How Much Concrete for a 30x40 Garage Slab?
For a 30x40 garage slab poured 5 inches thick, plan on about 18.52 cubic yards of concrete. That comes from 1,200 square feet times 0.417 feet of depth, divided by 27. How much concrete for a 30x40 garage slab at other thicknesses is in the table below.
Concrete calculator →The math for a 30x40 garage slab
- Area: 30 ft × 40 ft = 1,200 sq ft.
- Thickness in feet: 5 in ÷ 12 = 0.417 ft.
- Volume: 1,200 × 0.417 = 500.0 cubic feet.
- Cubic yards: 500.0 ÷ 27 = 18.52 yd³.
- With 10% extra: 20.37 yd³.
The concrete calculator runs the same steps for any size and thickness.
Related: How Much Concrete for a 12x20 Garage Slab?
Concrete for a 30x40 garage slab by thickness
| Thickness | Cubic feet | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 400.0 | 14.81 | 16.30 | 667 | 889 |
| 5 in | 500.0 | 18.52 | 20.37 | 834 | 1,112 |
| 6 in | 600.0 | 22.22 | 24.44 | 1,000 | 1,334 |
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
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 12x24 Garage Slab?
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 garage 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 garage 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.
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 12 feet apart control where cracks form.
Questions people ask
How many yards of concrete for a 30x40 garage slab at 4 inches?
About 14.81 cubic yards, or 16.30 with 10 percent extra.
How many yards for a 30x40 garage slab at 6 inches?
About 22.22 cubic yards.
How many 80 lb bags for a 30x40 garage slab?
About 667 bags at 4 inches thick, before any waste allowance.
How many square feet is a 30x40 garage slab?
1,200 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
- How Much Concrete for a 30x32 Garage Slab?
- How Much Concrete for a 30x36 Garage Slab?
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