How Much Concrete Do I Need?
For a rectangular slab, convert the thickness to feet, multiply length × width × thickness, then divide cubic feet by 27 to get cubic yards.
Concrete slab formula
Length (ft) × width (ft) × thickness (ft) ÷ 27 = cubic yards.
Example: 20 × 30 slab, 4 inches thick
Four inches is 0.333 feet. So 20 × 30 × 0.333 = about 200 cubic feet. Divide by 27 and the slab is about 7.41 cubic yards before waste.
With a 10% planning allowance, 7.41 × 1.10 = about 8.15 cubic yards. Ready-mix suppliers may have minimum-load, short-load or truck-capacity rules, so confirm the actual order with the plant.
What changes the number?
Uneven subgrade, thickened edges, footings, over-excavation, slopes and actual formed dimensions can all change concrete volume. Measure the job you are actually pouring instead of relying only on plan dimensions.
Calculate your slab
Enter the dimensions, waste allowance and truck capacity into the free concrete calculator.
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