What is a PD?
PD stands for Pupillary Distance; the distance measured in millimeters between the center of one pupil to the center of the opposite pupil. This measurement is important to ensure that your lenses will work properly. PD differs amongst individuals and once you're an adult this measurement will not change. Once your PD is determined you won't have to get this measurement again even if your prescription changes. This measurement is often omitted from a person's prescription, making it difficult to purchase glasses online. You can ask your local optician for this measurement or you can do it yourself at home with a few materials. All you'll need is a mirror and an mm ruler.
Follow these simple steps to find your PD:
1. While looking straight ahead into the mirror place the ruler against the bridge of your nose.
2. Line up the 0mm mark in the center of your right(or left) pupil; the dash that lands in the center of the opposite pupil is your PD.
3. Repeat this procedure 2 to 3 times to get the most accurate measurement possible.
See diagram below or watch this video