The invention relates to a device (1) for determining method data of a method for cosmetically treating hair on a person's head. The inventive device comprises a microprocessor (3), at least one storage device (4), in which at least one application program (5) is contained, a data entry device (6), a data processing device (7), at least one method data output device (8), and a display device (9). The device (1) is provided as a battery-operated hand-held device (2). The storage device(s) (4) is/are contained in at least one optionally exchangeable module (10, 10.1). The application program (5) contains at least one data table (11) and application texts (12) which communicate with the data entry device (6), the data processing device (7), the method data output device (8), and with the display device (9). A dialog menu-assistance via the display device (9) is provided as a data entry device (6) by means of a menu program (13), and data is entered by means of a cursor function.