Use a millimeter ruler
Stand about 8 inches away from a mirror
Close your right eye and align the 0 mm mark with the center of your left pupil
Keep the ruler steady and open your right eye
Read the measurement at the center of your right pupil
That number is your pupillary distance in millimeters
For single PD, measure from the center of one pupil to the center of the other
For dual PD, measure from the center of each pupil to the bridge of the nose
Repeat the measurement a few times and average the results
Ask an optician to measure it if you want the most accurate result