How Much Does a 20x20 Concrete Slab Cost?
A 20x20 concrete slab costs $2,400–4,800 installed at the typical $6–12 per square foot. It covers 400 square feet — the classic two-car garage floor or a large backyard patio — and takes 5.43 cubic yards of concrete at 4 inches thick, including 10% waste. Thickness is the decision that matters most at this size: 4 inches for a patio, 5 inches for a garage you actually park in.
A 20x20 concrete slab costs $2,400–4,800 installed — 400 square feet at the typical $6–12 per square foot for a broom finish. The low end is a 4-inch patio in a low-cost market; the high end is a 5–6-inch reinforced garage floor in an expensive metro. For your exact dimensions and region, run the numbers in the concrete cost calculator.
What a 20x20 Slab Is Usually For
Twenty by twenty is the standard footprint for a two-car garage — two parked cars with room to open doors — and a generous backyard patio that fits a dining set and a lounge area. The two uses price differently because they need different thickness and reinforcement:
| Use | Thickness | Reinforcement |
|---|---|---|
| Patio (foot traffic, furniture, grill) | 4 in | #3 bar 18–24 in o.c. or wire mesh |
| Garage, occasional parking | 4 in | #4 bar 12–18 in o.c. |
| Garage, daily parking or workshop | 5 in | #4 bar 12–18 in o.c. |
| Garage with a car lift | 6 in (thicker at lift pads) | #4 bar 12 in o.c. |
Each extra inch of thickness adds roughly $0.75–1.25 per square foot — about $300–500 on a 20x20 slab — so a 4-inch patio sits near the low end of the $6–12 range and a 6-inch reinforced garage floor near the high end.
For the garage-specific breakdown, see the garage floor thickness guide and the garage floor cost calculator. For a patio with a decorative finish, the patio cost calculator prices stamped and colored options.
How Much Concrete a 20x20 Slab Takes
Volume is length × width × thickness. Order about 10% extra for uneven subgrade and spillage.
| Thickness | Cubic Yards (exact) | With 10% Waste | 80-lb Bags | Ready-Mix at $165–205/yd | Weight in Place |
|---|---|---|---|---|---|
| 4 in | 4.94 | 5.43 | 245 | $896–1,114 | 20,000 lb |
| 5 in | 6.17 | 6.79 | 306 | $1,120–1,392 | 25,000 lb |
| 6 in | 7.41 | 8.15 | 367 | $1,344–1,670 | 30,000 lb |
A 4-inch 20x20 slab is a single ready-mix truck; a 6-inch slab is close to a full load. At any thickness this is a ready-mix job — hand-mixing 245+ bags would take a crew all day and the first batches would set before the last ones were placed.
20x20 Slab Cost in Four Cities
Local labor rates swing the installed price more than anything you choose on site. These ranges come from our patio cost by city data, multiplied out to 400 square feet:
| Metro | Installed $/sq ft | 20x20 Slab (400 sq ft) |
|---|---|---|
| Dallas, TX | $4.97–8.71 | $1,988–3,484 |
| Chicago, IL | $5.87–9.01 | $2,348–3,604 |
| Seattle, WA | $8.88–17.84 | $3,552–7,136 |
| Atlanta, GA | $10.25–19.80 | $4,100–7,920 |
Base, Rebar, and Joints
- Gravel base: 4 inches of compacted crushed stone under 400 sq ft is 4.94 cubic yards — about 6.9 tons, or $123–247 at $25–50 per yard. See gravel under concrete.
- Rebar: #4 bar at 18 inches each way is about 14 bars in each direction, roughly 560 linear feet ($420–840 at $0.75–1.50/ft). Size it with the rebar calculator.
- Control joints: At 4–5 inches thick, joints every 10 feet divide a 20x20 slab into four 10x10 panels — the cleanest layout for crack control.
- Garage specifics: A garage slab usually gets a thickened edge under the walls and a vapor barrier if the space will be finished or heated.
DIY vs. Contractor for a 20x20 Slab
Doing it yourself, materials for a 4-inch slab run roughly $1,440–2,200: ready-mix ($896–1,114), gravel base ($123–247), and rebar ($420–840), plus forms and tool rental. Against $2,400–4,800 installed, that's a meaningful saving — but 400 square feet is well past the size where a first-timer can finish concrete well. Our DIY vs contractor guide suggests keeping a first pour under 50–75 square feet. On a garage floor, a poor finish, bad drainage slope toward the door, or missed joints are permanent.
If you're weighing other sizes, see the 20x30 slab cost for a larger garage or shop, or the 10x20 slab cost for a shed or single parking pad.