1. Calculate the Concrete Volume First, determine the Wet Volume of the slab by multiplying its dimensions. V wet=Length Width>Thickness Important: Always convert the thickness to the same unit as length and width (e.g., if the slab is 5 inches thick, use 5/12=0.416 ft or 0.125 m). Convert to Dry Volume Concrete ingredients (cement, sand, aggregate) are measured in a dry state. When water is added, the volume shrinks due to the filling of air voids. To account for this, We use a safety factor of