20 × 16 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 | 4.35 | 196 | 261 | $1,176 to $1,568 | $615 to $976 |
| 5 in | 5.43 | 245 | 326 | $1,470 to $1,960 | $706 to $1,032 |
| 6 in | 6.52 | 294 | 392 | $1,764 to $2,352 | $847 to $1,239 |
| 8 in | 8.69 | 392 | 522 | $2,352 to $3,136 | $1,130 to $1,651 |
At this size you are ordering a truck. Plan the pour for early morning and have three people on hand: one at the chute, two screeding.
Having it poured instead
Contractors charge about $6 to $12 per square footfor a finished 4-inch slab in 2026, which puts a 20 × 16 slab at$1,920 to $3,840 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:
- Concrete Network: Concrete Prices 2026, How Much Is a Yard of Concrete?
- Angi: Concrete Cost Per Yard (2026)
- The Home Depot: Quikrete 80 lb Concrete Mix (bag yield and price)
- Estimators.us: Concrete Slab Cost in 2026, $6 to $12 per sq ft
- CalcShed: Concrete Slab Cost in 2026, per sq ft and by state
Frequently asked questions
How many bags of concrete for a 16x20 slab?
At 4 inches thick, 196 bags of 80 lb mix or 261 bags of 60 lb mix, including 10% extra for waste. That is 4.35 cubic yards of concrete.
How much does a 16x20 concrete slab cost?
Poured by a contractor, about $1,920 to $3,840 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 $1,176 to $1,568 in bags or $615 to $976 delivered by truck, plus forms, gravel, and reinforcement.
How thick should a 16x20 slab be?
4 inches for a large patio or a workshop floor. 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.
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 × 20 slab (a one-car garage floor or a driveway apron)
- 24 × 24 slab (a two-car garage floor)