Your PD is the distance between each of your pupils. This measurement is necessary to ensure the correct positioning of your lenses within the frame you have chosen. Your PD can easily be measured in three simple steps.
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This video explains what a PD (Pupillary Distance) is and how to measure it.
You can do it yourself simply by looking in the mirror. If you need your glasses on to do this, you can draw dots on the lenses where your pupils are (using a magic marker or highlighter); then take the glasses off and measure the distance between them.
You can also get this information from your doctor or from a licensed optician.
If you have two PD numbers on your prescription, you have either been given both the Distance/Reading or Right Eye/Left Eye.
Here's what you do:
As a Guideline: Most adults' PDs are between 55-65 mm. Most kids' PDs are between 42-54 mm.