According to one embodiment, an image forming apparatus includes a document sensor to detect a width of a document stacked on an automatic document feeder (ADF), a sheet sensor to detect a width of a sheet stacked on a manual feed tray, memory to store, correspondingly to sheets of various sizes, print patterns based on which an entire area of a document image can be printed according to a document size, a controller which receives detection information from the sensors as input information, selects print patterns from the memory based on the input information, and displays the selected print patterns on a monitor, and a selector by which a user selects, from the print patterns displayed on the monitor, one pattern based on which a document image is printed on a sheet stacked on the ADF or the manual feed tray.