Abstract: The invention relates to a device that comprises: an element (1) for placement of a foot, which is provided with means (2-4, 7, 8) for positioning the foot (13) of a user and a mark (6) corresponding to a true measurement on the element (1); a camera (11) for capturing an image of the element (1) with the foot (13) placed in the required position; and a processor (12) for processing the image obtained in accordance with a conventional augmented reality program (AR), in order to obtain the true measurement of the length of the foot and to determine a size in accordance with conventional established sizes. The invention facilitates the purchase of footwear, both in a store and online.