An image forming apparatus includes a discharge tray; a printing unit; a stacking controlling unit that determines a front side value related to toner amount on a front side of a sheet and a back side value related to toner amount on a back side, determines an absolute value of difference between the front side value and the back side value for each sheet, determines a correction value of each sheet based on the absolute value and correction value data, adds the correction value to a predetermined value for determining a corrected count value of each sheet, integrates the corrected count value, and detects full of the discharge tray when the integrated value reaches a predetermined upper limit value; and a storage unit for storing the correction value data in which the correction value is determined corresponding to the absolute value.