An image forming apparatus includes a sensor configured to detect a length of a sheet, which is placed on a platen, in a predetermined direction and a controller. The controller is configured to convert an image signal based on a conversion condition, control the image forming unit to form an image based on the converted image signal, and execute calibration in which the conversion condition is generated. The controller is configured, in a case in which the calibration is to be executed, to acquire information related to a size of a sheet on which a test chart is to be formed, select a sheet based on the information, form the test chart on the selected sheet acquire reading data output from the reader, and generate the conversion condition based on the reading data.