A display device includes a housing, a flexible display panel, a light receiving sensor, and an area calculator. The flexible display panel is coupled to the housing and displays a reference image and an output image. The flexible display panel has an exposed area with a size that varies with movement of the flexible display panel in a first direction relative to the housing. The sensor senses the reference image and generates sensing data based on the reference image. The area calculator calculates the exposed area of the flexible display panel based on the sensing data.