How Much Concrete for a 12x20 Slab?
For a 12x20 slab poured 4 inches thick, plan on about 2.96 cubic yards of concrete. That comes from 240 square feet times 0.333 feet of depth, divided by 27. How much concrete for a 12x20 slab at other thicknesses is in the table below.
Concrete calculator →The math for a 12x20 slab
- Area: 12 ft × 20 ft = 240 sq ft.
- Thickness in feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 240 × 0.333 = 80.0 cubic feet.
- Cubic yards: 80.0 ÷ 27 = 2.96 yd³.
- With 10% extra: 3.26 yd³.
The concrete calculator runs the same steps for any size and thickness.
Related: How Much Concrete for a 12x20 Driveway?
Concrete for a 12x20 slab by thickness
| Thickness | Cubic feet | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 80.0 | 2.96 | 3.26 | 134 | 178 |
| 5 in | 100.0 | 3.70 | 4.07 | 167 | 223 |
| 6 in | 120.0 | 4.44 | 4.89 | 200 | 267 |
Bag counts assume about 0.60 cubic feet per 80 lb bag and 0.45 per 60 lb bag, rounded up.
Thickness for this job
Four inches is the usual thickness for patios, walks and shed floors. Slabs that carry cars or a heavy hot tub are often poured at 5 to 6 inches, and edges are sometimes thickened where loads concentrate.
Related: How Much Concrete for a 12x20 Patio?
Truck or bags?
Concrete is ordered by the cubic yard, and a yard is 27 cubic feet. Ready-mix plants usually sell in quarter- or half-yard steps, so round up to the next step rather than down. Running short in the middle of a pour leaves a cold joint, which is a weak seam in the slab.
Base and forms
Under the slab, most residential pours sit on about 4 inches of compacted gravel. The base spreads the load, drains water away and keeps the slab from cracking as the soil freezes and thaws. Compact the base in thin lifts, and dampen it just before the pour so it does not pull water out of the mix.
Keeping cracks under control
Cut or tool control joints so the slab cracks where you want it to. A common rule is spacing them, in feet, at no more than two to three times the slab thickness in inches, so about 8 feet apart for a 4-inch slab, and about a quarter of the slab depth deep.
Questions people ask
How many yards of concrete for a 12x20 slab at 4 inches?
About 2.96 cubic yards, or 3.26 with 10 percent extra.
How many yards for a 12x20 slab at 6 inches?
About 4.44 cubic yards.
How many 80 lb bags for a 12x20 slab?
About 134 bags at 4 inches thick, before any waste allowance.
How many square feet is a 12x20 slab?
240 square feet.
Should I add extra for waste?
Yes. Adding 5 to 10 percent covers uneven base, spillage and form bulges.
How thick should the base under the slab be?
About 4 inches of compacted gravel is common for residential slabs, more on soft or poorly drained soil.
Related guides
- How Much Area Does a Yard of Concrete Cover?
- How Many Bags of Concrete in a Yard? 40, 60, 80 lb
- Gravel Base Under a Concrete Slab: Depth and Tons
- How Many Yards in a Concrete Truck? Mixer Sizes
- Concrete Short Load Fee: What It Is and How to Cut It
- 60 lb vs 80 lb Concrete Bags: Yield, Count, and Cost
- Rebar vs Wire Mesh in a Slab: Which One to Use
- Cost to Pour a Concrete Slab: Per Square Foot
- How Much Concrete for a 12x15 Slab?
- How Much Concrete for a 12x18 Slab?
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