An X-ray apparatus includes a collimator comprising a lamp and configured to adjusting an irradiation region of X-rays radiated from an X-ray source; an image acquirer configured to acquire an object image by imaging an object while the lamp is turned on; and a controller configured to acquire an object distance based on the object image and acquire a thickness of the object based on a detector distance and the object distance. The object distance is a distance between the X-ray source and the object, and the detector distance is a distance between the X-ray source and an X-ray detector.