How to measure pupillary distance?
In case its PD is not specified in the medical prescription for your lenses, you can easily measure it on your own. Watch this instructional video.
Please measure the distance 3-4 times to make it as accurate as possible.
The normal PD for an adult is: 54mm to 74mm
The normal PD for a child is: 44mm to 53mm
It is also possible that your optical advisor has this value. You can just call your optician to get your PD, and that way you will get the accurate measurement.
In any case, every time you get a new prescription, ask the professional to measure your PD or pupillary distance, since they should always write it down on your prescription.