From slab dimensions to a concrete order estimate

A 20 ft × 12 ft slab at 4 inches thick contains about 2.963 cubic yards before overage. The easy mistake is mixing inches and feet.

1. Put all three dimensions in feet

The thickness is 4 ÷ 12 = 0.3333… ft. Keep the unrounded fraction while calculating. Length and width are already in feet.

2. Calculate volume, then convert to yards

One cubic yard contains 27 cubic feet because a 3 ft cube is 3 × 3 × 3. Dividing by 3 instead of 27 gives the wrong volume.

3. Add an explicit allowance

With an illustrative 10% allowance, 2.96296… × 1.10 = 3.25926… cubic yards. The slab calculator displays 3.259 cubic yards. The 10% is an editable example, not a requirement for every pour.

4. Keep the estimate separate from the order

Ask the supplier about order increments, delivery minimums, short-load charges, and mix specifications. If a supplier uses quarter-yard increments, rounding 3.259 up to the next increment gives 3.5 cubic yards. Do not assume every supplier uses that increment.

Account for what the rectangle leaves out

Thickened edges, footings, irregular excavation, slopes, and separate pads require their own quantities. Do not count the same volume twice. This calculator does not select slab thickness, reinforcement, or concrete strength.

Quantity is only one part of the cost

Use the concrete cost calculator with the supplier's quoted unit price. Its separate waste input should be zero if your entered volume already includes all overage. Add quoted delivery, pumping, and other charges separately.

Estimate your slab

Prepared by JobSiteMath. Worked examples are checked against the calculator tests. Read our methodology.