Finding your Pupillary Distance (PD) is often the last step before ordering new eyeglasses, especially online. The PD is a precise measurement, separate from the refractive power of your prescription. Understanding this measurement dictates how your lenses are manufactured and fitted into your chosen frames. This number, expressed in millimeters, ensures your new corrective eyewear provides the clearest, most comfortable vision possible.
Defining Pupillary Distance
Pupillary distance (PD) is the measurement of the space between the centers of your two pupils, expressed in millimeters (mm). This distance tells the optical lab where to align the optical center of the lens to match the center of your eye. An accurate PD is fundamental to manufacturing prescription glasses.
There are two primary ways this measurement is recorded: single and dual PD. A single PD (binocular PD) is one number representing the total distance between the pupils. This is often sufficient for single-vision lenses.
A dual PD (monocular PD) is a more precise measurement that records the distance from the center of each pupil to the bridge of the nose separately. This results in two numbers, such as 32/30 (right eye/left eye). Dual PD is preferred for progressive or high-power prescriptions because it accounts for slight facial asymmetry.
Locating PD on an Eyewear Prescription
The first place to look for this measurement is on the physical paper your eye doctor provided after your exam. Scan the document for the abbreviations “PD,” “D.P.,” or the full phrase “Pupillary Distance.” It is typically written in a dedicated box, or sometimes placed near the bottom or side of the form, separate from the main grid listing your sphere (SPH), cylinder (CYL), and axis numbers.
If you have a dual PD, it will appear as two distinct numbers, sometimes separated by a slash (e.g., 31/30). You may also find two different PD measurements listed: “Distance PD” for far vision and “Near PD” for reading. The eyes naturally converge when focusing close up, so the near PD is usually 3 to 4 millimeters shorter than the distance PD.
The PD may not always be included on the prescription because it is considered a dispensing measurement, not a refractive measurement. Refractive measurements determine lens power, while dispensing measurements determine how the lens is physically fitted to the wearer. If you cannot find it, this omission is common.
The Role of PD in Eyewear
The necessity of the PD measurement lies in its relationship to the optical center of the lens. Every corrective lens has an optical center, the exact point through which light passes without distortion. For the clearest vision, this optical center must be positioned directly in front of the center of your pupil. The PD provides the precise coordinates for the manufacturer to ensure this alignment.
If the PD is incorrect, the optical center shifts away from your pupil. This misalignment forces the eye to look through a non-optical part of the lens, introducing an unwanted prismatic effect. This effect causes light to bend incorrectly, forcing your eyes to work harder to compensate.
This visual stress can lead to uncomfortable symptoms, even if the rest of your prescription is accurate. Common complaints include eye strain, headaches, fatigue, and blurry or double vision. The impact of an inaccurate PD is significantly more noticeable for individuals with higher-strength prescriptions.
Methods for Obtaining Your PD Measurement
If the PD measurement has been omitted, the most reliable option is to contact the eye care professional who performed your examination. Most practices keep this measurement on file and can provide it upon request, as it was likely measured during the exam using a specialized tool called a pupillometer. Some regions require providers to release this measurement to the patient.
If contacting your provider is not possible, you can attempt an at-home measurement using a ruler marked in millimeters.
Measuring PD with a Mirror
You can measure your single PD by standing about eight inches away from a mirror. Line up the zero mark of the ruler over the center of one pupil, then read the measurement that aligns with the center of the other pupil.
Measuring PD with Assistance
A more accurate method involves having a friend assist you. This ensures you are looking straight ahead and not converging your eyes. Focus on an object roughly ten to twenty feet away while your assistant measures the distance between the centers of your pupils.
Digital Measurement Tools
Various smartphone applications and online tools are also available that utilize a webcam or phone camera to calculate the PD. These digital tools often require a standard-sized reference object, like a credit card, to calibrate the scale. Measure several times to ensure consistency and accuracy, as a small error can significantly impact your vision.