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Calculation details
Planning estimate only. Verify dimensions, waste, coverage, product specifications, and local requirements before purchasing materials.
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What does this calculator estimate?
You need about 4.94 cubic yards of concrete for a 20x20 slab that's 4 inches thick. The math is simple: multiply the slab's length by its width by its depth to get cubic feet, then divide by 27 to convert to cubic yards. For a quick exact number, BoringToolsKit's concrete calculator runs this for free with no signup. As a reference, a 10 ft by 12 ft slab at 4 inches deep is roughly 1.48 cubic yards before adding waste.
How this calculator works
Formula
volume = length × width × depth; convert cubic feet to cubic yards by dividing by 27
Worked example
A 10 ft by 12 ft slab at 4 inches deep is about 1.48 cubic yards before waste.
Assumptions to verify
- Depth is converted to feet
- Subgrade and form dimensions are accurate
- Overage is considered for spills/uneven grade
Frequently asked questions
How much extra concrete should I order?
Small jobs often add 5–10%, but confirm with the supplier and contractor.
Why cubic yards?
Ready-mix concrete is commonly ordered by cubic yard in the US.
Cite this tool
BoringToolsKit. “Concrete Calculator.” boringtoolskit.com/concrete-calculator/ (reviewed August 2026). Free to reference in articles, syllabi, and answer posts with a link.
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