Concrete for a 16×20 slab
A 16×20 ft slab at 4″ thick needs about 196 × 80 lb bags of concrete mix (with 10% waste) — or 4.5 cubic yards of ready-mix. Estimated materials cost ≈ $743 (ready-mix). Big enough to live on, small enough that the short-load fee still decides bags vs a truck.
| Area | 320 sq ft |
|---|---|
| Volume | 106.67 ft³ = 3.95 cu yd |
| 80 lb bags (no waste) | 178 |
| 80 lb bags (10% waste) | 196 |
| 60 lb bags (10% waste) | 261 |
| Ready-mix | 4.5 cu yd |
| Finished slab weight | ≈ 14,933 lb |
| Recommended | Order ready-mix |
What a 16×20 slab is usually for
- One-car detached garage
- Large patio
- Workshop floor
- Equipment or implement store
16×20 is a common detached single-garage kit size and one of the smallest footprints that gives you a car plus usable bench and storage space along one wall. At 320 square feet the concrete cost is now a modest share of the whole project — the base preparation, the forms, the reinforcement and the labour all start to outweigh the mix itself.
Adjust it
Change the size, thickness, bag size, or waste % below:
How much concrete do I need?
| Bags (with overage) | |
|---|---|
| Bags (no overage) | |
| Ready-mix (delivered) | |
| Estimated materials cost |
Estimate for standard concrete mix (e.g. Quikrete #1101 / Sakrete High-Strength). Bag counts round up and include your overage %. Cost is an estimate (as of June 2026, US national average) — confirm bag price and ready-mix delivery (often a 1 yd³ minimum + short-load fees) with your supplier. Not a substitute for a structural engineer on load-bearing work.
Bags or a truck for this 16×20 patio
At 3.95 cubic yards, this slab sits clearly past the point where bags stop being sensible. Here is the comparison with the fees included:
| Route | What you order | Estimated cost |
|---|---|---|
| Bags (80 lb) | 196 bags at ≈ $6.50 each | ≈ $1,274 |
| Ready-mix delivered | 4.5 yd³ at ≈ $165/yd³ | ≈ $743 |
Ready-mix is the better call at this size, even carrying the short-load fee. Beyond the price, a truck delivers the whole slab as one continuous placement — no cold joints between batches, no risk of the first section setting while you mix the last, and a consistent mix throughout. Mixing 196 bags to match it is a full day of heavy work with a worse result.
The short-load fee is what people miss. Suppliers charge it below roughly 4 cubic yards, typically $50–$150, because sending a truck out costs them the same whether it is full or not. Always ask for the delivered total including fees rather than the per-yard rate. The full reasoning is in bags vs ready-mix concrete.
Control joints so a 16×20 patio cracks where you choose
Concrete shrinks as it cures and it will crack — the only question is whether it cracks where you decided or where it chose. Control joints are the decision. The rule of thumb is a maximum spacing in feet of about two to three times the slab thickness in inches, so at 4″ that is 8–12 ft.
| Panel grid | 2 × 2 = 4 panels |
|---|---|
| Panel size | ≈ 8 × 10 ft |
| Cuts needed | 2 (36 linear ft) |
| Cut depth | 1″ (a quarter of the slab) |
That layout keeps every panel at 1.25:1 or better, inside the 1.5:1 limit that stops a long thin panel cracking across its middle. Cut or tool the joints the same day — within roughly 6 to 12 hours of finishing, once the surface will carry a saw without ravelling but before shrinkage has done its work.
How thickness changes a 16×20 slab
Thickness is the one input that changes the order most, and it is chosen by what will sit on the slab rather than by budget. Here is the same 320 sq ft footprint at each common depth:
| Thickness | 80 lb bags | Ready-mix | Est. materials | Typical use |
|---|---|---|---|---|
| 3.5″ | 172 | 4 yd³ | ≈ $660 | What a nominal 4″ pour often actually measures |
| 4″ (this page) | 196 | 4.5 yd³ | ≈ $743 | Patios, sheds, walkways, foot traffic |
| 5″ | 245 | 5.5 yd³ | ≈ $908 | Cars, light trailers, frequent vehicle use |
| 6″ | 294 | 6.75 yd³ | ≈ $1,114 | Trucks, RVs, machinery, heavy loads |
Going from 4″ to 6″ on this slab adds roughly $371 in materials — about 50% more. That is the entire cost of the decision, and it is why the advice for anything carrying a vehicle is to just pour the extra inches. Removing and replacing a cracked slab costs many times the difference.
The 3.5″ row is not a recommendation, it is a reality check: a “4 inch” slab poured on a subgrade that is a little high in places genuinely finishes at 3.5″ in those spots. If your base is not dead flat, order to the thicker figure. More on this in concrete slab thickness: 4″ vs 6″.
Pouring a 16×20 patio: the practical plan
| Wheelbarrow loads | ≈ 36 (at 3 ft³ each — weight-limited, not volume-limited) |
|---|---|
| Mixer batches | ≈ 98 at 2 bags per batch |
| Rough mixing time | ≈ 8.2 hours of continuous mixing, one person |
| Finished weight | ≈ 14,933 lb on the ground |
Be honest about that number.196 bags is past the point where a one-person mix-and-pour produces a good slab. Concrete starts stiffening within about 30 to 60 minutes in warm weather, so a long mixing session leaves cold joints between what you placed first and what you placed last — visible lines that are also weak lines. If you are set on bags, get help and run two mixers; otherwise price the truck.
Budget for the tools as well as the concrete: a mixer runs about $70/day, a plate compactor for the base about $80/day, and a concrete saw for the joints about $60/day.
What it would cost to hire out
A professionally installed basic slab runs roughly $4–$8 per square foot including labour, so for 320 sq ft:
| Route | Estimated total | What you spend |
|---|---|---|
| DIY | ≈ $843–$1,043 | Concrete, forms, base, reinforcement, tool rental — plus your weekend |
| Hire a contractor | ≈ $1,280–$2,560 | Everything, done to a finish, usually in a day |
The gap is labour, and it is real — but so is the risk transfer. A contractor who gets the finish wrong fixes it; if you do, you own it. See DIY vs hiring a pro and slab cost by size for the full breakdown of where each route makes sense.
Watch out on a slab this size
Do not economise on the base to save on concrete. Four to six inches of compacted gravel under this slab does more for its lifespan than an extra inch of thickness would, and it costs a fraction as much. A slab is only ever as flat as what it was poured on.
The arithmetic
Volume = 16 × 20 × (4 ÷ 12) = 106.67 ft³, then ÷ 27 = 3.95 cubic yards. An 80 lb bag yields 0.60 ft³, so 106.67 ÷ 0.60 = 178 bags before waste; add 10% and round up to 196 bags. Ready-mix rounds up to the quarter yard, giving 4.5 yd³. Every constant behind those steps is documented in how we calculate, and the reasoning is in how much concrete do I need?
Other slab sizes
Last updated: 2026-08-21.
Bag yields from the
QUIKRETE #1101
data sheet. Joint spacing, panel aspect and cut depth follow standard residential
practice (2–3 × slab thickness, 1.5:1 max panel ratio, cut to ¼ depth). Cost
figures are US-national-average estimates and vary by region and supplier.
Planning estimates only — see terms.