Measure the slab
For a rectangular slab, record length and width in feet and depth in inches. Measure in several places when the excavation is uneven.
Convert depth to feet by dividing inches by 12, then multiply length × width × depth to get cubic feet.
Convert volume to cubic yards
Ready-mix concrete is commonly sold by the cubic yard. Divide cubic feet by 27 to obtain cubic yards.
For example, a 10 ft by 10 ft slab at 4 inches deep contains about 33.33 cubic feet, or 1.23 cubic yards.
Plan for real-world conditions
Forms, excavation, spillage, and surface variation make the real requirement larger than the ideal geometric volume. A 5–10% allowance is common, but project conditions and supplier minimums should guide the final order.
Structural work should follow approved plans and local requirements. A volume estimate does not determine mix design, reinforcement, or load capacity.