This is the distance between your pupils in mm, which is needed to correctly position your lenses in your frame. This measurement won’t necessarily be written on the copy of your prescription. You can either ask your optician for this measurement or alternatively follow the steps of how to measure it yourself:
1. Stand around 6 inches away from a mirror.
2. Hold a ruler against your brow.
3. Close your right eye then align the ruler’s 0 mm with the center of your left pupil.
4. Look straight then close your left eye and open your right eye.
5. The mm line that lines up to the center of your right pupil is your PD.
See image below
If you are unable to obtain this measurement yourself, please be aware a generic pupillary distance will be used.