Your pupillary distance (PD), the distance between your pupils, is important in the manufacturing of your lenses correctly to give you the best and clearest vision. Measurements can vary but on average pupillary distance for women is approximately 62mm and for men, it's 64 mm. When calculating for reading glasses we subtract 3mm for a Near PD of 60mm which is typical for many people.
Your eye doctor often will measure your PD as part of your eye exam using a pupillary distance ruler or a corneal reflex pupillometer. You can request this information but if you don't have it here's a good way to measure it yourself.
This is How You Can Measure Your PD:
All you need is a millimeter ruler and a mirror.
Stand about 10-12 inches away from a mirror
Hold a ruler against your brow
Close your right eye then align the ruler's 0 mm with the center of your left pupil
Look straight then close your left eye and open your right eye
The mm line that lines up to the center of your right pupil is your PD
Repeat this just to double-check
*A do-it-yourself PD will not be as accurate as the measurement taken by a qualified optician or eye doctor who uses specific PD measuring tools.