Concrete Slab Calculator
Size a slab pour with volume, bags and rebar guidance.
How much concrete does a slab need?
A concrete slab needs length times width times thickness in feet, divided by 27, to get cubic yards. A 12 ft by 14 ft slab at 4 inches thick takes about 2.07 cubic yards. Stepping that same slab up to 6 inches raises it to 3.11 cubic yards, a 50 percent increase in concrete.
Concrete Slab Calculator
Inputs
Results
Cubic Yards
2.07 yd³
Concrete to order
Slab Area
168 sq ft
80 lb Bags
94
Gravel Base
2.07 yd³
Compacted sub-base
Mesh Sheets
4
3.5 x 8 ft sheets
Estimates update instantly as you type. Confirm against local code before ordering materials.
A slab is the most common concrete pour a contractor handles, and getting the volume right is the difference between a clean single delivery and a scramble for more material mid-pour. This calculator takes the slab length, width, and thickness and returns cubic yards, total cubic feet, the finished surface area, and bag counts for smaller jobs.
Slab thickness drives cost more than most people expect. Moving from a 4 inch to a 6 inch slab increases your concrete volume by 50 percent for the same footprint, so it pays to confirm the structural requirement before ordering. Use the waste allowance to cover the uneven base and edge thickening that nearly every slab needs.
Slab volume formula
A slab is a flat rectangular pour. Volume is calculated the same way as any concrete pour, then converted to cubic yards for ordering from a ready-mix supplier.
Cubic Yards = (Length ft × Width ft × Thickness ft) ÷ 27Concrete needed by slab size
Cubic yards required for common slab footprints at two thicknesses.
| Slab size | Area | At 4 in | At 6 in |
|---|---|---|---|
| 10 × 10 ft | 100 sq ft | 1.24 yd³ | 1.85 yd³ |
| 12 × 14 ft | 168 sq ft | 2.07 yd³ | 3.11 yd³ |
| 20 × 20 ft | 400 sq ft | 4.94 yd³ | 7.41 yd³ |
| 24 × 24 ft | 576 sq ft | 7.11 yd³ | 10.67 yd³ |
Order full yards when you are close to a half-yard threshold to avoid short-load fees.
How to use it
- 1Enter the slab length and width in feet.
- 2Enter the slab thickness in inches (4 in is typical for patios).
- 3Review volume and the recommended bag count.
- 4Order ready-mix for pours larger than about one cubic yard.
Key terms explained
- Slab on grade
- A concrete slab poured directly on prepared ground, used for floors, patios, and pads.
- Thickened edge
- A deeper perimeter section that adds strength where the slab carries walls or loads. It adds volume beyond the flat field.
- Control joint
- A tooled or sawn groove that tells the slab where to crack as it cures, preventing random cracking.
- Vapor barrier
- A plastic sheet under the slab that blocks ground moisture. It does not change volume but is part of the assembly.
Worked examples
Patio slab
A 12 ft × 14 ft patio at 4 in thick requires about 2.07 cubic yards.
Shed base
A 10 ft × 12 ft shed pad at 4 in deep needs roughly 1.48 cubic yards.
Pro tips from the field
- 1Specify slab thickness from the structural plan, not habit; an extra inch across a large pad is real money.
- 2Plan control joints at roughly 24 to 36 times the slab thickness in inches, spaced evenly.
- 3Pour against a screeded, compacted base to keep your actual volume close to the calculated figure.
- 4For slabs over 400 square feet, schedule enough finishers, surface workability drops fast once placement starts.
Common mistakes
- Pouring thinner than 4 inches on load-bearing slabs.
- Skipping a compacted gravel base under the slab.
- Not accounting for a thickened edge or footing.
Where it gets used
Shop and garage floors
Estimate interior floor slabs including the thickened perimeter in one pass.
Equipment pads
Size pads for HVAC units, generators, and tanks where thickness is critical.
Driveways
Plan residential driveway slabs and confirm whether a 4 or 5 inch section is required.
Frequently asked questions
Concrete Slab Calculator for Patios and Pads
Plan a slab pour with confidence. The FoxCalc slab calculator converts your length, width and thickness into the exact volume of concrete you need, plus a bag count for smaller jobs.
Proper slab thickness and a compacted base are critical to preventing cracking and settling over time. Use these numbers as a planning baseline and verify against local code requirements.
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