The question of how to measure eyeglasses probably didn't occur to people prior to being able to order eyeglasses over the Internet. They went to the eye doctor's office and picked out a pair of frames after the doctor examined their eyes. Today, with a prescription from the eye doctor in hand, picking out new frames can be done online, eliminating the fitting process. You will get your eyes tested, After the tests are performed, the doctor will write a prescription for your eyeglass lenses, and take measurements, like the distance between your pupils. This measurement, abbreviated as PD, is essential to ensuring the optical centers of your lenses will sit directly in front of your pupils. This number is measured in millimeters, and the average distance is about 63 mm. This measurement is extremely important for strong prescriptions. If PD is incorrectly measured, eyestrain and headaches may occur, and vision will not be optimal.When ordering glasses online, check your old glasses for the numbers you will find either behind the bridge (nose-piece) or on the inside of the arm. The numbers will look like this: 50/17 135 and may be separated by a box shape. These numbers represent the frame size you will need to order. If this is your first pair of glasses, visit an eyeglass store and ask to be fitted, copy the number down, or simply order from the store.