How Much Concrete for a 20x20 Slab (2026): Yards, Bags, and Cost

20 × 20 slab by thickness

Every figure includes 10% waste. Bag counts use the manufacturers' stated yields (0.6 cu ft per 80 lb bag, 0.45 per 60 lb).

Thickness	Cubic yards	80 lb bags	60 lb bags	Bags cost	Ready-mix cost
4 in	5.43	245	326	$1,470 to $1,960	$706 to $1,032
5 in	6.79	306	408	$1,836 to $2,448	$883 to $1,290
6 in	8.15	367	489	$2,202 to $2,936	$1,059 to $1,548
8 in	10.86	489	652	$2,934 to $3,912	$1,412 to $2,064

Anything a car parks on should be 5 to 6 inches with wire mesh or rebar. A 4-inch garage floor is the classic cracked-in-three-years mistake.

Building a 20 × 20 slab

This is a vehicle slab, so the spec goes up. Five inches minimum, #4 rebar on an 18 to 24-inch grid on chairs, 4 inches of compacted gravel, and a vapor barrier if it's a garage. Where the slab edge has no stem wall under it, thicken the perimeter: a turned-down edge about 12 inches wide and 12 inches deep carries the wall or the wheel load at the edge. For a driveway apron, put a 1/2-inch expansion joint where the apron meets the garage slab and another at the sidewalk, and slope it 1/4 inch per foot toward the street.

Pour day at this size

About 6.8 yards at 5 inches, one truck. Cut joints at 10 feet each way for four 10 × 10 panels. Garage slabs get a light broom finish so tires don't squeal and feet don't slip when wet; a smooth trowel finish looks nicer and is the wrong call. Keep cars off it for a week, not three days.

Joint spacing and depth follow ACI 302.1R (joints every 24 to 36 times the slab thickness, cut one quarter deep); anchor bolt spacing follows IRC R403.1.6; load figures come from manufacturers' published weights. Local code wins where it differs.

Having it poured instead

Contractors charge about $6 to $12 per square footfor a finished 4-inch slab in 2026, which puts a 20 × 20 slab at$2,400 to $4,800 installed. That covers forming, gravel base, pour, and finish; thicker slabs, reinforcement, and site prep on sloped or hard-to-reach ground push it toward the top of the range.

How we get these numbers

Volume is length × width × thickness in cubic feet, divided by 27, plus 10% waste. Bag yields come from the manufacturers' technical data sheets. Ready-mix runs $130 to $190per yard delivered, with a $50 to $150short-load fee under 5 yards at most plants. Prices refreshed at least twice a year (last: September 2026). Sources:

Frequently asked questions

How many bags of concrete for a 20x20 slab?

At 5 inches thick, 306 bags of 80 lb mix or 408 bags of 60 lb mix, including 10% extra for waste. That is 6.79 cubic yards of concrete.

How much does a 20x20 concrete slab cost?

Poured by a contractor, about $2,400 to $4,800 at 2026 rates of $6 to $12 per square foot for a 4-inch slab, including forming, pouring, and finishing. Doing it yourself, the concrete alone is $1,836 to $2,448 in bags or $883 to $1,290 delivered by truck, plus forms, gravel, and reinforcement.

How thick should a 20x20 slab be?

5 inches for a one-car garage floor or a driveway apron. Go to 5 or 6 inches if vehicles will park on it, and use wire mesh or rebar in any slab that carries a load.

How much rebar for a 20 × 20 garage slab?

#4 bar on a 24-inch grid both ways is 11 bars each direction, 22 bars of 20 feet, plus chairs to hold it at mid-depth. Go to 18 inches if the slab will see a loaded pickup. Wire mesh alone is not enough once a car parks on it every day.

Does a driveway apron need an expansion joint at the garage?

Yes, a 1/2-inch strip where the apron meets the garage slab and another at the sidewalk or street. The apron moves with frost and sun; the joints keep that movement from cracking the slab on either side.

Other common slab sizes

10 × 10 slab (a shed base, a small patio, or a hot tub pad)
12 × 12 slab (a patio, a larger shed, or a carport pad)
16 × 12 slab (a standard backyard patio)
20 × 16 slab (a large patio or a workshop floor)
24 × 24 slab (a two-car garage floor)

How much concrete for a 20 × 20 slab