How to use this calculator
- Choose US customary or metric measurements.
- Enter the length, width, and thickness of each rectangular section.
- Add sections for separate slabs, then choose a waste allowance.
- Calculate and review volume and bag estimates.
How the calculation works
The calculator converts every measurement to feet, multiplies length by width by thickness, and adds all sections. The waste percentage is applied to the combined base volume. Bag counts are rounded up because partial bags cannot normally be purchased.
Formula
Volume = length × width × thickness Cubic yards = cubic feet ÷ 27 Adjusted volume = base volume × (1 + waste % ÷ 100)
Worked example
A 10 ft × 10 ft slab at 4 in thick has a base volume of 33.33 cubic feet, or 1.23 cubic yards. With 10% waste, the estimate becomes 36.67 cubic feet, or 1.36 cubic yards.
Project considerations
- Confirm the finished slab dimensions and consistent thickness.
- Use the yield printed on the actual bag; nominal yields vary.
- Uneven subgrade, spillage, and form dimensions can change the amount required.
Frequently asked questions
How many cubic feet are in a cubic yard?
One cubic yard equals 27 cubic feet.
Why include waste?
A modest allowance can account for uneven excavation, spillage, and small measurement differences. Choose a percentage appropriate to the actual site.
Are the bag quantities exact?
No. They use visible nominal yield assumptions. Check the manufacturer’s current yield and mixing instructions before buying.