How Much Concrete for a 12x20 Garage Slab?
A 12 by 20 foot garage slab is 240 square feet. How much concrete for a 12x20 garage slab at the common 5-inch thickness: about 3.70 cubic yards. Most people order 4.25 yards to cover waste and an uneven base.
Concrete calculator →The math for a 12x20 garage slab
- Area: 12 ft × 20 ft = 240 sq ft.
- Thickness in feet: 5 in ÷ 12 = 0.417 ft.
- Volume: 240 × 0.417 = 100.0 cubic feet.
- Cubic yards: 100.0 ÷ 27 = 3.70 yd³.
- With 10% extra: 4.07 yd³.
The concrete calculator runs the same steps for any size and thickness.
Related: How Much Concrete for a 12x20 Slab?
Concrete for a 12x20 garage 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.
Choosing the thickness
Garage floors are commonly 4 to 6 inches thick; 5 inches is a practical middle for cars and light trucks, and 6 inches suits heavier vehicles or a shop with equipment. Check local code, since some areas set a minimum.
Related: How Much Concrete for a 12x20 Driveway?
Placing the order
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 garage slab.
Getting the ground ready
Uneven ground is the main reason pours run short. Every half inch of extra depth across the garage slab adds concrete you did not plan for. Screed the base flat, check it with a string line and set the forms to finished height before the truck arrives.
Joints and reinforcement
Reinforcement does not stop cracks, but it holds them tight. Wire mesh or rebar set mid-depth on chairs is common for a garage slab that carries vehicles; fiber-reinforced mix is an option for light-duty slabs. Joints about 10 feet apart control where cracks form.
Questions people ask
How many yards of concrete for a 12x20 garage slab at 4 inches?
About 2.96 cubic yards, or 3.26 with 10 percent extra.
How many yards for a 12x20 garage slab at 6 inches?
About 4.44 cubic yards.
How many 80 lb bags for a 12x20 garage slab?
About 134 bags at 4 inches thick, before any waste allowance.
How many square feet is a 12x20 garage slab?
240 square feet.
How many 80 lb bags make a cubic yard?
About 45. Each 80 lb bag yields roughly 0.6 cubic feet, and a cubic yard is 27 cubic feet.
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
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