Your PD is the distance from the center of one pupil to the center of the other in millimeters. This information is necessary to properly center your prescription correctly in your frames. It's important to note that the average PD is between 57 and 65mm. You may have two PD values - one for each eye. It is best to get this measured by your eye care professional. If your doctor did not include your PD measurement with your prescription, follow these easy steps to find your PD before you start your order: 1) Place a ruler or credit card against your forehead and line up an edge with your right pupil, 2) Looking straight ahead, mark the location of your left pupil on the ruler or credit card, and 3) Repeat multiple times and take an average measurement to ensure an accurate PD.