Pupillary Distance (PD) is the measurement, in millimeters, between the centers of your pupils. This number is a fundamental part of an eyeglass prescription, determining the precise point on each lens through which you will look. If the optical centers of your lenses are not correctly aligned with your pupils, you may experience eye strain, headaches, or blurred vision, even with the correct prescription power. Obtaining an accurate PD is necessary for ordering prescription eyewear, particularly from online retailers who require you to input this specific measurement yourself.
Utilizing Optical Professionals and Retailers
A reliable method for obtaining your pupillary distance is requesting it from the professional who performed your most recent eye examination. Although PD is technically part of the glasses fitting process, not the refractive eye exam, it is usually recorded in your patient file. The Federal Trade Commission (FTC) encourages providers to furnish this measurement, as patients are generally entitled to a copy of their health records.
In a few specific states, including Kansas, Massachusetts, Alaska, and New Mexico, the PD is legally mandated to be included directly on the eyeglass prescription provided to you after the exam. Even where not mandatory, many large retail optical chains or independent shops will measure your PD for free as a customer service gesture. You can simply call ahead and ask if they offer a complimentary pupillary distance check without requiring an eye exam or a purchase.
Free Digital Measurement Tools and Apps
Modern technology offers a convenient and accurate way to measure your PD using a smartphone or computer webcam. Several free applications and online tools utilize your device’s camera to capture an image of your face and calculate the distance between your pupils. The most effective of these tools often require you to hold a standard object, such as a credit card or a key, against your forehead for scale calibration.
The app’s software uses the known dimensions of the reference object to accurately determine the millimeter measurement in the photograph. Certain applications, particularly on newer iPhones, leverage advanced depth-sensing technology for greater accuracy. Studies show that the best-performing apps can achieve an error of 0.5 millimeters or less compared to professional instruments, which is sufficient for basic prescriptions. Always follow the app’s instructions precisely, ensuring you are in a well-lit area and holding the device steady for the most reliable results.
Simple At-Home Manual Measurement
You can measure your pupillary distance at home using a millimeter ruler and a mirror or the help of a friend. To measure yourself, stand approximately eight inches away from a mirror and hold the ruler horizontally across the bridge of your nose. Close your right eye and align the ruler’s zero mark with the center of your left pupil.
Without moving your head or the ruler, open your right eye, close your left eye, and read the millimeter mark that aligns with the center of your right pupil; this number is your total PD. Because manual self-measurement can introduce small errors, it is advisable to repeat the process two or three times to confirm the consistency of your result and use the average number.
If you have a friend assisting, they should perform the measurement while you look straight ahead at a fixed point roughly ten to twenty feet away. Looking at a close object or mirror can cause your eyes to converge slightly, resulting in a narrower measurement.