Concrete Slab Calculator
Enter the length, width and thickness of your slab and get the concrete volume in cubic yards and cubic feet, the number of pre mixed bags, and an estimated cost. It works for patios, shed bases, garage floors and any flat rectangular pour.
Four inches is the standard thickness for most residential slabs. The calculator includes a 10 percent waste allowance by default, and both prices can be edited to match your local supplier.
Slab Concrete Calculator
Concrete for slabs, patios, shed bases and floors, with bag counts and an estimated cost.
Cost estimate uses the ready mix price of $165 per cubic yard and $5.50 per 80 lb bag set for this calculator. Estimates only, confirm quantities with your supplier before ordering.
Quick answers for common slab sizes
Figures are for a 4 inch thick slab and include 10 percent extra for waste.
| Slab size | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 8 x 8 ft | 0.87 | 40 | 53 |
| 10 x 10 ft | 1.36 | 62 | 82 |
| 10 x 12 ft | 1.63 | 74 | 98 |
| 12 x 12 ft | 1.96 | 88 | 118 |
| 12 x 16 ft | 2.61 | 118 | 157 |
| 16 x 16 ft | 3.48 | 157 | 209 |
| 20 x 20 ft | 5.43 | 245 | 326 |
The slab formula, with an example
Volume = length (ft) x width (ft) x thickness (ft). Divide cubic feet by 27 for cubic yards. For a 12 x 12 ft slab at 4 inches: 12 x 12 x 0.333 = 48 cubic feet, which is 1.78 cubic yards. With 10 percent waste the order amount is 1.96 cubic yards, or 88 bags of 80 lb mix.
Bags or a ready mix truck
A slab under about 1 cubic yard is a comfortable bag job. Once a slab passes 2 cubic yards, like a 16 x 16 ft slab at 3.48 yards with waste, a ready mix truck is cheaper and much faster than mixing 157 bags by hand.
Frequently asked questions
How many bags of concrete do I need for a 12 x 12 slab?
At 4 inches thick with 10 percent waste, a 12 x 12 ft slab needs 88 bags of 80 lb mix or 118 bags of 60 lb mix, which is 1.96 cubic yards.
What thickness is best for a shed slab?
Four inches on a compacted gravel base is enough for most sheds and workshops. Go to 5 or 6 inches if heavy equipment or a vehicle will sit on it.
Does the calculator include a gravel base?
No, it calculates the concrete only. A gravel base is placed and compacted before the pour and is ordered separately, usually 4 inches deep.
How do I calculate a slab that is not a rectangle?
Split the shape into rectangles, calculate each one, and add the volumes together. For a round pad, use the main calculator on the homepage with the post hole shape set to your diameter and depth.
Related calculators
- Concrete Calculator
- How Many Bags for a 10 x 10 Slab
- How Many Bags for a 12 x 12 Slab
- Concrete Patio Calculator
Estimates only. Quantities and prices are guides, not quotes. Confirm final amounts with your supplier before you order.