Measure the external height, width and depth of the filing cabinet. Multiply the three dimensions to determine the total volume of the cabinet. For example, a filing cabinet with a height of 5 feet, a width of 2 feet and a depth of 3 feet has a total volume of 30 cubic feet -- 5 x 2 x 3 = 30. The result is the volume of space occupied by the cabinet; however this is not the same as the storage volume within the cabinet.
Measure the internal height, width and depth of each drawer within the cabinet. Determine the volume of each drawer using the equation volume = height x width x depth.
Add together the volumes of all the drawers to establish the total internal storage volume. For example, if a cabinet contains three drawers each with dimensions of 1.5 x 2 x 3 feet, the total volume is 27 cubic feet -- 3 x ( 1.5 x 2 x 3) = 27.