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.
Building a 20 × 16 slab
A 16 × 20 slab is 320 square feet, and the joint layout is the design decision. Cut it into four 8 × 10 panels: one joint across the 20-foot length at 10 feet, one across the width at 8 feet. For a workshop floor, add a 6-mil polyethylene vapor barrier over the gravel so moisture doesn't wick up through the slab and under whatever you store on it; a patio can skip it. Wire mesh is fine for a workshop with benches and tools. If a tractor or a car will come inside, use the 5-inch row and #4 rebar instead.
Pour day at this size
At 4.3 yards this is one truck and three people minimum. Check the access first: a mixer needs about 10 feet of clear width and ground that holds 30 tons, and if the slab is behind the house with a 3-foot gate, you are either wheelbarrowing 60 trips or paying for a pump truck. Pour early, keep a hose on the finished surface for a week if it's hot, and keep everyone off it for 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 × 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.
Can I pour a 16 × 20 slab with bags?
You can, but it's 196 bags of 80 lb mix, close to 8 tons of material to haul, mix, and place before the first batch sets. Order the truck. The short-load fee under 5 yards is the only reason bags look close on paper.
Does a 16 × 20 workshop slab need a vapor barrier?
Yes, if the space will be enclosed. A 6-mil poly sheet over the compacted gravel, seams lapped 6 inches, keeps ground moisture from coming up through the slab. Without it, boxes on the floor get damp and tools rust faster.
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)