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Concrete Slab Calculator

Size up a slab pour in cubic yards, the way ready-mix is actually ordered, and see the bag count too, so you can compare both routes before you commit.

Diagram of a concrete slab showing its length and width from above, with the thickness marked on a side view
Measure the pour's length and width; thickness is how deep the concrete goes.

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Enter your measurements

How slab yardage is calculated

Volume is length × width × thickness (thickness converted from inches to feet), multiplied by the number of slabs, then divided by 27 to get cubic yards.

Ready-mix is sold in quarter-yard steps, so the order quantity is your exact yardage plus a waste allowance, rounded up to the nearest 0.25 cu yd. Running short mid-pour costs far more than a quarter yard of extra concrete. Most suppliers also charge short-load fees on small orders.

Typical thicknesses: 4 in for patios, walkways and shed bases; 5–6 in for driveways that carry vehicles; follow local code and engineering for anything structural. Reinforcing the pour? The Rebar Calculator lays out the grid and counts the sticks.

Sources

The figures behind this calculator come from published manufacturer data and industry standards:

Worked examples

10 × 10 ft patio slab, 4 in

1.23 cu yd exact. 10% waste, rounded to quarter-yard steps.

Order 1.5 cu yd

20 × 10 ft driveway, 6 in

3.70 cu yd exact. 10% waste, rounded up.

Order 4.25 cu yd

Two 8 × 8 ft slabs, 4 in

1.58 cu yd combined. 10% waste, rounded up.

Order 1.75 cu yd

Frequently asked questions

How many cubic yards is a 10×10 slab?
At 4 inches thick, 1.23 cubic yards exact. Order 1.5 cu yd once waste and quarter-yard rounding are included. At 6 inches thick it is 1.85 cu yd exact, so order about 2.25 cu yd.
Why does the order amount round up to quarter yards?
Ready-mix plants batch and price in quarter-yard increments. Ordering the exact figure risks running short; the rounded figure is what a supplier will actually quote.
How thick should my slab be?
4 inches covers most patios, paths and shed bases. Driveways and garage floors are usually 5–6 inches, often with rebar or mesh. Structural slabs follow your local code.
Can I mix a slab from bags instead?
You can. The calculator shows the bag count for the same pour, but above roughly a cubic yard the mixing effort grows fast. A 10×10 slab at 4 in is already 62 bags.

Related calculators

Concrete Bag Calculator

How many 40, 60 or 80 lb bags of concrete you need, and what they'll cost.

Concrete Volume Calculator

Cubic yards and cubic feet for any pour shape.

Concrete Block (CMU) Calculator

Blocks and mortar bags for a block wall.

Rebar Calculator

Rebar pieces and grid layout for a slab.

Estimate disclaimer: results are planning estimates based on standard manufacturer yields and coverage figures. Site conditions, product choice and installation method change actual quantities. Verify before you buy, and follow local building code for structural work. This tool does not replace professional engineering advice.

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