To calculate the size of a plenum box, you need to know the volume of the space. The volume is calculated by multiplying the length times width times height. For example, a standard 20 feet x 30 feet x 10 feet plenum space is 100 cubic yards.
The size of a plenum box can be calculated by multiplying length, width and height. The following formula will calculate a standard 20 feet x 30 feet x 10 feet plenum box:
length * width * height = cubic yards