Concrete Slab Calculator
Enter the length, width and thickness of your slab, or just the square footage. You get cubic yards, bags, cost and a rebar or mesh estimate.
How to calculate concrete for a slab
Slab volume = length × width × thickness. Keep all three in feet, then divide by 27 for cubic yards.
Example: a 12 ft × 16 ft patio at 4 inches thick is 12 × 16 × 0.333 = 64 cubic feet, or 2.37 cubic yards. With 10% waste you order 2.75 yards, or 118 bags of 80 lb mix.
Concrete for common slab sizes
All rows are 4 inches thick. The last two columns include 10% for waste.
| Slab size (ft) | Square feet | Cubic yards | Cubic yards + 10% | 80 lb bags + 10% |
|---|---|---|---|---|
| 8 × 8 | 64 | 0.79 | 0.87 | 40 |
| 10 × 10 | 100 | 1.23 | 1.36 | 62 |
| 10 × 12 | 120 | 1.48 | 1.63 | 74 |
| 12 × 12 | 144 | 1.78 | 1.96 | 88 |
| 12 × 16 | 192 | 2.37 | 2.61 | 118 |
| 16 × 16 | 256 | 3.16 | 3.48 | 157 |
| 20 × 20 | 400 | 4.94 | 5.43 | 245 |
| 24 × 24 | 576 | 7.11 | 7.82 | 352 |
Choosing a slab thickness
| Use | Typical thickness |
|---|---|
| Walkway, patio, shed pad | 4 in |
| Driveway for cars | 4 to 5 in |
| Driveway for heavier vehicles | 5 to 6 in |
| Structural slab or footing | Set by local code |
These are common residential figures, not a design. Soil, climate and load all matter, so confirm with your building department.
If you only know the square footage
Switch the calculator to “Square feet”, enter the area and the thickness, and it does the rest. By hand, divide the area by 81 for a 4-inch slab or by 54 for a 6-inch slab to get cubic yards.
L-shaped and multi-part slabs
Pick the L-shape option and enter the overall size and the cut-out corner. For anything more complex, add each rectangle as its own area and the calculator totals them.
Frequently asked questions
How much concrete do I need for a 10×10 slab?
1.23 cubic yards at 4 inches thick. With 10% for waste, order 1.5 yards or buy 62 bags of 80 lb mix.
How much concrete do I need for a 20×20 slab?
4.94 cubic yards at 4 inches thick, or 7.41 cubic yards at 6 inches. Add 10% for waste before you order.
How thick should a concrete slab be?
Four inches is the usual thickness for patios, walkways and shed pads. Driveways are commonly 4 to 6 inches. Check your local building code for anything that carries a structure or heavy vehicles.
How do I calculate concrete for an L-shaped slab?
Work out the full rectangle and subtract the missing corner, or split the slab into two rectangles and add them. The calculator has an L-shape option that does this for you.
Do I need rebar in a 4-inch slab?
It depends on the use and your local code. Many patios use wire mesh or no steel at all, while driveways and slabs carrying loads are usually reinforced. The calculator can estimate rebar or mesh for the slab you enter.