Pupillary Distance, or PD, is the distance in millimeters between the center of one pupil to the other pupil. Your PD is usually included in your prescription and can also be measured easily if not. When measuring you will need to find a PD ruler that you can print out. You will then place the ruler directly and horizontally over the center of the right pupil. To add ease, place the ruler against your forehead for added stability and stand in front of a mirror. Once the ruler is stable and while looking straight ahead, the distance from the center of your right pupil to the center of your left pupil. If you are unsure of your measurement, repeat a couple times for better accuracy.