Linear actuator comprising • a reversible DC-motor, which through a transmission drives a spindle and thus applies a linear movement to a spindle nut retained against rotation, where the spindle nut is connected to an activation element, which can be displaced between two end positions on the spindle, • an electric control, • a housing surrounding the components of the actuator, and • a positon detection system for determining the position of the spindle nut on the spindle, wherein the position detection system is encased in the housing and further comprises • at least one magnetic sensor (13,14) arranged in a position on a length parallel to the axial direction of extent of the spindle and • a magnet arranged in connection with the spindle nut or the activation element, characterized in that the position detection system determines the position of the spindle nut on the spindle by counting rotations of a movable element in the actuator, whose movement relates to the rotations of the spindle, and indicating the position of the spindle as a counter value on a scale describing the dynamic range of the possible displacement of the activation element, and in that the position detection system is further configured to • receive and process a signal from the at least one magnetic sensor (13, 14), where the signal indicates a known position of the spindle nut during its travel on the spindle, and • subsequently calibrate the counter value of the position detection system such that it corresponds to a predetermined counter value of the known position of the spindle nut during its travel on the spindle. • a reversible DC-motor, which through a transmission drives a spindle and thus applies a linear movement to a spindle nut retained against rotation, where the spindle nut is connected to an activation element, which can be displaced between two end positions on the spindle, • an electric control, • a housing surrounding the components of the actuator, and • a positon detection system for determining the position of the spindle nut on the spindle, wherein the position detection system is encased in the housing and further comprises • at least one magnetic sensor (13,14) arranged in a position on a length parallel to the axial direction of extent of the spindle and • a magnet arranged in connection with the spindle nut or the activation element, characterized in that the position detection system determines the position of the spindle nut on the spindle by counting rotations of a movable element in the actuator, whose movement relates to the rotations of the spindle, and indicating the position of the spindle as a counter value on a scale describing the dynamic range of the possible displacement of the activation element, and in that the position detection system is further configured to • receive and process a signal from the at least one magnetic sensor (13, 14), where the signal indicates a known position of the spindle nut during its travel on the spindle, and • subsequently calibrate the counter value of the position detection system such that it corresponds to a predetermined counter value of the known position of the spindle nut during its travel on the spindle.