A concrete slab calculator works by calculating the amount of concrete needed in cubic feet by multiplying together the values in feet for the length, width and depth of the concrete slab. The result is divided by 27 to get cubic yards.

To estimate the amount of concrete needed for a slab, a concrete calculator takes values entered by a user for the length, width and depth of the slab in feet. The calculator multiplies the values together to derive the volume of the slab in cubic feet. Pre-mixed concrete is sold in cubic yards, so this result is divided by 27, the number of feet in a cubic yard, to calculate the volume of the slab in cubic yards.

To determine the number of bags of pre-mixed concrete needed, the calculator uses a value for the density of the concrete in pounds and multiplies it by the calculated result to determine the total pounds of concrete. Dividing the result by the size of the bag gives the number of bags needed.

For example, a concrete slab that is 10 feet long, 3 feet wide and 6 inches deep is 15 cubic feet or 0.556 cubic yards in volume. Using a value of 133 pounds per cubic foot for the concrete density, the weight of concrete needed is 1,995 pounds. This is approximately 33 60-pound bags or 25 80-pound bags of concrete.