Measure the inside width of one drawer and call this value W. For example, if the interior space of the drawer is 12 inches across, then W = 12.
Measure the inside length of one drawer and call this value L. For instance, if the inside of the drawer is 16 inches front to back, then L = 16.
Measure the inside depth of one drawer and call this value D. For example, if the drawer is 10 inches from top to bottom, D = 10.
Multiply W, L and D to compute the volume of one drawer in cubic inches. Following the example, the volume of one drawer is 1,920 cubic inches because 12 x 16 x 10 = 1,920.
Multiply the drawer volume by the number of drawers to obtain the total capacity of the filing cabinet. For example, if your filing cabinet has three drawers, the total capacity is 5,760 cubic inches because 1,920 x 3 = 5,760.