Eyeglasses are a major part of a person's style, and when they are worn regularly, it is crucial to get the PD, or pupillary distance, correct. The pupillary distance is the distance between the pupils (black centers) of your eye, and the corrective lens in the eyeglasses should be centered correctly over the pupil for accurate vision. Without getting this distance correct, you could go home with shiny new glasses that look too big or too small for your face, and suffer eyestrain from an incorrectly placed lens.
Look at your prescription for previous glasses. The pupillary distance may have been measured previously and be detailed on the prescription. Some opticians may not put pupillary distance on prescriptions, to prevent you using the measurement to buy eyeglasses online, and also to encourage you to return to their shop for new glasses.
Ask your previous optician for your pupillary distance measurements if they are not mentioned on your prescription. If your optician has them on file, she may be able to give the measurements to you over the phone.
Self-measure the pupillary distance if you cannot obtain it from your optician. You can do this yourself, or if your eyesight is not good enough, ask a friend to help.
Stand in front of a mirror and hold up a ruler to your face. Align the zero millimeter mark with the center of the pupil of your right eye. (Pupillary distance is always measured in millimeters).
Keep your head and the ruler still and close your right eye. With only your left eye open, read the mark on the ruler that lies just under the center of your left pupil.
Repeat the measurements three times to get a consistent result. This result is your distance PD. Take away 3mm from the result to get your near PD.
Ask a friend to measure the distance for you if you cannot see the numbers clearly in the mirror. This person should sit about 18 inches away from you (the length of one and a half rulers). Keep both your eyes open and ask your assistant to close one eye. The distance she measures will be the near PD. To get the distance PD, add 3mm millimeters to the result.
Another way of obtaining PD is to put on your current eyeglasses, and have a friend mark the centers of your pupils on the glass with a non-permanent marker. Then you can measure the pupillary distance from dot to dot using a ruler.