An image forming apparatus includes a transfer unit configured to transfer a toner image based on image data onto a recording medium, a fixing device configured to thermally fix the toner image transferred onto the recording medium, a stack tray configured to stack a recording medium conveyed from the fixing device, and a controller configured to calculate based on the image data an amount of a toner on a top surface of the recording media stacked on the stack tray and an amount of a toner on a bottom surface of the recording media to be subsequently conveyed and discharged onto the stack tray and to control a recording medium discharge interval based on a result of calculating the amounts of the toners.