Concrete for a 20×20 Slab at 4" Thick
A 20 ft × 20 ft slab poured 4 inches thick is 400 sq ft and needs 4.94 cubic yards of concrete — order 5.33 CY to cover 8% waste, or about 240 80-lb bags for a small pour. Adjust the dimensions below for your project.
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Want the full guide — tools, step-by-step, common mistakes and FAQ? See the Concrete Slab Calculator.