An image forming apparatus includes a developer container for containing a developer; a developer amount detector for detecting the amount of the developer contained in the developer container; a memory; and a calculator for calculating a display level of the remaining amount of the developer. At least the developer container and the memory are in a unit which is detachably mountable to a main assembly of the image forming apparatus. The memory stores information relating to a lifetime of the unit, and a device calculates the display level in accordance with the amount detected by the developer amount detector and the information relating to the lifetime of the cartridge.