A method for defining a working area of a rock drilling rig including the steps of determining at least one current input parameter, determining a working area that defines a reach of the drilling unit in a control unit on the basis of the at least one current input parameter and at least one fixed reach parameter, displaying a graphical representation of the working area on a display, and re-determining the working area and updating the graphical representation of the working area dynamically in response to a change in the input parameters. The current input parameter may be, for example, an orientation of the rock drilling rig or a hole parameter.