Prices updated January 2026

20×20 Concrete Slab Cost

A 20×20 slab covers 400 square feet. Here's the concrete, bags and cost for 2026 — a common size for a two-car garage.

Concrete (4", waste in)

5.43 yd³

245 × 80 lb bags

Materials (ready-mix)

$739 – $1,207

delivered, 4" slab

Installed by a pro

$2,200 – $4,800

4" slab, national average

20×20 slab cost calculator

Adjust thickness, waste or region — the cost updates live.

Width (ft)

Length (ft)

Thickness (in)

Waste & region

Waste factor (%)

Order extra for spillage & uneven subgrade. 10% is typical.

Region (cost)

Installed cost estimate

$2,200 – $4,800

400 sq ft · 5.43 yd³ concrete

Bags of concrete needed (rounded up)

245

80 lb

326

60 lb

392

50 lb

489

40 lb

Ready-mix (delivered): $739 – $1,207
Bagged mix (80 lb, DIY): $1,348 – $2,205
Installed by a pro: $2,200 – $4,800

Estimate only — not a quote. Bag counts are rounded up and include a 10% waste allowance.

Installed $2,200 – $4,800

20×20 slab cost by thickness

Thickness	Concrete	80 lb bags	Ready-mix	Installed
4"	5.43 yd³	245	$739 – $1,207	$2,200 – $4,800
5"	6.79 yd³	306	$909 – $1,465	$2,500 – $5,100
6"	8.15 yd³	367	$1,079 – $1,723	$2,800 – $5,400

Includes a 10% waste allowance; bag counts rounded up. National US averages, January 2026.

You'll also need

Sized for this exact slab — tap any item to open its calculator with your measurements already filled in.

Gravel base

5.68 yd³

4" compacted base under the slab

≈ $170 – $369

Open calculator

Rebar

28 sticks

#4 bar, 18" grid (28 bars)

≈ $196 – $364

Open calculator

Bedding sand

1.24 yd³

~1" leveling layer

≈ $35 – $72

Open calculator

20×20 concrete slab: what to expect

A 20×20 slab covers 400 square feet. At the standard 4" thickness it takes about 5.43 cubic yards of concrete once you allow 10% for waste, which is roughly 245 × 80 lb bags if you mix by hand. Stepping up to 6" — sensible if it'll carry a vehicle — raises that to about 8.15 cubic yards.

Budget around $2,200 – $4,800 to have a 4" slab this size poured and finished by a contractor, or roughly $739 – $1,207 in ready-mix if you're doing the labor yourself. On top of the concrete you'll need about 5.68 yd³ of gravel base and 28 sticks of rebar.

Compare nearby sizes

Planning something bigger or smaller? Jump straight to the cost for a nearby size.

Size	Concrete (4")	80 lb bags	Installed cost
12×24	3.91 yd³	176	$1,584 – $3,456
16×20	4.35 yd³	196	$1,760 – $3,840
18×20	4.89 yd³	220	$1,980 – $4,320
20×24	6.52 yd³	294	$2,640 – $5,760
24×24	7.82 yd³	352	$3,168 – $6,912
20×30	8.15 yd³	367	$3,300 – $7,200

See all common slab sizes →

Prices updated January 2026 National US averages · reviewed quarterly

Volumes use standard bag yields and a 10% waste allowance; costs are national US averages that vary by region, mix and finish. See our methodology for exactly how each figure is worked out, or check the reference sources:

Slab cost by project

Concrete Slab Cost Cost per sq ft + calculator Garage Slab Cost Typical 20×20 / 24×24 Driveway Slab Cost Concrete vs asphalt Patio Slab Cost 10×10 to 20×20 Shed Base Cost Small pads

Frequently asked questions

How much does a 20×20 concrete slab cost?

A 20×20 slab (400 sq ft) costs about $2,200 – $4,800 installed at 4" thick in 2026, or $2,800 – $5,400 at 6". DIY in materials, the ready-mix runs about $739 – $1,207 at 4".

How many yards of concrete for a 20×20 slab?

At 4" thick a 20×20 slab needs about 5.43 cubic yards including 10% waste (4.94 yd³ net). At 6" it needs about 8.15 cubic yards.

How many bags of concrete for a 20×20 slab?

At 4" you'd need roughly 245 × 80 lb bags, 326 × 60 lb bags or 489 × 40 lb bags. That's a lot of mixing — past about a cubic yard, ready-mix is usually cheaper and easier.

What base and rebar does a 20×20 slab need?

Plan on about 5.68 cubic yards of compacted gravel base and roughly 28 sticks of #4 rebar on an 18" grid. The "You'll also need" cards size these for you.