Concrete slab cost guide (garage, shed, floor)
A slab-on-grade is the foundation for a garage, shed, workshop or floor, and its budget follows a clean formula. This guide shows the cost math, a garage-slab example, and how thickness by use changes both strength and price.
The cost formula
A slab is priced as an area rate plus the base and reinforcement it needs, with a contingency:
Total = ( area × price per sq ft + sub-base + rebar ) × (1 + contingency%)
For a garage or shed slab you often add a thickened edge (a deeper perimeter that carries the wall load). The concrete slab cost tool covers the general case and garage / shed slab cost adds the thickened-edge line.
A worked example
A 400 sq ft slab (20 × 20) at $7 per square foot: 400 × $7 = $2,800. Add $250 for the sub-base and $200 for rebar, then a 10% contingency: ($2,800 + $250 + $200) × 1.10 = $3,250 × 1.10 = $3,575.
Now a 24 × 24 garage slab (576 sq ft) at $7: 576 × $7 = $4,032, plus a $400 thickened edge, then 10%: ($4,032 + $400) × 1.10 = $4,432 × 1.10 = $4,875. The thickened edge is the main thing that separates a garage slab from a plain pad.
The pieces
- Area × price per sq ft: concrete, labor, forming and finishing. Plain slabs sit at the lower end of the flatwork range; enter your own quote.
- Sub-base: compacted gravel for drainage and load spreading — standard under any slab. See the gravel / sub-base calculator.
- Rebar or mesh: crack control; quantity from grid geometry in the rebar calculator.
- Thickened edge: a deeper perimeter beam under load-bearing walls, common on garage and shop slabs.
- Contingency: a buffer for subgrade surprises and overruns.
Thickness by use
Slab thickness follows the load it carries — these are labeled planning bands, not a structural design:
| Use | Typical thickness |
|---|---|
| Residential slab / patio / floor | 4" |
| Garage / shed slab | 4–5" |
| Driveway / heavy vehicles | 5–6" |
Thickness matters to the budget because concrete is volumetric: going from 4 to 6 inches is 50% more concrete for the same footprint. See slab-on-grade thickness reference. Load-bearing decisions — footings, span, reinforcement design — belong to a licensed engineer and your local code.
A garage or shed floor is a slab, not a floor covering
Worth being clear: this guide is about pouring the concrete slab itself. What you later put on top — epoxy coating, tile, an interior floor finish — is a separate trade and a separate budget. Here we price the structural flatwork: the pour, its base and its reinforcement. If your slab is an existing one you want to refresh, that is a resurfacing or sealer job (see crack repair and resurfacing), not a new pour.
Scope that changes the price
Excavation and grading for a level pad, removing an old slab, truck access and a pump for tight sites, vapor barrier under a heated or living-space floor, and local labor rates all move the number beyond the square-foot rate. Compare bids on matched thickness, base, reinforcement and edge detail.
Vapor barrier and insulation under a floor
A slab that will become a finished floor — a basement, a heated shop, a room addition — often needs two things a bare shed pad does not. A vapor barrier, typically a polyethylene sheet laid over the sub-base before the pour, stops ground moisture from wicking up through the concrete and ruining flooring, coatings or stored goods above. Rigid foam insulation under and around the edge of the slab keeps a heated floor from bleeding warmth into the ground and is standard where a slab is conditioned living space. Both are inexpensive relative to the pour and nearly impossible to add later, so they belong in the plan and the budget from the start. Whether your slab needs them comes down to what goes on top of it — a decision to make before, not after, the concrete arrives.
Curing, joints and a flat floor
The finish quality of a slab floor is set in the first days. Control joints (saw-cut or tooled) steer shrinkage cracking onto planned lines rather than a random pattern under your future flooring. A proper cure — keeping the surface damp or covered so it does not dry too fast — is what lets the slab reach its strength and resist dusting and scaling. And flatness matters more for a floor than for a yard pad: a garage or shop slab that will take shelving, a lift or a coating should be finished flat and level, which is skilled labor worth paying for. When you compare slab bids, ask how joints, curing and finish flatness are handled, not just the thickness and the square-foot price — that is where a floor you can actually use is won or lost.
The bottom line
Slab cost = (area × your $/sq ft + sub-base + rebar, plus a thickened edge for garages) × a contingency. A 400 sq ft slab lands near $3,575 and a 576 sq ft garage slab near $4,875 in the examples above. Build yours in concrete slab cost or garage / shed slab cost. Results are planning estimates; get itemized quotes and defer load-bearing design to a licensed engineer.