Concrete slab calculator
Estimate concrete for any slab.
For shed foundations, garage floors and general-purpose pours. Enter your slab's dimensions to get cubic yards, bags, an optional gravel base, and estimated cost.
Enter your dimensions
National average is $150–$200 per cubic yard, delivered — edit it to match your local supplier.
Results
- Volume needed
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- Recommended order
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- Gravel base (4″)
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- Estimated cost
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Bagged mix alternative
- 40 lb bags—
- 60 lb bags—
- 80 lb bags—
Bag counts include your waste % and round up to whole bags.
Estimates only — structural slabs should follow an engineer's spec or your local building code.
One formula, every slab
Length × width × thickness ÷ 27.
Every rectangular slab — patio, driveway, shed floor, or garage pad — uses the same volume formula. What changes is the thickness and base you should use for the job. This calculator defaults to a general-purpose 4 inches with an optional gravel base; adjust both to match what you're actually building.
Quick reference
Concrete for common slab sizes.
At 4 inches thick. Garage floors and driveways at 5 or 6 inches are in thefull slab size chart; reinforcing the slab? Use the rebar calculator.
| Slab (4″ thick) | Cubic yards | Order (+10%) | 60 lb bags | 80 lb bags |
|---|---|---|---|---|
| 4 × 4 ft | 0.20 yd³ | 0.22 yd³ | 14 | 10 |
| 8 × 8 ft | 0.79 yd³ | 0.87 yd³ | 53 | 40 |
| 10 × 10 ft | 1.23 yd³ | 1.36 yd³ | 82 | 62 |
| 10 × 12 ft | 1.48 yd³ | 1.63 yd³ | 98 | 74 |
| 12 × 12 ft | 1.78 yd³ | 1.96 yd³ | 118 | 88 |
| 10 × 20 ft | 2.47 yd³ | 2.72 yd³ | 163 | 123 |
| 12 × 20 ft | 2.96 yd³ | 3.26 yd³ | 196 | 147 |
| 16 × 16 ft | 3.16 yd³ | 3.48 yd³ | 209 | 157 |
| 20 × 20 ft | 4.94 yd³ | 5.43 yd³ | 326 | 245 |
| 24 × 24 ft | 7.11 yd³ | 7.82 yd³ | 470 | 352 |
Bag counts include 10% waste and round up to whole bags.
FAQ
Common questions
How thick should a concrete slab be?
4 inches works for sheds, walkways and general-purpose slabs. Garage floors and slabs supporting vehicles typically use 5–6 inches. Structural slabs should follow an engineer's spec or local code.
How much concrete do I need for a 10×10 slab?
At 4 inches thick, a 10×10 ft slab needs about 1.23 cubic yards before waste — enter 10 for both length and width above to see the exact number with your waste percentage and bag-count alternative included.
Do I need a gravel base under a slab?
For a slab on grade (not over an existing foundation), a compacted gravel base — commonly 4 inches — improves drainage and helps prevent cracking from ground movement or frost heave.
What's the difference between a slab, a footing and a patio calculator?
They all use the same volume formula — length × width × thickness. This page is for general-purpose slabs (sheds, garage floors, equipment pads). If you're pouring a patio or driveway specifically, those calculators start with more typical defaults for that project.
Building something specific?