How Much Is a 20 x 20 Concrete Slab?

Slab Thickness, Concrete Volume, and PSI Ratings

Determining the structural thickness of your 400-square-foot slab dictates both material volume and vehicle load capacity. A standard four-inch-thick slab is sufficient for residential foot-traffic patios, hot tub pads, and lawn equipment sheds, requiring approximately 5.0 cubic yards of ready-mix concrete. Ready-mix concrete delivered via transit mixer costs between $125 and $165 per cubic yard, representing roughly $650 to $850 in raw concrete charges.

For a two-car garage, heavy SUV parking, or workshop housing machinery, a six-inch-thick slab engineered with 4,000 PSI commercial-grade concrete is mandatory. A six-inch 20x20 slab requires 7.5 cubic yards of concrete (roughly $1,000 to $1,300 in raw material). Thicker slabs resist heavy vehicle wheel loads and hydraulic floor jack point-loads without cracking, especially when paired with thickened twelve-inch monolithic perimeter footings.

Compare material requirements, ready-mix yardage, and installed costs for 20x20 concrete slabs:

Site Preparation, Subbase, and Reinforcement Grid

Professional contractors spend over half their project time on ground preparation before the concrete truck arrives. Native topsoil must be stripped and excavated four to eight inches deep, followed by laying three to four inches of compacted crushed stone or #57 gravel ($400 to $900). A well-compacted gravel subbase ensures uniform load distribution and facilitates sub-slab drainage, preventing frost heave in cold northern climates.

Internal reinforcement choices govern structural longevity. While inexpensive welded wire mesh ($0.30/sq ft) provides basic crack control, professional structural engineering favors a grid of #4 (half-inch) steel rebar tied on eighteen-inch centers ($0.75 to $1.25/sq ft). Rebar must be elevated on plastic dobies or steel chairs so it sits suspended in the middle third of the slab, providing maximum tensile strength against thermal contraction cracking.

Review typical line-item labor and material expenses for a professional 20x20 concrete pour:

Finishing Textures, Control Joints, and Curing Care

The final surface finish dictates the slab's aesthetic appeal and slip resistance. A traditional exterior broom finish ($0.50/sq ft) produces fine textured ridges that provide traction when wet, making it the preferred choice for outdoor patios and driveways. Smooth steel-troweled finishes ($1.00/sq ft) deliver an ultra-flat, polished sheen ideal for interior garage floors slated for epoxy coatings.

Saw-cut control joints are mandatory to manage inevitable concrete shrinkage. Within twenty-four hours of pouring, contractors saw-cut one-inch-deep joints dividing the 20x20 slab into four ten-by-ten-foot quadrants. These deliberate weakened planes guide hairline shrinkage cracks underneath the saw cut where they remain hidden. Keeping the slab damp under curing blankets for seven days ensures full 4,000 PSI strength development.