How Much Concrete for a 20x20 Garage Slab?
How much concrete for a 20x20 garage slab depends on thickness. The garage slab covers 20 × 20 = 400 square feet, and at 5 inches thick that is about 6.17 cubic yards, or 6.79 yards with a 10 percent allowance. Mixed from bags, it is roughly 278 bags of 80 lb mix.
Concrete calculator →The math for a 20x20 garage slab
- Area: 20 ft × 20 ft = 400 sq ft.
- Thickness in feet: 5 in ÷ 12 = 0.417 ft.
- Volume: 400 × 0.417 = 166.7 cubic feet.
- Cubic yards: 166.7 ÷ 27 = 6.17 yd³.
- With 10% extra: 6.79 yd³.
The concrete calculator runs the same steps for any size and thickness.
Related: How Much Concrete for a 20x20 Slab? Yards by Depth
Concrete for a 20x20 garage slab by thickness
| Thickness | Cubic feet | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 133.3 | 4.94 | 5.43 | 223 | 297 |
| 5 in | 166.7 | 6.17 | 6.79 | 278 | 371 |
| 6 in | 200.0 | 7.41 | 8.15 | 334 | 445 |
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 20x20 Patio?
Ordering ready-mix
A cubic yard of concrete is 27 cubic feet and weighs roughly 4,000 pounds. Most ready-mix trucks carry up to about 9 to 10 yards, though local trucks and rules vary. If your total is under a yard or two, bagged mix or a short-load supplier may be cheaper.
Base and forms
Prep matters as much as the yardage. Strip sod and soft soil, compact the subgrade, then add a compacted stone base, commonly about 4 inches. A level, firm base also keeps your thickness even, which keeps the yardage close to the math.
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 20x20 garage slab at 4 inches?
About 4.94 cubic yards, or 5.43 with 10 percent extra.
How many yards for a 20x20 garage slab at 6 inches?
About 7.41 cubic yards.
How many 80 lb bags for a 20x20 garage slab?
About 223 bags at 4 inches thick, before any waste allowance.
How many square feet is a 20x20 garage slab?
400 square feet.
How much does a cubic yard of concrete weigh?
Roughly 4,000 pounds, or about 2 tons, for normal-weight concrete.
When is bagged concrete practical?
Usually for jobs under about a cubic yard. Beyond that, mixing dozens of bags by hand gets slow and batches may not blend well.
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 16x20 Garage Slab?
- How Much Concrete for a 16x24 Garage Slab?
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