Pd, which is short for pupillary distance, refers to the distance from the center of the pupil in one eye to the center of the pupil in the other eye. The correct value of your pupillary distance is usually required when you fill in the prescription which will ensure the correct centration of your prescription lenses in front of your eyes, or else you will feel fatigued, dizzy or even sick when you are wearing the glasses. That is why PD is so important for you when getting a pair of prescription glasses. You can measure your pupillary distance with a straight ruler, a pencil, and a mirror. First of all, you face the mirror, and place the ruler on the bridge of your nose, then bring the start of the ruler directly below the center of one eye's pupil. Secondly, look straight into the mirror, hold the ruler and mark the location on the ruler of the other eye's pupil. Here pay attention that you should keep the ruler steady and parallel to the floor. After that, measure the distance in millimeters between the two marks. And this is your pupillary distance. It is better to repeat this process a couple of times to ensure you have an accurate measurement. You can also take an online test to figure your pupillary distance, please refer to the following website.