A stacking device includes a stacking unit, a regulation member, an interlocking member, a pivoting member provided on the interlocking member, a sensor having an output, and a determination unit. The stacking unit stacks a sheet on the stacking unit. The regulation member moves in a first direction and a second direction opposite to the first direction and regulates a position of the sheet by abutting on an edge of the sheet stacked in the stacking unit. The interlocking member interlocks with the regulation member. The pivoting member rotates. The sensor output changes based on positions of the interlocking member and the pivoting member. The determination unit determines a size of the sheet based on the output of the sensor.