Your interpupillary distance (PD) is the distance between your pupils in millimeters and an important factor in all aspects of your binocular vision; in fact eye doctors always measure this distance to account for in many of their binocular testings as well as the prescriptions they write for glasses. It should also be noted that an abnormal PD may induce binocular vision complications and/or be indicative of certain anatomical and physiological anomalies.
All you need to measure your own PD is a large mirror and a PD stick (fancy eye doctor term for a small ruler!).
Get close enough to the mirror so you can clearly see your pupils.
Keep your head upright and straight, place the ruler horizontally on the bridge of your nose in a way that its parallel with the floor, and slide it up and down your nose until the millimeter markings are visible right under your pupils.
Close your left eye and line up the -0- mark of the ruler with the center of your right pupil as accurately as you can. It is very important that your left eye is closed for the most accuracy; also any slight movement of your head or the ruler after this point will alter your measurements so don't move, next step will be quick!
Now close your right eye, open the left one, and read the millimeter marking that's exactly lined up with the center of your left pupil; your reading is your interpupillary distance. This distance normally ranges between 55 to 70 millimeters but deviations from this range does not necessarily indicate an anomaly.