Embodiments are disclosed that relate to calibrating an eye tracking system for a computing device. For example, one disclosed embodiment provides, in a computing device comprising a gaze estimation system, a method of calibrating the gaze estimation system. The method includes receiving a request to log a user onto the computing device, outputting a passcode entry display image to a display device, receiving image data from one or more eye tracking cameras, and from the image data, determining a gaze scanpath representing a path of a user's gaze on the passcode entry display image. The method further includes comparing the gaze scanpath to a stored scanpath for the user, and calibrating the gaze estimation system based upon a result of comparing the gaze scanpath to the stored scanpath for the user.