Abstract: Disclosed is a manually operable calculator for determining a fourth value on the basis of three variables. According to one example, the calculator is used to determine the total storage available for a recording medium consisting of several units, such as hard disks or magneto optical disks. Specifically, values for total storage are displayed in response to inputting the capacity of each disk, the number of disks and the data recording rate. According to the invention, the calculator comprises a slide member upon which two rotating wheels are mounted along a common axis of rotation. The slide and wheels each have indicia corresponding to the three variables printed thereon. The indicia on the wheels is selected by rotation while the indicia on the slide is selected by moving the slide within a sleeve having openings formed therein.