How Much Concrete for a 16x20 Slab? Yards by Thickness
How much concrete for a 16x20 slab comes to about 3.95 cubic yards at 4 inches thick, 4.94 cubic yards at 5 inches, and 5.93 cubic yards at 6 inches. The slab covers 320 square feet. With a normal 5 to 10 percent allowance, a 4-inch slab is usually ordered as 4.25 to 4.5 yards, and a 6-inch slab as about 6.5 yards. That size is a common footprint for a workshop, a large shed, a patio, or a one-car garage.
Concrete calculator →Step-by-step calculation
- Area: 16 ft × 20 ft = 320 square feet.
- Thickness in feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 320 × 0.333 = 106.7 cubic feet.
- Cubic yards: 106.7 ÷ 27 = 3.95 cubic yards.
Use the same steps for any thickness, or enter the size in the concrete calculator.
Volume table for 16x20
| Thickness | Cubic feet | Cubic yards | Order with ~10% extra |
|---|---|---|---|
| 4 inches | 106.7 | 3.95 | about 4.5 yd |
| 5 inches | 133.3 | 4.94 | about 5.5 yd |
| 6 inches | 160.0 | 5.93 | about 6.5 yd |
| 8 inches | 213.3 | 7.90 | about 8.75 yd |
The 8-inch row is useful when estimating a slab with thickened edges or a heavy equipment pad.
Bag count, for comparison
A 4-inch 16x20 slab would take about 178 bags of 80-pound concrete or about 237 bags of 60-pound concrete before waste. Even with a rented mixer and a crew of four, that is very hard to place before the first sections start setting, which leads to cold joints and uneven finish. Ready-mix is the normal choice at this size.
Picking the thickness
- Patio or shed with light storage: 4 inches is typical.
- Workshop with heavy tools or a garden tractor: 5 inches gives more margin.
- One-car garage or vehicle parking: 6 inches is a safer choice, and many local codes set a minimum for garage floors.
Soil matters too. Clay that swells and shrinks, or fill that was never compacted, calls for a thicker slab and a better base, not just more concrete.
Reinforcement and joints
A 16x20 slab should have control joints to make cracks happen in straight lines. A common rule is to space joints no more than 2 to 3 times the slab thickness in feet, so about 8 to 12 feet apart for a 4-inch slab. A single joint across the middle of the 20-foot side splits it into two 16x10 panels. Rebar or welded wire, set on chairs so it ends up in the middle of the slab, helps keep cracks from opening. See rebar vs wire mesh in a slab.
Ordering and delivery
At about 4.5 yards, you may be just under or just over a plant’s short-load threshold, so ask. A standard mixer truck carries up to about 9 to 11 yards, so one truck covers even an 8-inch 16x20 slab. Have forms set, base compacted, and reinforcement in place before the truck arrives, since waiting time is often billed. If you want a price check, multiply your yardage by the local concrete cost per yard and add fees.
Common mistakes that throw off the order
- Measuring the plan, not the forms. Forms that drift out even a little add volume across a 320 square foot slab.
- Ignoring low spots in the base. A half-inch average dip adds about half a cubic yard.
- Forgetting thickened edges or footings. These can add a yard or more.
- Ordering exact. Running short means a second truck, another fee, and a cold joint.
Bottom line
A 16x20 slab needs about 3.95 cubic yards at 4 inches, 4.94 at 5 inches, and 5.93 at 6 inches. Add 5 to 10 percent for waste and grade variation, and round up to the plant’s next increment. That puts a typical 4-inch slab at about 4.5 yards and a 6-inch garage slab at about 6.5 yards, both easily covered by one truck.
Questions people ask
How many yards of concrete for a 16x20 slab?
About 3.95 cubic yards at 4 inches thick and 5.93 at 6 inches thick, before adding 5 to 10 percent for waste.
How many bags of concrete for a 16x20 slab?
About 178 bags of 80-pound concrete at 4 inches thick. That is too many to mix by hand for most crews, so ready-mix is common.
How thick should a 16x20 garage slab be?
Many builders use 6 inches for vehicle loads, and local code may set a minimum. Four inches works for a patio or light shed.
Will one concrete truck cover a 16x20 slab?
Yes. A typical mixer truck carries about 9 to 11 yards, more than a 16x20 slab needs even at 8 inches.
Where should control joints go on a 16x20 slab?
A common layout is one joint across the 20-foot length, making two 16x10 panels, or a grid with panels around 8 to 10 feet on a 4-inch slab.
Related guides
- How Much Concrete for a 20x20 Slab? Yards by Depth
- Concrete Cost per Yard: Ready-Mix Prices and Add-Ons
- Rebar vs Wire Mesh in a Slab: Which One to Use
- How Many Yards in a Concrete Truck? Mixer Sizes
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