Concrete calculator
Concrete Calculator
Calculate concrete for rectangular slabs in cubic feet, cubic yards, and common bag sizes.
Results
- Base cubic feet
- 33.33 cu ft
- Base cubic yards
- 1.23 cu yd
- Cubic yards including waste
- 1.36 cu yd
- 40 lb bags
- 123
- 60 lb bags
- 82
- 80 lb bags
- 62
10% waste allowance
Save this calculation to a project estimate.
What this calculator does
This calculator estimates concrete volume for rectangular slabs, pads, walks, and similar flat pours. Enter the finished length and width in feet, slab thickness in inches, and the number of identical pours.
The result shows base volume, waste-adjusted volume, and estimated 40 lb, 60 lb, and 80 lb bag counts. Use the project button when you want to carry the calculated cubic yards into a broader estimate.
Formula used
Base cubic feet equals length in feet times width in feet times thickness in inches divided by 12, then multiplied by quantity. Cubic yards equal cubic feet divided by 27. The selected waste factor is applied before bag counts are rounded up.
How waste allowance works
Waste allowance increases the base volume to account for small measurement differences, uneven excavation, spillage, and ordering cushion. It does not replace checking the supplier's minimum order, truck access, or job-specific placement needs.
Bag yield assumptions
Bag estimates use approximate yields of 0.30 cubic feet for 40 lb bags, 0.45 cubic feet for 60 lb bags, and 0.60 cubic feet for 80 lb bags.
Measurement reminder
Measure the actual formed area and thickness, not only the plan dimensions. Verify project measurements, site conditions, and supplier requirements before ordering concrete or bagged mix.
Frequently asked questions
- How do I convert cubic feet to cubic yards?
- Divide cubic feet by 27. Concrete is commonly ordered by cubic yard, so the calculator shows both units.
- How much waste should I add for concrete?
- Use the waste setting as a planning cushion for small differences in form size, depth, and placement. Larger or irregular pours may need a higher cushion than a simple rectangular slab.
- Are concrete bag counts exact purchasing quantities?
- No. Bag counts use approximate yields and are rounded up. Check the bag label, mix type, and supplier guidance before purchasing.