Abstract: A calculating device installed in a calculator capable of performing arithmetical operations for discount is disclosed. The calculating device comprises a microprocessor for controlling the operations of the calculator; a discount calculating unit electrically connected to the microprocessor, the discount calculating unit being used to perform arithmetical operations for discount; an input unit electrically connected to the microprocessor, wherein the input unit comprises a plurality of discount operation keys installed on the calculator, and wherein when the discount operation keys are pressed orders for calculating arithmetical operations for discount are sent to the microprocessor; a random access memory electrically connected to the microprocessor, the random access memory being used to store data generated during the arithmetical operations for discount; and a display screen electrically connected to the microprocessor for displaying the data generated during the arithmetical operations for discount.