete volume is approximately:
20 × 15 × 0.3333 ≈ 100 cubic feet
Convert cubic feet to cubic yards:
100 ÷ 27 ≈ 3.70 cubic yards
With a 10% waste allowance:
3.70 × 1.10 ≈ 4.07 cubic yards
This gives an estimated ordering quantity of approximately 4.07 cubic yards.
Why Add a Concrete Waste Allowance?
Theoretical volume does not always equal the amount of concrete that must be ordered. Real projects can have uneven excavation, variations in thickness, spillage, equipment losses, or small measurement differences.
A waste allowance provides additional material beyond the calculated geometric volume.
| Waste Allowance | General Planning Situation |
|---|---|
| 5% | Well-measured, straightforward project |
| 10% | Common starting estimate for many projects |
| 15% | More variable dimensions or site conditions |
These are general planning values rather than a universal ordering rule. Actual requirements depend on the project and site conditions.
Projects You Can Estimate
The rectangular volume calculation can be useful for estimating concrete for many simple projects, including:
- Concrete slabs
- Garage and driveway sections
- Patios
- Walkways
- Concrete pads
- Rectangular footings
- Foundation sections
Irregular or multi-section projects should be divided into simpler measurable shapes and calculated separately.
How to Calculate Concrete for an Irregular Project
If the project does not form one simple rectangle, divide it into smaller rectangular sections whenever practical.
Calculate the volume of each section and add the volumes together before applying the waste allowance.
Total Volume = Volume 1 + Volume 2 + Volume 3 + ...
How to Use the Concrete Calculator
- Measure the project length in feet.
- Measure the project width in feet.
- Measure the average concrete thickness in inches.
- Enter a suitable waste allowance.
- Click Calculate Concrete.
- Review the calculated volume in cubic feet and cubic yards.
- Use the waste-adjusted estimate when planning the amount of concrete required.
Concrete Measurement Tips
- Measure the project dimensions as accurately as possible.
- If thickness varies, use a realistic average thickness for an initial estimate.
- Divide irregular shapes into simpler sections.
- Check whether the supplier sells concrete by cubic yard, cubic meter, or another unit.
- Confirm the final quantity with your concrete supplier or construction professional before ordering.
Important Note About Concrete Estimates
This calculator provides a geometric volume estimate for planning purposes. Actual concrete requirements can vary because of excavation conditions, uneven surfaces, changes in thickness, formwork, spillage, and other site factors.
Structural concrete work should be designed and reviewed according to applicable building requirements and by an appropriately qualified professional when necessary.
Frequently Asked Questions
How do I calculate how much concrete I need?
Multiply the length by the width and thickness to find the volume. Convert the result to the unit used by your supplier and consider an appropriate waste allowance.
How many cubic feet are in a cubic yard?
One cubic yard contains 27 cubic feet.
Why is concrete thickness entered in inches?
Concrete slab thickness is commonly specified in inches. The calculator converts the thickness to feet so it can be used with dimensions measured in feet.
Should I add extra concrete?
A waste allowance can help account for real-world variations such as uneven surfaces, thickness differences, and spillage. The appropriate allowance depends on the project.
Can this calculator estimate a driveway or patio?
Yes. It can estimate simple rectangular sections of driveways, patios, slabs, pads, and similar projects.
Is the concrete estimate exact?
No. It is a planning estimate based on the dimensions you provide. Actual quantities can vary at the job site.