An image forming apparatus includes a stack portion on which a sheet is to be stacked, a regulating portion, a first detection device which includes a rotary variable resistor, a second detection, and a control unit. The regulating portion regulates a position of an edge of the stacked sheet. The first detection device outputs a detection signal based on the rotary variable resistor. The rotary variable resistor rotates in accordance with the sheet edge position regulated by the regulating portion. The second detection device detects presence or absence of the stacked sheet. The control unit calculates a calculated sheet width of the stacked sheet based on the output detection signal. The control unit sets the calculated sheet width after a first predetermined time has elapsed since the second detection device detects that the sheet is stacked on the stack portion as a detected sheet width of the stacked sheet.