What Is Pupillary Distance?

By: Anthony Tangeman

In a previous blog post Michael Walach described how to measure Pupillary Distance, or PD. Now we are here to tell you exactly what P.D. is and why it is important.

You can likely infer what PD means from its name: it is the distance between the centers of the pupils in millimeters. While a simple concept, this measurement is crucial in order to have glasses perfectly suited for your eyes.

The purpose of measuring PD is to place the optical center of the lens directly in front of your pupil. The optical center is a point where light rays go through a lens without any deviation. Proper placement of the optical centers allows both of your eyes to focus easily without any strain.

Because of this, an inaccurate PD measurement can throw off an entire eyeglasses prescription. The good news is that the measurement is not difficult to take. If you have questions about taking the measurement, please see the above link to Michael’s article on measuring PD.

It is also important to note that PD may be written with two or four numbers numbers instead of one. This can mean that your measurement is displayed as monocular PD (one number for each eye), or near and distance PD (two numbers if binocular PD, four numbers if monocular).

Binocular PD is a single measurement between the centers of the eye pupils in millimeters. Monocular measurement is essentially PD measured from the center of the bridge of the nose to the center of the pupil. Monocular measurement allows a more fine-tuned placement of the optical center in each eye.


PD is the starting point towards a proper eyeglasses prescription. When measuring your own PD, make sure to be as accurate as possible. The optical center, when positioned correctly, allows eyes to focus and see the world clearly without strain. For questions about PD or anything else eye-care related, feel free to contact GoSmartEyewear.