A virtual reality headset includes multiple illumination sources emitting light towards a user's eye and an image capture device capturing light reflected by the user's eye. The image capture device captures images of light from the illumination sources reflected by the user's corneas when the user looks at a specific location in the virtual reality headset. Based on locations of light having at least a threshold intensity in the captured images, the position of the center of user's eye's pupil is determined in three dimensions and used to determine a distance between the center of user's eye's pupil and a reference point relative to the illumination sources. Distances between centers of pupils of the user's eyes and reference points are used to determine a distance between the centers of the pupils of the user's eyes and a distance from the corneas to an optical system of the headset.