About the concrete calculator
Concrete is ordered by volume. For a slab, multiply length by width by thickness (converted to feet) and divide by 27 to get cubic yards. For round columns or post holes, the calculator uses the cylinder volume. A 10% allowance covers spillage, uneven sub-grade and over-excavation.
For small jobs, bagged concrete is easier; for anything above roughly 1 cubic yard, ready-mix delivery usually saves time.
Formula
Slab volume (yd³) = length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27. Column volume = π × (diameter ÷ 2)² × depth × quantity.
Frequently asked questions
How many 80 lb bags make a cubic yard?
About 45 bags of 80 lb concrete mix make one cubic yard (27 ÷ 0.6).
How thick should a concrete slab be?
Patios and walkways are usually 4 inches thick. Driveways and garage floors are typically 5 to 6 inches.