Worked example
How Much Concrete for a 12x12 Slab?
This example starts with a 12 ft by 12 ft slab at 4 in thick and embeds the concrete calculator so you can adjust thickness, yield, and waste.
Calculator inputs
Concrete Slab Calculator Results
Results update in your browser as you edit inputs. They are planning estimates, not complete shopping lists.
Enter project dimensions to calculate material quantity.
Inputs used in this example
- Slab size: 12 ft by 12 ft.
- Thickness: 4 inches.
- Bag yield preset: 80 lb bag at about 0.6 cubic feet.
- Waste factor: 5%.
Expected output
The calculator estimates about 48 cubic feet before waste, 50.4 cubic feet after 5% waste, 1.87 cubic yards after waste, and about 84 bags at 0.6 cubic feet per bag.
When to adjust this example
- Increase waste for uneven excavation, overdig, or form boards that are not square.
- Change bag yield if your bag label lists a different cubic-foot yield.
- This example does not include forms, rebar, mesh, vapor barrier, base gravel, delivery, permits, or finishing tools.
Formula explanation
- Convert 4 inches to 0.333 feet.
- Multiply 12 x 12 x 0.333 to get about 48 cubic feet.
- Apply the 5% waste factor.
- Divide waste-adjusted cubic feet by selected bag yield and round up.
Main calculator
Use the full Concrete Slab Calculator to change dimensions, waste factor, and optional user-entered unit price.
FAQ
Can I use this for a thicker slab?
Yes. Change the slab thickness field and the embedded calculator will update the volume and bag count.
Does this include structural design?
No. It estimates concrete quantity only. Check local requirements and qualified professionals for structural or permitted work.