Place a Millimeters Ruler on the bridge of your nose.Measure the distance between the left pupil to the right pupil while looking into the distance (You can do this yourself, while looking into the mirror or ask someone to help you).
Another way to gather this information is by asking your licensed optician.
An acceptable prescription must always have the PD written on it.
Typical adult's pupillary distance measurements:
Pupillary sistance measurements are from 54 to 66.