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.
Concrete needed
4.94 cu yd
Slab area
400 sq ft
Concrete needed
4.94 cubic yards5.33 CY with 8% waste — order this much
80-lb bags
240 bagsor 320 × 60-lb bags
Ready-mix cost (est.)
$933~$175/CY delivered
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How this is calculated
Concrete volume = area × thickness. We convert to cubic yards (the unit ready-mix is sold in), then add 8% for spillage and over-ordering — the same waste factor used in professional estimating.