How to use this calculator
- Choose US customary or metric measurements.
- Enter each slab’s finished length, width, and thickness.
- Add separate slabs when the project has more than one rectangular pad.
- Set a waste allowance and calculate the combined volume, area, and bag equivalents.
How the calculation works
Each slab is calculated separately as a rectangular prism. The volumes and top surface areas are combined, then waste is applied to volume. Bag equivalents use nominal yields and round up to whole bags.
Formula
Slab volume = length × width × thickness Combined volume = slab 1 + slab 2 + … Adjusted volume = combined volume × (1 + waste % ÷ 100)
Worked example
A 12 ft × 10 ft slab at 4 in thick contains 40 cubic feet. With 10% waste, the estimate is 44 cubic feet, or about 1.63 cubic yards.
Project considerations
- Use finished form dimensions rather than nominal project labels.
- Thickness can vary when the subgrade is uneven.
- Bag equivalents use 0.30, 0.45, and 0.60 cubic feet for 40 lb, 60 lb, and 80 lb bags; verify the product label.
Frequently asked questions
How is this different from the general concrete calculator?
This tool focuses on slab planning and includes combined top surface area and slab count alongside concrete volume.
Can I enter slabs with different thicknesses?
Yes. Add each slab separately and enter its own thickness.